A Review on the Effect of High Pressure Processing (HPP)

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molecules Review ReviewA Review on the Effect of High Pressure Processing A Review on the Eﬀect of High Pressure Processing (HPP) on Gelatinization and Infusion of Nutrients (HPP) on Gelatinization and Infusion of Nutrients Akash Kaushal Balakrishna, Md Abdul Wazed and Mohammed Farid * Akash Kaushal Balakrishna , Md Abdul Wazed and Mohammed Farid * Department of Chemical and Materials Engineering, University of Auckland, Private Bag 92019, Auckland 1142,Department New Zealand; of Chemical [email protected] and Materials Engineering, (A.K.B.); University [email protected] of Auckland, Private Bag (M.A.W.) 92019, Auckland* Correspondence: 1142, New [email protected]; Zealand; [email protected] Tel.: +64-9-9234807 (A.K.B.); [email protected] (M.A.W.) * Correspondence: [email protected]; Tel.: +64-9-9234807 Academic Editors: Carmen Cuadrado and Karim Allaf Academic Editors: Carmen Cuadrado and Karim Allaf Received: 13 April 2020; Accepted: 18 18 May May 2020; 2020; Published: Published: 20 20 May May 2020 2020

Abstract: High pressure processing (HPP) is a novel technology that involves subjecting foods to high hydrostatic pressures of the order of 100–600 MPa. This te technologychnology has been proven successful for inactivation of numerous microorganisms, spor sporeses and enzymes in foods, leading to increased shelf life. HPP is not limited to cold pasteurization, but has many other applications.applications. The focus of this paper is to explore other applications of H HPP,PP, suchsuch asas gelatinization,gelatinization, forced water absorption and infusion of nutrients. The The use use of of high high pressure pressure in in producing producing cold cold gelatinizing gelatinizing effects, eﬀects, imparting unique properties to food and improving food quality will be also discussed, highlighting the latest published studies and the innovative methods adopted.

Keywords: highhigh pressure processing (HPP); gelatinization; infusion; quality improvement

1. Introduction Introduction High pressure processing (HPP) or high hydrostatic pressure (HHP) are terms that describe subjecting foodsfoods to to pressures pressures in in the the order order of thousands of thousands of atmospheric of atmospheric pressures. pressures. The foods The arefoods usually are vacuumusually vacuum packed andpacked placed and intoplaced a basket, into a whichbasket, iswhich then loadedis then intoloaded the into HPP the machine, HPP machine, as shown as shownin Figure in1 Figure. 1.

End Closures Packed foods

Pressure Vessel Pressure Transmitting Liquid Carrier Basket (water)

Figure 1. SchematicSchematic representation representation of a high pressure processing (HPP) vessel.

The twotwo openingsopenings are are closed closed with with end end closures closures and and a pressure a pressure transmitting transmitting liquid, liquid, which iswhich usually is water,usually is water, used to is pressurizeused to pressurize the vessel. the The vessel. combined The combined effect of a effect high pressureof a high pump(s) pressure with pump(s) a pressure with aintensifier(s) pressure intensifier(s) is used to generate is used the to requiredgenerate pressure. the required The foods pressure. are then The held foods at a are particular then held pressure at a particularfor a specified pressure duration. for Thisa specified is followed duration. by a quick This decompression is followed and by unloading.a quick decompression Under compression, and unloading.there is an increaseUnder compression, in water temperature there is an of increase up to 3 ◦inC water for every temperature 100 MPa increaseof up to in3 °C pressure. for every Lipid 100 based foods experience a slightly higher increase in temperature with pressure [1]. The temperature

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Molecules 2020, 25, 2369; doi:10.3390/molecules25102369 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 18

Molecules 2020, 25, 2369 2 of 18 MPa increase in pressure. Lipid based foods experience a slightly higher increase in temperature with pressure [1]. The temperature returns to close to the initial temperature upon depressurization. HPP hasreturns been to widely close to investigated the initial temperature for increasing upon the depressurization. shelf life of foods HPP hasthrough been widelythe inactivation investigated of microorganismsfor increasing the [2], shelf spores life [3] of foodsand enzymes through [4] the present inactivation in foods. of HPP microorganisms can also be used [2], sporesfor functional [3] and andenzymes physical [4] presentmodifications in foods. of foods HPP [5,6]. can alsoIt has be also used been for tested functional for creating and physical pressure-shift modifications freezing of andfoods thawing, [5,6]. It which has also is beenoutside tested of the for scope creating of this pressure-shift review. This freezing review andcovers thawing, the application which is outsideof HPP forof theinducing scope oflow this temperature review. This gelatinization, review covers changing the application food properties of HPP for and inducing enhancing low food temperature quality. Somegelatinization, of the prior changing review food works properties have been and exclusivel enhancingy done food on quality. high Somepressure of the gelatinization prior review of works pure starchhave been [7,8] exclusively and on high done pressure on high pressureinfusions gelatinization [9]. This article of pure is starcha unique [7,8 ]review and on highof the pressure latest developmentsinfusions [9]. Thisin these article fields is a and unique includes review discussions of the latest on developmentsother aspects of in gelatinization these fields and of starch includes in foods,discussions gelatinization on other aspectsin other of compounds, gelatinization texture of starch and in foods,property gelatinization enhancement in otherin whole compounds, foods, nutritional,texture and flavour, property aroma enhancement related infusions, in whole foods,etc. nutritional, flavour, aroma related infusions, etc.

2. High High Pressure Pressure Gelatinization Gelatinization

2.1. Starch Starch Gelatinization Gelatinization Starch isis a polymerica polymeric carbohydrate carbohydrate that consiststhat cons ofists two typesof two of molecules,types of molecules, amylose and amylose amylopectin and amylopectin(Figure2). When (Figure starch 2). When granules starch swell granules up in swell the presence up in the of presence water and of water heat, theand processheat, the is process called isgelatinization. called gelatinization. During this During process, this water process, molecules water aremolecules adsorbed are onto adsorbed the amylose onto the and amylose amylopectin and amylopectinmolecules, breaking molecules, the breaking intermolecular the intermolecular hydrogen bonds hydrogen present bonds in starch. present On in cooling, starch. On starch cooling, loses starchwater andloses reformation water and reformation of hydrogen of bondshydrogen takes bonds place; takes this place; is called this retrogradation. is called retrogradation. High pressure High pressureprocessing processing causes cold causes gelatinization cold gelatinization of starches. Peaof starches. starch gelatinizes Pea starch at 25gelatinizes◦C and pressures at 25 °C above and pressures400 MPa. above The presence400 MPa. ofThe water presence is necessary, of water withoutis necessary, which without there willwhich be th noere e ﬀwillect be of no pressure effect ofon pressure starch [10 on]. starch [10].

Figure 2. SchematicSchematic representation representation of of a a starch starch granule granule with with its its structure structure and and types (redrawn from O’Neill and Field [11] [11] and Raguin and Ebenhöh, 2017 [12]). [12]).

2.1.1. Gelatinization Gelatinization Mechanism Mechanism Previous studies have suggested that the use use of of high high pressure pressure results results in in lowering lowering the the gelatinizing gelatinizing temperaturetemperature of of the the starch starch and and that that the gelatinization the gelatinization temperature temperature is a non-linear is a non-linear function functionof pressure of [13,14].pressure The[13 higher,14]. The the pressure, higher the the pressure, greater the the redu greaterction theis. It reduction has been reported is. It has that been the reported mechanisms that ofthe gelatinization mechanisms ofin gelatinizationthe pressure treated in the pressurestarch and treated heat starchtreated and starch heat are treated different starch [15]. are Starch, diﬀerent when [15]. Starch, when heated with water, swells up and loses the ellipsoidal lamellar ring structure along with

Molecules 2020, 25, x FOR PEER REVIEW 3 of 18 Molecules 2020, 25, 2369 3 of 18 heated with water, swells up and loses the ellipsoidal lamellar ring structure along with its itscrystallinity; crystallinity; whereas whereas pressure pressure gelatinization gelatinization main maintainstains the the shape shape and and structure structure of the granulesgranules andand lamellaelamellae asas shownshown inin FigureFigure3 .3. Similar Similar conclusions conclusions were were reported reported in in the the study study comparing comparing thermal thermal gelatinizationgelatinization with with pressure pressure gelatinization gelatinization of wheat of wheat starch starch [16]. The[16]. SEM The analysis SEM analysis of the starch of the granules starch gelatinizedgranules gelatinized using heat using and pressure heat and revealed pressure that revealed the gelatinization that the gelatinization caused by heat caused results by in heat disruption results (structurein disruption loss) (structure of the granules, loss) of whereas the granules, the pressure whereas treated the onespressure undergo treated distortion ones undergo (structure distortion change). Heat(structure treatment change). acted Heat by gelatinizingtreatment acted only by the gelatinizing less stable crystallites,only the less whereas stable crystallites, the pressure whereas treatment the actedpressure on bothtreatment the less acted stable on andboth more the less stable stable crystallites. and more stable crystallites.

Partial gelatinization Continued Complete gelatinization heat Granule shape lost

Continued pressure

Partial gelatinization Complete gelatinization Granule shape retained

FigureFigure 3.3.Schematic Schematic representation representation of heat of heat and pressureand pressu gelatinizationre gelatinization (redrawn (redrawn Yamamoto Yamamoto et al., 2009 et [15 al.,]). 2009 [15]). Another study showed that gelatinization occurs at diﬀerent pressures and temperatures for the diﬀerent starches [17]. Potato starch is highly resistant to pressure, whereas that of wheat is much more Another study showed that gelatinization occurs at different pressures and temperatures for the sensitive. The degree of gelatinization increases with temperature at any pressure and vice versa and different starches [17]. Potato starch is highly resistant to pressure, whereas that of wheat is much the action caused by pressure is time-dependent. more sensitive. The degree of gelatinization increases with temperature at any pressure and vice 2.1.2.versa Propertiesand the action of Pressure caused Gelatinizedby pressure Starchis time-dependent.

2.1.2.High Properties pressure of Pressure treatment Gelatinized of starch resultsStarch in starch with unique properties. Along with the preservation of the granule structure, the starch gel formed by HPP treatment of tapioca starch showed greaterHigh hardness pressure [18 ].treatment When tapioca of starch starch results was pressurizedin starch with at 600 unique MPa inproperties. the temperature Along rangewith the of preservation of the granule structure, the starch gel formed by HPP treatment of tapioca starch 30 ◦C to 80 ◦C, the hardness was more evident in the gels treated at lower temperature and became less prominentshowed greater with thehardness increase [18]. in temperature.When tapioca At starch high pressure,was pressurized treatment at time600 MPa decreased in the thetemperature hardness ofrange gels, of but 30 the°C to eff 80ect °C, was the very hardness small andwas becamemore ev evidentident in onlythe gels after treated long processingat lower temperature times of more and thanbecame 30 min.less prominent The starch–starch with the andincrease starch–water in temperat interactionsure. At high in pressure, the HPP treatment induced geltime were decreased much higher.the hardness Post storage of gels, analyses but the of effect the gels was were very conducted small and after became a storage evident time only of 28 after days long under processing storage conditionstimes of more of refrigeration than 30 min. (4 TheC) starch–starch and freezing (and18 starch–waterC). In terms ofinteractions textural change in the andHPP amylopectin induced gel ◦ − ◦ recrystallization,were much higher. the Post HPP storage induced analyses starch of gels the showedgels were better conducted stability after after a storage storage time compared of 28 days to theirunder thermally storage conditions treated counterparts. of refrigeration The (4 properties °C) and freezing of wheat (−18 starch °C). treatedIn terms with of textural HPP were change similar. and HPPamylopectin produced recrystallization, denser gels, and the these HPP gels induced had a lower starch grade gels of showed retrogradation better whenstability compared after storage to the heatcompared treated to gel their [19 ].thermally Starch granule treated preservation, counterparts. limited The properties granule expansion of wheat andstarch low treated amylose with release HPP werewere othersimilar. important HPP produced properties denser observed gels, and in HPP thes treatede gels had granules. a lower Another grade of study retrogradation on wheat starch when compared to the heat treated gel [19]. Starch granule preservation, limited granule expansion and showed that the pressure treated wheat starch had properties of higher hardness, gumminess and low amylose release were other important properties observed in HPP treated granules. Another chewiness up to a gelatinization percentage of about 40%, after which these properties were greater study on wheat starch showed that the pressure treated wheat starch had properties of higher in the heat treated starch [16]. Colussi et al. [20] examined the effect of cyclic pressure treatments on hardness, gumminess and chewiness up to a gelatinization percentage of about 40%, after which potato starch gels. The SEM analysis revealed that cyclic HPP caused eruptions (bursting of granules), these properties were greater in the heat treated starch [16]. Colussi et al. [20] examined the effect of abrasions (surface deformities) and disruption of starch granules with varying severity of treatments. cyclic pressure treatments on potato starch gels. The SEM analysis revealed that cyclic HPP caused The gel structure became more compact with smaller spaces within fragment networks after HPP. eruptions (bursting of granules), abrasions (surface deformities) and disruption of starch granules The pressure treated samples after a seven-day retrogradation period showed a further increase in the with varying severity of treatments. The gel structure became more compact with smaller spaces density and compactness. within fragment networks after HPP. The pressure treated samples after a seven-day retrogradation Use of HPP on maize starch led to an increase in moisture content for a given water activity period showed a further increase in the density and compactness. with pressurization time following an adsorption-desorption isotherm [21]. The effect of the shift

Molecules 2020, 25, x FOR PEER REVIEW 4 of 18 Molecules 2020, 25, 2369 4 of 18 Use of HPP on maize starch led to an increase in moisture content for a given water activity with pressurization time following an adsorption-desorption isotherm [21]. The effect of the shift in the waterin the sorption water sorption isotherm isotherm was higher was for higher both for higher both water higher activity water activityand lower and water lower activity water (Figure activity 4).(Figure The shift4). Thewas shiftalso correlated was also correlatedto pressurization to pressurization time; and longer time; trea andtment longer time treatment led to a higher time led shift. to Thea higher effect shift.of high The pressure eﬀect oftreatment high pressure time from treatment 5 to 60 time min fromalso led 5 to to 60 a 2.7 min fold also increase led to ain 2.7 water- fold holdingincrease capacity in water-holding of maize capacitystarch. The of maize extent starch. of starch The hysteresis extent of starchobserved hysteresis after the observed HPP treatment after the wasHPP lower treatment (depending was lower on the (depending treatment ontime) the until treatment the water time) activity until reached the water 0.44. activity Subsequently, reached 0.44.the degreeSubsequently, of hysteresis the degree in pressure of hysteresis treated in starch pressure became treated much starch higher became in comparison much higher to in unpressurized comparison to starch.unpressurized starch.

0.3 Moisture Sorption Isotherm (MSI)

0.25

0.2

0.15

0.1

No Pressurization 0.05 Pressurization for

Moisture Moisture content of(g moisture/ gof drystarch) 60 minutes 0 0 0.2 0.4 0.6 0.8 1 water activity (aw) FigureFigure 4. 4. ChangeChange in in the the hysteresis hysteresis of of starch starch observed observed after after high high pressure pressure treatment treatment (plotted (plotted using using the the datadata from from Santos Santos et al., 2014 [21]). [21]).

WaxyWaxy rice, which has no amylose content, showed structural disr disruptionuption and and granular granular structure structure disappearancedisappearance at at very very high pressures of 600 MPa.MPa. HPP HPP increased increased the the lamellar lamellar repeat repeat distance proportionalproportional to to pressure pressure [22]. [22]. The The relative relative crystall crystallinityinity decreased decreased as the as thepressure pressure increased increased from from 100 MPa100 MPa to 600 to 600MPa. MPa. Starches Starches can can be behighly highly resistan resistantt to topressure. pressure. Chestnut Chestnut flour flour suspensions suspensions did did not not completelycompletely gelatinizegelatinize inin the the range range of of 400 400 MPa MPa to 600 to MPa600 MPa [23]. [23]. There There was no was change no change in granule in granule particle particlesize or hydrationsize or hydration properties. properties. X-ray di X-rayffraction diffraction diagrams diagrams given bygiven native by starchesnative starches are mainly are mainly of two oftypes two [types24]. These [24]. areThese the are A type,the A generally type, generally exhibited exhibited by cereal by starchescereal starches and the and B type, the B usually type, usually shown shownby tubers by andtubers high and amylose high amylose starches. starches. C type diCff typeraction diffraction pattern, pattern, which is which a combination is a combination of A type of and A typeB type and diagrams, B type diagrams, is typical is for typical legume for starches. legume starches. The V form The crystalline V form crystalline complex complex rarely seen rarely in native seen instarches native is starches observed iswhen observed there when is a complex there is formation a complex of amyloseformation with of fattyamylose acids with and monoglycerides.fatty acids and monoglycerides.Sorghum starch didSorghum not show starch any did change not show in the any crystallinity change in the from crystallinity the native from starch the up native to 480 starch MPa upand to continued 480 MPa toand show continued the A typeto show crystalline the A patterntype crystalline [25]. The pattern starch underwent[25]. The starch gelatinization underwent at gelatinization600 MPa and thenat 600 showed MPa Band type then crystalline showed pattern. B type Properties crystalline such pattern. as water Properties absorption such capacity as water and absorptionthermal stability capacity increased, and thermal whereas stability oil absorption increased, capacity,whereas swellingoil absorption power capacity, and viscosity swelling decreased power andwith viscosity increasing decreased pressure with levels. increasing pressure levels. AA non-conventional non-conventional starch starch extracted extracted from from the s seedseeds of of mangoes mangoes and and referred referred to to as as mango mango kernel kernel starchstarch (MKS) (MKS) was was investigated investigated for its its changes changes on on treatment treatment with HPP [26]. [26]. The The HPP HPP treated treated MKS MKS had had strongerstronger starch starch aggregations aggregations and and lower lower retrogradation retrogradation tendencies. tendencies. The The SEM SEM images images of of MKS MKS granules, granules, when treated treated with with high high pressure, pressure, showed hardly any structural structural changes, but the the surface surface of of the the starch starch granulesgranules lost lost its its smoothness smoothness and became rough. AA summary summary of of the the highlights of the studies on the properties of starches can be seen in Table1 1.

Table 1. Important highlights of the studies on different HPP treated starches.

Type of Pressure Temperature Time Property Assessed Outcome Reference Starch

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Table 1. Important highlights of the studies on diﬀerent HPP treated starches.

Type of Starch Pressure Temperature Time Property Assessed Outcome Reference

Harder gels at low temperatures. Hardness • • Long treatment led to a slight decrease in hardness Post storage changes in hardness • Tapioca 600 MPa 30–80 ◦C 10–30 min • Higher starch-starch and starch water interactions Vittadini et al. [18] Comparison with thermally treated starch • • Better storage eﬃciency •

Hardness • Greater hardness in gels Retrogradation rate • Wheat 600 MPa 25 ◦C 15 min • Lower rate of retrogradation Douzals et al. [19] Comparison with thermally treated starch • •

Hardness • Gumminess Hardness, gumminess and chewiness were high • • Wheat 300–600 MPa 25 ◦C 10 min Chewiness only till gelatinization <40% Liu et al. [16] • Comparison with thermally treated starch •

Eruptions, abrasions and disruptions of • starch granules Cyclic pressure treatments 400 MPa (3 cycles) followed by • Highly compact gel Potato 21 ◦C Each cycle 10 min Post retrogradation analysis • Colussi et al. [20] 600 MPa (6 cycles) • Post retrogradation, gel was more compact • and dense

Upward shift of the MSI • Ability to higher moisture at the same Maize 300 MPa 20 ◦C 5–60 min Moisture adsorption-desorption analysis • Santos et al. [21] water activity

With increase in pressure Lamellar repeat distance in starch granules • Lamellar repeat distance increased Waxy rice 100–600 MPa 25 ◦C 20 min • Li et al. [27] Crystallinity Crystallinity decreased • •

Degree of gelatinization Pressure resistant • • Granule particle size or No change in granule particle size or Chestnut 400–600 MPa 35–38 ◦C 10 min • • Ahmed et al. [23] hydration properties hydration properties

Gelatinization B type crystalline pattern at 600 MPa • • Water absorption capacity and Increase in water absorption capacity and Room • • Sorghum 120–600 MPa 20 min thermal stability thermal stability Liu et al. [25] temperature Oil absorption capacity, swelling power Decrease in oil absorption capacity, swelling • • and viscosity power and viscosity

Lower aggregation and retrogradation tendencies • Retrogradation Insigniﬁcant structural changes in the starch • • Mango kernel 300–600 MPa 38 ◦C 10 min Structural analysis of the starch granules granules, however signiﬁcant changes in the Kaur et al. [26] • smoothness of the granule surfaces Molecules 2020, 25, 2369 6 of 18

2.1.3. Influence of Additives on Gelatinization When gelatinization of maize starch was studied in the presence of hydrocolloids, it was found that the hydrocolloids ensure the availability of water to the starch granules, but some hydrocolloids, like xanthan gum, can result in lower levels of gelatinization [28]. The reason could be the shielding effect provided by the hydrocolloids at high pressure. Teixeira et al. [28] also reported that high pressure changes the maize starch crystal from the A type XRD pattern to give patterns with characteristics of both A type and B type starch. The extremely high pressure of 700 MPa, long time of pressure treatment or the presence of hydrocolloids favoured a V-crystalline complex. The presence of NaCl premixed in corn starch solutions after high pressure treatment did not affect the granular structure of the starch, but it decreased the stability of the emulsion of solutions prepared with it [29]. A study on the influence of pH and osmolarity (of salts) in HPP gelatinization of waxy starches of rice and corn revealed that the rate of gelatinization and its extent decreased by increasing the osmolarity of the salts, whereas the influence of pH was minor [30]. Simonin et al. [30] further stated that the reason behind this behavior was explained by the Hofmeister series; thus: “Anions coupled with constant cations increase the solubility of protein or decrease the starch gelatinization temperature in the order SO4− < CH3COO− < Cl− < Br− < NO3− < ClO4− < I− < SCN−.” Additionally, the salts used in the experiment were kosmotropes (salts that structure water around macromolecules and increase the viscosity of the solution), which meant the availability of the solution to the starch granules would be delayed. The gel strength and extent of swelling were found to be in direct correlation with the extent of gelatinization.

2.1.4. Digestibility The digestion of different components of starch takes place at a different rate in the human intestine [31]. The starch can thus be classified: rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) components. As the name suggests, RDS digests very quickly and SDS digests slowly in the small intestine (Figure5). RS escapes digestion and undergoes fermentation in the large intestine. The pressure treated samples of starch have a higher digestibility if the pressure treatment of about 500 MPa is carried out at temperatures above 45 ◦C[32]. Subjecting starch to high pressure causes gelatinization but a long time of exposure to high pressure gives it a new form that is less digestible. The gelatinized state is expected to have the highest digestibility. Waxy rice starch, when treated under different high pressure conditions, underwent microstructural and morphological changes that altered its digestibility properties. At 600 MPa, the RDS starch, and at 400 MPa, the SDS starch, reached their highest digestibility respectively [22]. When similar tests at a pressure range of 120 MPa to 600 MPa were carried out on sorghum starch, the RDS successively reduced, and SDS and RS levels successively increased with pressure [25]. The glycemic index value is another term in assessing foods to determine the rate of their digestibility. It is determined by estimation of the increase in blood glucose with time [31]. Even though potato starch cannot be fully gelatinized by pressures up to 600 MPa, applying 6 cyclic pressures of 400 MPa for 10 min each followed by retrogradation of 7 days produced starch that underwent lower in-vitro hydrolysis [33]. This would mean a slower release of glucose, leading to food of low glycemic value (a food that would lead to longer duration of fullness). Thus, the properties of the final product can be easily adjusted to the desired characteristics with the use of HPP by varying pressure, temperature and duration of treatment. Molecules 2020, 25, 2369 7 of 18 Molecules 2020, 25, x FOR PEER REVIEW 7 of 18

FigureFigure 5. 5.DigestionDigestion of ofdifferent different kinds kinds of of starches starches in in the the digestive digestive system system and their glycemic responseresponse in inthe the bloodbloodglucose glucose (redrawn (redrawn from from Miao Miao et et al., al., 2015 2015 [ 34[34]] and and Sorndech Sorndech et et al., al., 2018 2018 [ 35[35]).]). 2.1.5. Applications 2.1.5. Applications Corn starch gains some unique properties after high pressure treatment. A study was conducted Corn starch gains some unique properties after high pressure treatment. A study was conducted on high pressure treatment of corn starch and its use as a carrier for molecules of nutritional on high pressure treatment of corn starch and its use as a carrier for molecules of nutritional value value [36,37]. Three types of treatments were conducted for comparison. Alkali treatment (with [36,37]. Three types of treatments were conducted for comparison. Alkali treatment (with NaOH), NaOH), HPP treatment (at 400 MPa for 35 min and an initial temperature of 38 ◦C) and combined HPP treatment (at 400 MPa for 35 min and an initial temperature of 38 °C) and combined alkali and alkali and pressure treatment were carried out. The microstructure analysis showed that all treatments pressure treatment were carried out. The microstructure analysis showed that all treatments increased the porosity of the starch granules. The alkali treatment produced higher numbers of increased the porosity of the starch granules. The alkali treatment produced higher numbers of pores pores with larger pore volume when compared to HPP treated corn starch. The combined treatment with larger pore volume when compared to HPP treated corn starch. The combined treatment produced many macropores and hence, a higher pore volume than native starch. Structure analysis produced many macropores and hence, a higher pore volume than native starch. Structure analysis showed that when zinc and magnesium were loaded into the granules, the alkali treatment proved to showed that when zinc and magnesium were loaded into the granules, the alkali treatment proved be a better method than HPP. The combined treatment did not act synergistically and gave moderate to be a better method than HPP. The combined treatment did not act synergistically and gave results. Since the microstructure analysis proved to be positive for the combined treatment, further moderate results. Since the microstructure analysis proved to be positive for the combined treatment, study is required. Trials using other nutrients with different molecular sizes may be carried out. further study is required. Trials using other nutrients with different molecular sizes may be carried High pressure treated corn starches have also shown enhanced emulsion stabilization properties. out. High pressure treated corn starches have also shown enhanced emulsion stabilization properties. During the homogenization process of sunflower oil mixed with the treated corn starch, the starch During the homogenization process of sunflower oil mixed with the treated corn starch, the starch disintegrated [29]. The broken fragments of starch contributed to the stabilization of the oil droplets. disintegrated [29]. The broken fragments of starch contributed to the stabilization of the oil droplets. The emulsion stability increased with the concentration of the starch. The creaming stability also The emulsion stability increased with the concentration of the starch. The creaming stability also increased with the concentration of the starch. The addition of NaCl to the process allowed the increased with the concentration of the starch. The addition of NaCl to the process allowed the formation of droplets of larger size in the emulsion. The presence of NaCl did not affect the flow formation of droplets of larger size in the emulsion. The presence of NaCl did not affect the flow behaviour of the emulsion but the creaming stability was significantly reduced. behaviour of the emulsion but the creaming stability was significantly reduced. HPP gelatinization has been used in manufacturing foods and studies have shown both positive HPP gelatinization has been used in manufacturing foods and studies have shown both positive and negative impact on food quality. HPP on cake batters did not give any favourable results [38]. and negative impact on food quality. HPP on cake batters did not give any favourable results [38]. The cake made from the pressure treated batter had traits such as less expansion on baking, darker The cake made from the pressure treated batter had traits such as less expansion on baking, darker crust colour and much harder texture. It was stated that for a good quality cake, a higher gelatinization crust colour and much harder texture. It was stated that for a good quality cake, a higher temperature is more desirable. In addition, a lower batter density provides a greater exchange area gelatinization temperature is more desirable. In addition, a lower batter density provides a greater for the rise during baking. Since HPP reduced the gelatinization temperature and the volume during exchange area for the rise during baking. Since HPP reduced the gelatinization temperature and the treatment, the process imparted undesirable characteristics to the cake. A method for developing volume during treatment, the process imparted undesirable characteristics to the cake. A method for resistant starch by treating the B Type starch was developed by making use of HPP treatment [39]. developing resistant starch by treating the B Type starch was developed by making use of HPP The technique involved mixing B type starch with water in the ratio 1:2 up to 1:10, vacuum packing in treatment [39]. The technique involved mixing B type starch with water in the ratio 1:2 up to 1:10, nylon-polyethylene bags and treatment at a pressure of 100–600 MPa at 20–50 C. Since this method vacuum packing in nylon-polyethylene bags and treatment at a pressure of 100–600◦ MPa at 20–50 °C. did not involve the use of any enzyme or reagents in the manufacturing process, it was claimed to Since this method did not involve the use of any enzyme or reagents in the manufacturing process, it have a ‘green’ cleaning process. The gelatinization using HPP at >400 MPa was uniquely used to was claimed to have a ‘green’ cleaning process. The gelatinization using HPP at >400 MPa was produce foods with granular pregelatinized starch. Certain pressure-sensitive starches can be added uniquely used to produce foods with granular pregelatinized starch. Certain pressure-sensitive to foods that are hermetically sealed and then HPP treatment can be conducted to get shelf-stable starches can be added to foods that are hermetically sealed and then HPP treatment can be conducted to get shelf-stable food products. This was carried out in another study, where the starch was used

Molecules 2020, 25, x FOR PEER REVIEW 8 of 18 in the process to create fat replacements, thereby reducing the calorie content of the foods [40]. This process is advantageous because digestibility of starch is closely linked to the degree of gelatinization and the extent of gelatinization can be easily controlled by adjusting the processing parameters of pressure, temperature and duration. From an industrial perspective, the initial study of setting the processing variables becomes necessary to achieve the levels of gelatinization or retrogradation while reviewing viable methods of commercialization. One such tool was presented by determining state diagrams of potato starch [41]. Molecules 2020, 25, 2369 8 of 18 The state diagrams showed treatment pressure versus starch content graphs enabling the classification offood processed products. This starch was carried into: out completely in another study, gelatinized, where the starch partially was used ingelatinized, the process completely gelatinized withto createretrogradation fat replacements, and thereby part reducingially gelatinized the calorie content with of retrogradation. the foods [40]. This process is advantageous because digestibility of starch is closely linked to the degree of gelatinization and the extent of gelatinization can be easily controlled by adjusting the processing parameters of pressure, 2.2. Gelatinizationtemperature of Collagen and duration. From an industrial perspective, the initial study of setting the processing variables becomes Gelatinizationnecessary is tonot achieve only the restricted levels of gelatinization to starch or retrogradation. Gelatin or while gelatine, reviewing commonly viable methods used of as a gelling agent in foods,commercialization. is made from One such the tool collagen was presented deriv by determininged from state animals. diagrams ofCollagen potato starch is [41 ].a triple-helical The state diagrams showed treatment pressure versus starch content graphs enabling the classiﬁcation heterotrimer andof processed is stabilized starch into: by completely hydrogen gelatinized, bonds partially [42]. Collagen gelatinized, is completely treated gelatinized with dilute with acid or alkali for several daysretrogradation so that the and hydrogen partially gelatinized bonds with and retrogradation. other interlinks present in the coils are cleaved such that water-soluble2.2. Gelatinization gelatin is of Collagenproduced. This is gelatin or gelatinized collagen (Figure 6). There have also been a few studiesGelatinization on the is not gelatinization only restricted to of starch. collagen Gelatin orusing gelatine, HPP. commonly Use of used HPP as a gellingin gelatinization of collagen with agentacid in as foods, the is pressure made from the transmitting collagen derived frommedium animals. dramatically Collagen is a triple-helical reduces heterotrimer the processing time. An investigationand iscompared stabilized by the hydrogen traditional bonds [42]. method Collagen is of treated acid with treatment dilute acid orwith alkali the for several high days pressure method so that the hydrogen bonds and other interlinks present in the coils are cleaved such that water-soluble [42]. Results showedgelatin is that produced. HPP This gelatinization is gelatin or gelatinized carried collagen out at (Figure500 MPa6). There reduced have also the been processing a few time from more than 26 hstudies to 15 onmin. the gelatinizationThough the of collagenpressure using treated HPP. Use collagen of HPP in had gelatinization a comparatively of collagen with lower yield than acid as the pressure transmitting medium dramatically reduces the processing time. An investigation the traditional comparedmethod, the the traditional physical method properties of acid treatment of th withe HPP the high treated pressure gels method were [42]. superior. Results showed The gel strength and rheologicalthat properties HPP gelatinization of high carried pressure out at 500 MPa proc reducedessed the gelatins processing timewere from better more thanthan 26 hthe to traditionally prepared gelatins.15 min. The Though hardness, the pressure gumminess, treated collagen chew had a comparativelyiness and loweradhesiveness yield than the of traditional the jelly made from method, the physical properties of the HPP treated gels were superior. The gel strength and rheological HPP treated gelatinproperties were of high greater pressure than processed the gelatins commerc were betterial gelatin, than the traditionallywhile cohesi preparedveness gelatins. and springiness were similar [43].The hardness, Increasing gumminess, acid chewiness strength and for adhesiveness extraction of the jellyincreases made from the HPP yield treated but gelatin slightly were reduces gel greater than the commercial gelatin, while cohesiveness and springiness were similar [43]. Increasing strength [44]. acid strength for extraction increases the yield but slightly reduces gel strength [44].

Figure 6. Gelatin made from collagen gelatinization (redrawn from von Endt and Baker 1991 [45]). Figure 6. Gelatin made from collagen gelatinization (redrawn from von Endt and Baker 1991 [45]). 3. Forced Water Absorption in Foods Using HPP 3. Forced Water AbsorptionStarch is one of thein primaryFoods components Using HPP in foods, but there has been some research on the eﬀect of HPP treatment on the properties of whole foods as well. The process of immersion of foods into liquids Starch is onefor softening of the isprimary called soaking. comp Theonents process in of soakingfoods, is but generally there long, has and been HPP some has been research used to on the effect fasten the process (Figure7). High pressure conditions can change many physico-chemical properties of HPP treatmentof foods on such the as properties the changes in of diﬀ usivities,whole nutritionalfoods as and well. anti-nutritional The process compound of immersion levels, etc. of foods into liquids for softening is called soaking. The process of soaking is generally long, and HPP has been used to fasten the process (Figure 7). High pressure conditions can change many physico-chemical properties of foods such as the changes in diffusivities, nutritional and anti-nutritional compound levels, etc.

Molecules 2020, 25, 2369 9 of 18 Molecules 2020, 25, x FOR PEER REVIEW 9 of 18 Molecules 2020, 25, x FOR PEER REVIEW 9 of 18

PressurizationPressurization FewFew minutesminutes

ApplicationApplication of of TransferTransfer of water AbsorptionAbsorption of ofwater water by by HighHigh Pressure Pressure foodsfoods due due to to High High Pressure Pressure

FigureFigureFigure 7.7. 7. RepresentationRepresentation Representation ofof of forcedforced forced water water absorptionabsorptiontion by by foods foods under underunder highhigh high pressure.pressure. pressure. When whole grain glutinous rice was subjected to HPP, it was observed that the diffusion WhenWhen whole whole grain grain glutinous glutinous ricerice waswas subjected toto HPP,HPP, itit waswas observed observed that that the the diffusion diffusion coefficient of water increased with temperature up to 300 MPa, after which temperature had no coefficientcoefficient of of water water increased increased withwith temperaturetemperature up toto 300300 MPa,MPa, after after which which temperature temperature had had no no significant effect [46]. The effective diffusion coefficient of water also increased from 20 C to 50 C after significantsignificant effect effect [46]. [46]. The The effective effective diffusiondiffusion coefficient ofof waterwater also also increased increased from from ◦20 20 °C °C to ◦to50 50°C °C which it decreased. The explanation stated for this phenomenon was the occurrence of gelatinization. afterafter which which it itdecreased. decreased. TheThe explanationexplanation stated forfor thisthis phenomenonphenomenon was was the the occurrence occurrence of of Thegelatinization. gelatinization The of the gelatinization rice granules of restricts the rice the granul transportes restricts (diffusion) the transport of water. (diffusion) However, irrespectiveof water. gelatinization. The gelatinization of the rice granules restricts the transport (diffusion) of water. of theHowever, change irrespective in the diff usionof the change coefficient, in the the diffusio quantityn coefficient, of water the absorbed quantity by of the water rice absorbed and the by rate However, irrespective of the change in the diffusion coefficient, the quantity of water absorbed by of itsthe absorptionrice and the increased rate of its with absorption pressure increase and temperature.d with pressure The and effect temperature. of presoaking The foreffect 30 min,of the rice and the rate of its absorption increased with pressure and temperature. The effect of followedpresoaking by high for 30 pressure min, followed treatment by high at 400pressure MPa treatment for 10 min at 400 and MPa final for soaking 10 min overnightand final soaking led to a presoaking for 30 min, followed by high pressure treatment at 400 MPa for 10 min and final soaking riceovernight variety that led to had a rice enhanced variety propertiesthat had enhanced [47]. The properties HPP processed [47]. The rice HPP had processed higher digestibilityrice had higher and overnight led to a rice variety that had enhanced properties [47]. The HPP processed rice had higher starchdigestibility with higher and swellingstarch with capabilities. higher swelling HPP led capabilities. to a higher HPP degree led of to water a higher penetration degree of in thewater rice, digestibility and starch with higher swelling capabilities. HPP led to a higher degree of water thuspenetration causing a higherin the rice, level thus of gelatinization causing a higher to the level starch of gelatinization (Figure8). The to ricethe starch was claimed (Figure to 8). have The morerice penetration in the rice, thus causing a higher level of gelatinization to the starch (Figure 8). The rice palatablewas claimed properties. to have A similarmore palata studyble conducted properties. by A Zhu similar et al. study [48], recommendedconducted by Zhu the combinationet al. [48], was claimed to have more palatable properties. A similar study conducted by Zhu et al. [48], of HPPrecommended and partial the milling combination of brown of HPP rice and to enhancepartial milling its processing of brown ericefficiency to enhance and quality.its processing Foxtail recommendedefficiency and the quality. combination Foxtail millet, of HPP a typeand ofpartial cereal milling grain, was of brown subjected rice to to long enhance durations its processing of HPP millet, a type of cereal grain, was subjected to long durations of HPP [49]. There was a drop in the efficiency[49]. There and was quality. a drop Foxtail in the effectivemillet, a diffusiontype of cereal coefficient grain, observed was subjected above 200to long MPa durations for germinated of HPP effective diffusion coefficient observed above 200 MPa for germinated and 600 MPa for non-germinated [49].and There 600 MPawas fora drop non-germinated in the effective grains. diffusion The degree coefficient of gelatinization observed above and water 200 MPa uptake for increasedgerminated grains. The degree of gelatinization and water uptake increased with pressure, temperature and time. andwith 600 pressure, MPa for temperature non-germinated and time. grains. In addition,The degree the ofnutritional gelatinization components and water such uptakeas antioxidants increased In addition, the nutritional components such as antioxidants and total phenolic activity were enhanced, withand pressure, total phenolic temperature activity wereand time.enhanced, In addition, wherea sthe anti-nutritional nutritional components compounds such asas tanninantioxidants and whereas anti-nutritional compounds such as tannin and phytic acid decreased with high pressure. andphytic total acid phenolic decreased activity with were high enhanced, pressure. whereas anti-nutritional compounds such as tannin and phytic acid decreased with high pressure. Milky Queen (Low amylose rice) Koshihikari (medium amylose rice) Hamasari (High amylose rice) 100 100 non HPP HPP 100 non HPP HPP Milky Queen (Low amylose rice) Koshihikari (medium amylose rice) 95 Hamasari (Highnon amylose HPP HPP rice) 100 95 10095 100 non HPP HPP non HPP HPP 90 non HPP HPP 95 90 9590 95 85 85 85 90 90 (%) 90 (%) 80 (%) 80 80 85 85 85 75 75 75(%) (%) 80 (%) 80 80 70 70 Degreee of gelatinisation 70 75 75 75 Degreee of gelatinisation Degreee of gelatinisation 25°C 55°C 25°C 55°C 25°C 55°C Soaking Temperature 70 Soaking Temperature Soaking Temperature 70 Degreee of gelatinisation 70 Degreee of gelatinisation Degreee of gelatinisation 25°C 55°C 25°C 55°C 25°C 55°C Figure 8. ComparisonSoaking Temperature of the degree of gelatinizationSoaking after Temperature cooking of three varieties ofSoaking rice with Temperature different levelsFigure of amylose 8. Comparison with their of HPP the treated degree counterparts of gelatinization (plotted after using cooking the data of from three Yamakura varieties et of al., rice 2005 with [47]).

different levels of amylose with their HPP treated counterparts (plotted using the data from InFigureYamakura another 8. Comparison study,et al., 2005 paddy [47]).of the grains degree of of the gelatinization Basmati variety after cooking were subjected of three varieties to HPP of with rice pressure,with temperaturedifferent and levels time of in amylose the range with of 350–550their HPP MPa, trea 30–50ted counterparts◦C and 5–30 (plotted min respectively using the [data50]. Thisfrom was doneYamakura toIn establish another et al.,the study, 2005 water paddy[47]). absorption grains of and the gelatinization Basmati variety kinetics. were subjected The eﬀect to of HPP presoaking with pressure, was also studied.temperature The non-soaked and time in grains the range showed of 350–550 a higher MPa, rate 30–50 of water °C and uptake, 5–30 min whereas respectively the presoaked [50]. This grainswas showeddoneIn anotherto a higherestablish study, degree the waterpaddy of gelatinization absorption grains of and the with gelatiniza Basmati the maximum tionvariety kinetics. were at theThe subjected highesteffect of to levelpresoaking HPP of with pressure was pressure, also and temperaturetemperaturestudied. The studied, and non-soaked time i.e., in 500the grains range MPa, showed 50of ◦350–550C. Temperaturea higher MPa, rate 30–50 of and water °C pressure and uptake, 5–30 were whereasmin shown respectively the to presoaked have [50]. a synergistic Thisgrains was actiondoneshowed to on establish water a higher absorption. the degree water of absorption Resultsgelatinization showed and withgelatiniza that the under maximumtion thekinetics. inﬂuence at the The highest effect of high levelof pressure,presoaking of pressure the was waterand also studied. The non-soaked grains showed a higher rate of water uptake, whereas the presoaked grains showed a higher degree of gelatinization with the maximum at the highest level of pressure and

Molecules 2020, 25, x FOR PEER REVIEW 10 of 18 Molecules 2020, 25, 2369 10 of 18 temperature studied, i.e., 500 MPa, 50 °C. Temperature and pressure were shown to have a synergistic absorptionaction on andwater di ffabsorption.usivity were Results much showed higher that compared under the to grainsinfluence soaked of high at atmosphericpressure, the pressure.water Presoakedabsorption grains and diffusivity exhibited were a higher much saturation higher compared moisture to content,grains soaked whereas at atmospheric the non-soaked pressure. grains Presoaked grains exhibited a higher saturation moisture content, whereas the non-soaked grains showed higher diffusivities (Figure9). A first-order kinetic model was used to describe gelatinization. showed higher diffusivities (Figure 9). A first-order kinetic model was used to describe gelatinization. Based on the energy of activation (Ea) and activation volume (∆V), adverse effects of pressure on Based on the energy of activation (Ea) and activation volume (ΔV), adverse effects of pressure on gelatinization was observed at extreme high pressure. Ravichandran et al. [50] further explained gelatinization was observed at extreme high pressure. Ravichandran et al. [50] further explained that that the progressing of gelatinization depends on the first order rate constant (k). k is related to Ea the progressing of gelatinization depends on the first order rate constant (k). k is related to Ea and ΔV and ∆V by the Arrhenius and Eyring equations respectively. An increase in the value of Ea leads by the Arrhenius and Eyring equations respectively. An increase in the value of Ea leads to a to a subsequent decrease in k. In thermal gelatinization, the value of E decreases with increase in subsequent decrease in k. In thermal gelatinization, the value of Ea decreasesa with increase in temperaturestemperatures above above 60 60◦ C.°C. This This eases eases thethe processprocess of gelatinization at at high high temperatures. temperatures. In Inpressure pressure gelatinization,gelatinization, an an increaseincrease in pr pressureessure leads leads to tothe the rise rise in E ina. The Ea. ease The of ease gelatiniza of gelatinizationtion is hence lower. is hence lower.This is This because is because an increase an increase in pressure in pressure restricts restrictsstarch granule starch swelling. granule swelling.Negative values Negative of ΔV values were of ∆Vobserved were observed which whichindicated indicated the sensitivity the sensitivity of the ofgrains the grainsto pressure. to pressure. This meant This meant the occurance the occurance of ofcompression of of grains grains during during pressure pressure treatment. treatment. The The negative negative value value of ofΔV∆ Vfurther further sustained sustained the the explanationexplanation of of the the restriction restriction of of starch starch granulegranule swellingswelling due due to to pressure. pressure. Hence, Hence, a lower a lower pressure pressure of of 450450 MPa MPa for for treatment treatment was was suggested. suggested. TheThe conclusion of of the the study study was was that that this this method method of ofusing using HPP HPP waswas a bettera better alternative alternative to to thermal thermal soakingsoaking becausebecause it reduces soaking soaking time time and and also also induces induces partial partial gelatinizationgelatinization in in the the paddy paddy grains. grains.

(a) Saturation moisture content (b) Diffusivity 60 10

9 55 8 /s) 2

m 7 50

-10 6

45 5

4 30 °C (Presoaked) 30 °C (Presoaked) 40 40 °C (Presoaked) 3 40 °C (Presoaked) 50 °C (Presoaked) 50 °C (Presoaked) Diffusivity (×10 Diffusivity Moisture content (% d.b.) (% content Moisture 2 35 30 °C (Non-soaked) 30 °C (Non-soaked) 40 °C (Non-soaked) 1 40 °C (Non-soaked) 50 °C (Non-soaked) 30 50 °C (Non-soaked) 0 300 350 400 450 500 550 600 300 350 400 450 500 550 600 Pressure (MPa) Pressure (MPa) FigureFigure 9. (9.a )( Watera) Water uptake uptake and and (b) water (b) water diﬀusivity diffusivity in presoaked in presoaked and nonsoaked and nonsoaked paddy grainspaddy subjectedgrains tosubjected HPP (plotted to HPP using (plotted the data using from the Ravichandran,data from Ravichandran, Purohit and Purohit Rao, and 2018 Rao, [50]). 2018 [50]).

DriedDried beans beans were were initially initially soaked soaked andand then treated with with a a high high pressure pressure of of 600 600 MPa MPa for for different different durations.durations. This This waswas thenthen comparedcompared with therma thermallylly treated treated (HT) (HT) (95 (95 °C)◦C) and and high high pressure/high pressure/high temperaturetemperature process process (HPHT) (HPHT) (600 (600 MPa,MPa, 9595 ◦°C)C) [[51]51].. Softening Softening of of the the beans beans is isan an important important process process beforebefore eating. eating. This This is is because because rawraw beansbeans are too ha hardrd to to be be disintegrated disintegrated by by mere mere chewing chewing and and the the softnesssoftness is is an an indicator indicator of of the the suitability suitability forfor consumption.consumption. The The results results showed showed that that the the high high pressure pressure processprocess alone alone was was notnot able to to reduce reduce the the hardness hardness of the of thebean bean even even after afterlong processing long processing times. The times. Thepresence presence of ofheat heat with with or orwithout without pressure pressure is necessary is necessary to soften to soften the bean. the bean. This Thiswas wasattributed attributed to tothermal thermal pectin pectin solubilization. solubilization. Although Although the the HPHT HPHT was was found found to tohave have a higher a higher degree degree of ofstarch starch hydrolysis than HT samples, the process did not impart enough enhancement of any property to hydrolysis than HT samples, the process did not impart enough enhancement of any property to make make the process economically viable. the process economically viable. HPP has been used to treat cut pieces of tubers and the process has been shown to alter some of HPP has been used to treat cut pieces of tubers and the process has been shown to alter some of the properties. When potato discs were subjected to moderate heat and pressure treatment (MHPT) the properties. When potato discs were subjected to moderate heat and pressure treatment (MHPT) (75–95 °C, 100 MPa), the firmness increased at the temperature range of 75–85 °C. MHPT also slightly (75–95increased◦C, 100 tissue MPa), sloughing the firmness while increased undergoing atthe colour temperature change similar range ofto 75–85that of◦ C.heat MHPT treatment also slightly[52]. increasedAmylase tissue acts on sloughing gelatinized while starch undergoing and causes coloursaccharification change similarof the starch. to that Sweet of heat potatoes treatment contain [52 ]. Amylaseinternal acts amylases. on gelatinized If the starch starch present and in causes sweetsaccharification potato is gelatinized, of the the starch. internal Sweet amylases potatoes can cause contain internal amylases. If the starch present in sweet potato is gelatinized, the internal amylases can cause saccharification and this would give a higher yield of reduced sugars in extraction. However, Molecules 2020, 25, 2369 11 of 18

the internal amylases are inactivated at high temperatures near 100 ◦C. An experiment on using HPP to cause gelatinization in sweet potato was carried out [53]. Treatments were conducted at different levels of pressure and temperatures for different durations. The process did not affect the amylase activity, but the reducing sugars extracted from the pressure treated samples were similar or slightly lesser in quantity than that extracted from untreated sweet potato pieces. This indicated insufficient gelatinization for saccharification. The process of HPP showed an inhibitory effect on the gelatinization and/or saccharification at lower pressures of 100 MPa and 200 MPa. The reduced sugar content in the samples treated at high pressures of 500 MPa and 70 ◦C were comparable, but were still lower than those treated at atmospheric pressure at 80 ◦C for the same duration. The reason behind this anomaly of the initial fall in the reducing sugar content with pressure remained unclear.

4. Pressure Assisted Infusion in Foods When the process of soaking (carried out at atmospheric pressure) is used to transfer some compounds of value to foods, the transfer occurs through the process of osmosis, where there are two components; the solvent (usually water) and the solute (the component intended for transfer). The concentration of solute in the soaking solution can be increased only up to a critical concentration. On exceeding that concentration, the moisture gets driven out, dehydrating the food that is not generally desirable. In such situations, high hydrostatic pressures can be used to force the transfer of the food value compounds. Using high pressure processing of foods can osmotically induce a mass transfer process to drive certain compounds of value into or out of foods according to the requirements. The diffusion of both the solvent and solute happens simultaneously during soaking, and the directions of diffusion can be the same or opposite depending on the osmotic potentials. This feature can be effectively made use of to increase the quality of foods in terms of nutritional content and flavour enhancement. If a food item is placed in a nutritive medium followed by HPP of the sample, the nutrients can be driven into the food while maintaining the moisture in them. The mechanism for the transfer is not well understood. A research on HPP treatment of brown rice suggested that HPP creates some microfractures on the surfaces of grain, facilitating water transport [54]. This can be taken as a plausible explanation for the transport of nutrients along with water. However, this is a low moisture content grain and developing cracks cannot be the mechanism for high moisture foods. When high moisture fruits were processed for nutrient transfer using HPP, two explanations have been hypothesized for the mechanism of transfer. High pressure conditions affect the cell structures of fruits or vegetables, thereby affecting their permeability and allowing them to uptake nutritional compounds from the surrounding medium [55,56]. However, for quercetin-infused cranberries, Mahadevan et al. [57] reported that even though the cell permeabilization does play some role in increasing the transfer, this effect is not dominant. Osmosis has been stated to be mainly responsible for the diffusion. This conclusion was supported by showing a similar degree of cell permeability in samples before and after HPP. One interesting point to note in this experiment is that the cranberries were impermeable to quercetin initially and had to be scarified before HPP treatment. This shows that, regardless of whether cell permeabilization is dominant or not, cell permeabilization is necessary for nutrient transfer. HPP can create cell ruptures in some foods while in others, external scarification may be necessary. Overall, the process of HPP preserves the main solid matrix of foods when transferring components to different phases.

4.1. Aroma Infusion HPP has had some application in binding different aromas to foods. Some common odorants were mixed with maize starch in native form, with HPP treated maize starch and with maize starch during HPP treatment at 650 MPa for 9 min [58]. Different maize starches with varying amylose content were studied for comparison. The binding extent of these different methods was then analyzed. Hylon VII, a high amylose maize starch, showed similar or slightly enhanced binding characteristics Molecules 2020, 25, 2369 12 of 18

Molecules 2020, 25, x FOR PEER REVIEW 12 of 18 after it was treated with HPP (Figure 10a). However, when it was mixed with odorants during HPP,during the HPP, binding the binding was poorer was forpoorer most for varieties most variet of theies odorants of the odorants excepting excepting hexanol, hexanol, guaiacol, guaiacol, methyl anthranilate,methyl anthranilate, and octanol. and Guaiacol octanol. and Guaiacol methyl anthranilateand methyl had anthranilate very poor bindinghad very characteristics poor binding for nativecharacteristics starch. Infor waxy native maize starch. starch In waxy with maize no amylose, starch with there no were amylose, a few there compounds were a few whose compounds sorption reducedwhose sorption in the HPP reduced treated in starch.the HPP Although treated starch. the overall Although average the sorption overall average of odorants sorption reduced of odorants for both typereduced of HPP for both treatments, type of hexanol,HPP treatments, guaiacol, hexanol, methyl anthranilate,guaiacol, methyl and anthranilate, octanol, along and with octanol, a few along more compounds,with a few showedmore compounds, enhanced sorption showed characteristics enhanced sorption during HPPcharacteristics treatment (Figureduring 10 HPPb). The treatment process of(Figure HPP also10b). led The to process gelatinization of HPP of also starch led andto gelatinization was inversely of related starch toand the was amylose inversely content. related to the amylose content.

120 Hylon VII

100

40 Average

Binding extent (g/100 g) extent Binding 20

0 Binding to native starch HPP treated starch Binding during HPP followed by binding treatment Hexanol Hexyl acetate Limonene Octanol Guaiacol Linalool Undecane γ-Heptalactone Iso-menthone Menthone 3-Hexenyl butyrate Octyl acetate Neral Geranial Methyl anthranilate Geranyl acetate β-Damascone γ-Decalactone δ-Dodecalactone Average binding (%)

(a)

120 Waxy starch

100

60 Average

Binding extent (g/100 g) extent Binding 20

(b)

Figure 10.10. Comparison of binding properties of didifferentﬀerent odorantsodorants to ((aa)) highhigh amyloseamylose maizemaize starchstarch (Hylon(Hylon VII),VII), (b(b)) no no amylose amylose maize maize starch starch (waxy (waxy starch) starch) treated treated in various in various ways ways (plotted (plotted using using the data the fromdata from Błaszczak, Błaszczak, Misharina, Misharina, Yuryev, Yuryev and Fornal,, and Fornal, 2007 [ 582007]). [58]).

4.2. High Pressure Impregnation (HPI) When any compound is used in the liquid form or in a dissolveddissolved form for infusion by pressure intointo aa solid solid food food (generally (generally a solid a solid porous porous matrix), matrix), it has been it has referred been to referred as high pressure to as high impregnation pressure orimpregnation HPI. Vatankhah or HPI. and Vatankhah Ramaswamy and [59 ]Ramaswamy used HPI for [59] the impregnationused HPI for ofthe ascorbic impregnation acid in apple of ascorbic cubes. Theyacid in found apple the cubes. ascorbic They acid found content the ascorbic in the apple acid content pieces increasedin the apple with pieces pressure. increased They with also pressure. checked theThey eﬀ alsoect of checked viscosity the in effect HPI byof makingviscosity use in ofHPI di ﬀbyerent making concentrations use of different of chitosan. concentrations The result of showedchitosan. a The result showed a decrease in the level of mass intake of chitosan with an increase in its concentration. The impregnation of chitosan also increased the pressure stability of the apple, which would otherwise collapse under high pressure due to the presence of small air pockets. Osmotic

Molecules 2020, 25, 2369 13 of 18 decrease in the level of mass intake of chitosan with an increase in its concentration. The impregnation of chitosan also increased the pressure stability of the apple, which would otherwise collapse under high pressure due to the presence of small air pockets. Osmotic dehydration (OD) was compared with vacuum impregnation (VI) and high pressure impregnation by adding sucrose and calcium lactate in unripened and ripened mango chunks [60]. All samples after OD, VI or HPI were subjected to further OD for up to 4 h before the examination. The effect of the presence of pectin methylesterase (PME), which is generally used for improving food firmness, was also evaluated. It was found that both VI and HPI led to an increase in the soluble solids gain and lower water loss in comparison to OD alone. HPI, however, had lesser pronounced effects than VI. The presence of PME for all cases led to a slight increase in firmness and work of shear. The soluble solids gain, with or without the presence of PME, increased in the unripened mango but decreased in ripened mangoes. This was because of two reasons; the sturdy cellular structure of unripened mangoes and the lower sugar content in them. When water leaves the fruit tissue, the susceptibility of the tissues structure to collapse in unripened mangoes is much lower than ripened ones. The collapse of tissue structure can lead to physical hindrance of the osmotic solution from entering the fruit. In addition, lower sugar content in unripened mangoes results in a greater concentration gradient with the osmotic solution, thereby promoting a higher soluble solids gain. PME-pretreated baby carrots with HPP treatment with calcium lactate gluconate (CLG) showed that the high pressures increased their calcium content by almost seven times [61]. The HPP-treated baby carrots had calcium contents of more than three times that obtained by soaking at atmospheric pressure or with vacuum infusion techniques. Experiments using Box-Behnken design showed that the pressure level, processing time and CLG concentration are directly related to the increase in the calcium content and hardness of the carrots. The application of cyclic pressures also causes a significant increase in calcium. The follow-up study using SEM imaging showed that HPP caused cell damage in the carrots [62]. The extraction of beta carotene from the carrots increased by three to five times in comparison to the untreated variety. The effects of individual processing variables, i.e., pressure, processing time, CLG concentration and PME pretreatment on the amount of beta carotene extracted were not significant. HPP also led to a mild to moderate darkening of the carrots after infusion. Another study using HPI was done to impregnate oil into selected fruits through water-based emulsions [63]. Oil and Tween 80, an edible emulsifier, were used in different ratios to form different emulsified oil mixtures. Twenty per cent of these emulsified oil mixtures were homogenized along with 80% water to form the infusate. It was found that the presence of hydrophilic emulsifiers in the solutions facilitated the transport of oil into the inner layers. Without the presence of emulsifiers, oil transfer in fruits was not very efficient. In addition, the higher the concentration of the emulsifier, the lower the transport, whereas higher oil concentration gave higher transfer. This method could prove to be a very efficient method for the fortification of foods with lipid-soluble vitamins, such as vitamin A, vitamin K, etc. in foods [63]. A unique study involving a combination of high pressure gelatinization and high pressure infusion was carried out with paddy rice [64]. Rice loses most of its thiamine, present in the bran and germ layers, during its processing from brown rice to white rice. Parboiled rice, however, has much of the thiamine retained in it. Rice undergoes gelatinization during parboiling and gelatinization facilitates the process of thiamine transfer. Since high pressure environments have shown to cause gelatinization, paddy rice was subjected to pressures of 450 MPa and 600 MPa at the temperatures of 50 ◦C and 70 ◦C for 15 min and 30 min. The results showed that HPP caused a higher transfer of thiamine as the pressure and temperature increased. Long processing times, however, led to its degradation. The HPP treated rice maintained the grain dimensions whilst parboiling rendered the rice shorter and broader.

4.3. Flavour Infusion HPP has been used to impart ﬂavours to foods. Villacís, Rastogi, and Balasubramaniam [65] demonstrated the use of HPP for adding salt and moisture to turkey breast while improving its textural properties. Lemus-Mondaca et al. [66] also used HPP to impregnate salt and water into jumbo squid to Molecules 2020, 25, 2369 14 of 18

Molecules 2020, 25, x FOR PEER REVIEW 14 of 18 enhance sensory characteristics. The Weibull model was reported as the best fit for the mass transfer of compounds into the jumbo squid. When the flavour from solid foods like lemon needs to be infused needs to be infused into any drink, the flavour imparting substance must be placed in the target into any drink, the flavour imparting substance must be placed in the target liquid. On the application liquid. On the application of high pressure, the flavour components will be transferred to the drink. of high pressure, the flavour components will be transferred to the drink. The brewing of coffee or The brewing of coffee or tea needs to be done at high temperatures. Cold brewed beverages take tea needs to be done at high temperatures. Cold brewed beverages take considerably longer times to considerably longer times to reach similar infusion levels. If a pot of hot brewed coffee takes five to reach similar infusion levels. If a pot of hot brewed coffee takes five to ten minutes for preparation, ten minutes for preparation, the cold brewed variety can take several hours [67]. Thus, HPP can make the cold brewed variety can take several hours [67]. Thus, HPP can make the infusion process faster the infusion process faster and much more effective. Hence, a design for the infusion of alcoholic and much more effective. Hence, a design for the infusion of alcoholic beverages with at least 20% beverages with at least 20% alcohol content was carried out with the HPP technology, as shown in alcohol content was carried out with the HPP technology, as shown in Figure 11[ 67]. This technology Figure 11 [67]. This technology can be effectively applied in the near future for the enhancement of can be effectively applied in the near future for the enhancement of flavours in the beverage industry. flavours in the beverage industry.

FigureFigure 11. 11. HighHigh pressurepressure infusioninfusion (HPI)(HPI) ofof flavourflavour intointo beveragesbeverages (redrawn(redrawn fromfrom U.S.U.S. PatentPatent No.No. 1515/975,989,/975,989, 2018 2018 [67 [67]).]).

5. Conclusions 5. Conclusions High pressure processing has many unexplored possibilities for food quality improvements. High pressure processing has many unexplored possibilities for food quality improvements. A A significant amount of work has been done in the area of high pressure gelatinization but with significant amount of work has been done in the area of high pressure gelatinization but with limited limited industrial-scale applications. The niche market for high pressure infusions and high pressure industrial-scale applications. The niche market for high pressure infusions and high pressure impregnations is an upcoming method, leading to a high potential for food quality enhancements. impregnations is an upcoming method, leading to a high potential for food quality enhancements. The ability to make use of the three parameters of pressure, temperature and time can be optimized for The ability to make use of the three parameters of pressure, temperature and time can be optimized the design of food products with specific properties. Since HPP also inactivates most pathogens, it can for the design of food products with specific properties. Since HPP also inactivates most pathogens, be very useful for the production of final packaged ready-to-consume foods that undergo gelatinization it can be very useful for the production of final packaged ready-to-consume foods that undergo and/or infusion during pressure treatment. The impregnation of vital nutrients such as vitamins, gelatinization and/or infusion during pressure treatment. The impregnation of vital nutrients such as flavours and stabilizers into foods are other beneficial applications of this method. Although aroma vitamins, flavours and stabilizers into foods are other beneficial applications of this method. binding to foods is an innovative idea for augmenting the sensory attributes of foods, the application of Although aroma binding to foods is an innovative idea for augmenting the sensory attributes of HPP in this field has not been extensively studied yet. The simultaneous quality enrichment, along with foods, the application of HPP in this field has not been extensively studied yet. The simultaneous microbial inactivation, would be very useful in developing processed foods and making the overall quality enrichment, along with microbial inactivation, would be very useful in developing processed expensivefoods and process making of the HPP overall viable. expensive process of HPP viable.

AuthorAuthor Contributions: Contributions:Conceptualization, Conceptualization, A.K.B. A.K.B. and and M.A.W.; M.A.W.; visualization, visualization, A.K.B. A.K.B. and M.A.W.;and M.A.W.; validation validation M.F.; resources,M.F.; resources, A.K.B.; A.K.B.; data curation, data curation, A.K.B.; writing—originalA.K.B.; writing—original draft preparation, draft preparation, A.K.B.; writing—review A.K.B.; writing—review and editing, and M.A.W. and M.F.; formal analysis, A.K.B.; project administration, M.A.W. and M.F. All authors have read and editing, M.A.W. and M.F.; formal analysis, A.K.B.; project administration, M.A.W. and M.F. All authors have agreed to the published version of the manuscript. read and agreed to the published version of the manuscript. Funding: The APC was funded by MDPI. Funding: The APC was funded by MDPI. Conﬂicts of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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