Relevance of Physical Properties in the Stability of Plant-Based Food Products

Indian Journal of Experimental Biology Vol. 51, November 2013, pp. 895-904

Review Article

Relevance of physical properties in the stability of plant-based food products

Elena Venir* & Enrico Maltini Department of Food Science, University of Udine, Via Sondrio 2/A, 33100 Udine, Italy

Plant tissues are composed of a watery solution of low molecular weight species, mainly sugars, salts and organic acids, and of high molecular weight hydrocolloids, contained in a water insoluble matrix of macromolecules, mostly carbohydrates. All these constituents interact with water, thus reducing its thermodynamic vapour pressure (aw), with small molecules interacting through polar binding, and large biopolymers through surface and capillary effects. Similarly, some constituents will greatly affect kinetic glass transition temperatures (Tg), while others will not. As regards stability, while microbial and chemical changes are mainly related to aw, structure-related changes such as collapse are dependent on the glass transition temperature, Tg. In simple systems such as juices, both thermodynamic and kinetic approaches, employed respectively for high and low moisture systems, have predictive ability, which can be unified in the concept of “critical aw”. However, in complex, multidomain, multiphase systems, such as vegetables and fruits, where insoluble polymeric phases are present, hydrocolloids such as soluble pectins will only slightly affect Tg and aw, but significantly increase the macro viscosity of the soluble fraction, thereby reducing the tendency to collapse. In such cases the use of Tg as a predictive tool must be considered with care. The interrelationships among these aspects are discussed in detail below.

Keywords: Glass transition, Plant food, Stability, Water activity

Food stability is influenced by chemical, physical and hydrocolloids4. The intracellular air spaces in enzymatic reactions, and is dependent on the system's parenchymous tissues of fresh plant food can be physical state−in relation to both thermodynamic and considered as structural elements, influencing the kinetic aspects−which can be described, respectively, perceived texture. In heterogeneous, multidomain, by water activity (aw) and glass transition temperature multiphase and more structured food systems (e.g. 1 (Tg) . In high-moisture homogeneous systems (e.g. dehydrated apple cubes), soluble/insoluble high concentrated juices, puree, etc.), the soluble molecular weight compounds, such as pectins and cell compounds, by interacting with water, are responsible wall components, have only a negligible effect on Tg for modifying aw, whereas in low-moisture dehydrated and aw, but can greatly increase the macro-viscosity of products (e.g. dried vegetables, fruit powders) the the system, and so delay time-dependent physical soluble components mainly affect mobility. In the changes to an extent that can no longer be predicted 5 latter case, the dynamics of change is influenced by only on the basis of Tg and aw . kinetic properties and may be better predicted by Tg 2 rather than by aw . The concept of “critical aw”, i.e. Interaction with water and food stability where the aw depresses Tg below environmental/ The physical properties of foods are affected by operative temperature3, can serve as a unifying water, which is more or less ubiquitous in food principle. Nevertheless, plant foods are mostly products. The influence of water on physical heterogeneous systems consisting of a water-insoluble properties is dependent on the state of water, which, cellular matrix of macromolecules, mostly in turn, depends on the interactions between water carbohydrates (including pectins and hemicelluloses) molecules and the constituents of food, with the and proteins, which are embedded in a watery solution chemical composition determining the extent and of low molecular weight species (mainly sugars, salts strength of interaction. and organic acids) and high molecular weight The thermodynamic parameter aw is mainly able to ______describe and predict the behaviour of reaction rates of *Correspondent author changes in intermediate high moisture systems, where Telephone: +39 0432 558100 (EV); +39 0432 558100 (EM) Fax. +39 0432 558160 (EV); +39 0432 558160 (EM) the soluble compounds interacting with water are E-mail: [email protected]; [email protected] responsible for lowering aw. 896 INDIAN J EXP BIOL, NOVEMBER 2013

In more dehydrated systems, which can be far from This description of water activity assumes that food thermodynamic equilibrium, metastable glasses may is in equilibrium with the surrounding atmosphere. form and aw can be less important with respect to the Under isobaric conditions and at moderate molecular mobility/macro viscosity of the system. In temperatures the deviation of water vapour from the such systems, the kinetic parameter Tg is the reference ideal gas is small, and water activity calculated by the parameter for the description and prediction of above equation varies from the thermodynamic value changes. by less than 5%11. Water activity is widely used as a reference Water activity is much more important for food parameter in most applications of food processing and stability than the total moisture content (MC). In fact storage for a variety of reasons: it is a determinant for MC alone does not provide information on the state of the growth of microorganisms; it is well related with water in a food, if it is “bound” or “free”, “inherent” most degradation reactions of a chemical, enzymatic, or “occluded”, etc., whereas aw gives an indication of and physical nature; it is easier to measure than the relationship of water with the binding sites of the moisture content, and measurement is non- food components. The equilibrium relationship destructive; the ‘‘monolayer’’ derived from the water between moisture content and the relative humidity of vapour sorption isotherm gives an indication of the the surrounding environment is described through the optimum moisture content in dried foods. moisture sorption isotherms12. Water sorption The role of aw in fruit-vegetable ingredients for isotherms may be either adsorption or desorption composite foods derives from the fact that a number of isotherms: adsorption occurs when food solids are formulations (ice cream, pie fillings, pastry, dessert, exposed to conditions where the vapour pressure of fruit yoghurt, etc.) are obtained from fruit/vegetable water in the atmosphere is higher than the water ingredients and moisture migration is critical for vapour pressure in the solid. Conversely, desorption obtaining end products which are attractive to the occurs when the water vapour pressure within the consumer. Indeed, moisture migration is frequently solid is higher than that of the atmosphere. implicated in quality loss in composite multiphase Water sorption isotherms systems when components or phases of different water 6−8 Sorption isotherm describes the thermodynamic activity are in contact . Water moves from the wet to relationship between water activity and the the dry components or phases, possibly leading to equilibrium moisture content of a food product, at irreversible loss in sensorial and microbiological 9 constant pressure and temperature (Fig. 1). quality and, of course, to shelf life reduction . According to the Brunauer’s classification13, the In multicomponent systems moisture loss or gain shapes of isotherms, which describe the sorption of from one region (or food component) to another will gases by solid materials, may have five basic forms. occur continuously until thermodynamic equilibrium is reached. Moisture migration in multi-domain foods obeys aw and not moisture content, with water activity being the controlling parameter.

Water activity (Thermodynamic property) The state of water in a solution or in a solid is expressed by the activity coefficient (aw = water activity), which is a thermodynamic measure of the chemical potential of water in the system. In food science water activity is referred to as a measure of the moisture availability in a product expressed as the ratio of the water vapour pressure of the substance (p) to that of pure water (vapour pressure) (p0) at the 10 Fig. 1Schematic representation of adsorption and desorption same temperature and pressure . isotherms showing the amount of water adsorbed as a function of steady state relative vapour pressure (RVP) at a constant = P temperature. The difference between these two curves shows a W hysteresis, indicative of the irreversibility of water sorption during P0 dehydration and rehydration. VENIR & MALTINI: PHYSICAL PROPERTIES IN STABILITY OF PLANT-BASED FOOD PRODUCTS 897

Water adsorption isotherms of food materials are type The extra assumption of the GAB model states that II, i.e. sigmoid curves, which often exhibit hysteresis the sorption state of the sorbate molecules in the between adsorption and desorption. The sigmoid, type layers beyond the first is different to the pure liquid II curve is also the typical shape of the desorption and state22. In common with the BET model, the GAB adsorption isotherms of plant tissues14. Some equation has the monolayer moisture content and crystalline material, e.g. sugars, may display a fairly energy constant parameters, and introduces an low adsorption of water until water activity becomes additional constant (k) which is related to the enthalpy sufficient for solubilization, after which the difference between water molecules in the pure liquid adsorption increases. Such sorption follows the type and in the second sorption stages22. III isotherm3. During moisture adsorption in foods, The value of the monolayer (m0) is of particular water molecules gradually mix together with solids importance because it refers to the amount of water via chemisorptions, physical adsorption, and strongly adsorbed in the binding sites, and it is multilayer condensation. considered to be a threshold value for food stability.

The sorption isotherm can be roughly divided into The interpretation of most of the reactions involved three zones, corresponding to: in degradation/spoilage during the processing or (I) strongly bound water, which includes structural storage of foods, whether of a biological nature water (H-bonded water) and monolayer water (enzymatic reactions and microbial growth) or a sorbed onto hydrophilic and polar groups of chemical one (oxidation and Maillard reactions), may food components (polysaccharides, proteins). be related to the structure of water and the water Water in this state is unfreezable and is not sorption isotherms. Indeed, the so called stability map available for chemical, enzymatic and was developed in the early seventies to describe the microbial reactions; rate of degradative reactions as a function of water (II) loosely bound water: water molecules bind less activity23. As a general rule, the rates of enzymatic firmly than in zone I, and are usually present in activity together with mold, yeast and bacteria small capillaries. This range of constituent growth, tend to increase as water activity increases. water content can be regarded as a continuous Conversely, lipid oxidation and Maillard reactions transition from bound to free water; show minimum and maximum and bell-shaped (III) solvent or free water: the properties of water in curves, respectively. this zone are similar to those of the free (bulk) With regard to water activity and the preservation water that is held in voids, large capillaries and of plant foods, Maltini et al.4 reported that in crevices15. oxidative reactions, water may act as a pro-oxidant or A variety of mathematical models have been anti-oxidant depending on the moisture content in the proposed to fit water sorption isotherms of foods, system. The protective effect of a monolayer of water among which Brunauer-Emmett-Teller (BET) and on foods containing oxidable sites was firstly Guggenheim-Anderson-De Boer (GAB) equations observed by Salwin24 during the storage of dehydrated have been extensively used. food items. He suggested that water molecules The BET13 adsorption model was the one mainly attached to the sensitive sites do prevent absorption of used to describe water sorption by food and oxygen directly and might possibly hydrate trace foodstuff12,16; it fits the sigmoidal sorption isotherms metal ions or decompose free radicals. Hence, water 17 in the range of aw of 0.05-0.5 . The BET model gives may act as an anti-oxidant at low aw (hydration of two constants: the monolayer moisture content (m0) trace metal ion, h-bonding of hydroperoxides, and the energy constant (CB). promoting free-radical recombination) and as a pro- 18,19 The GAB equation can be used in a wide range oxidant at high aw (increasing mobility, dissolution of of aw and has been found to adequately represent the precipitated catalyst, matrix swelling with exposure of experimental data in the range of water activities of new catalytic surfaces). These opposing effects give most practical interest in foods, i.e. 0.10–0.90. This rise to a minimum and maximum curve25. In fruit and equation has been recommended by the European vegetable products the oxidative changes mainly Project Group COST 90 on Physical Properties of affect carotene, lycopene, and phenol compounds and Foods as the fundamental equation for the can lead to the degradation of colour, flavour and characterization of water sorption of food materials20, 21. nutritional quality. 898 INDIAN J EXP BIOL, NOVEMBER 2013

Non enzymatic (Maillard) browning is mostly observed for the un-treated apple cubes (Fig. 3c) responsible for undesirable colour and flavour seems much more unusual. In these samples, changes in intermediate moisture systems, such as enzymatic oxidation before freeze drying certainly fruit concentrates and purees, and dehydrated occurred during the experimental time required for the vegetables such as potatoes. The Maillard reaction preparation of the cubes, since the polyphenol oxidase can also lead to desirable effects as seen in roast (PPO) activity in the dried cubes was still 0.350 -1 -1 coffee, cocoa beans, corn flakes, French fries, and so O.D.420 nm⋅min ⋅gd.m. . The rate of enzymatic reaction on. As is well known, the reaction rate of Maillard browning exhibits a bell-shaped curve as a function of water activity. Maximum browning has been observed in the range of water activities from 0.3−0.7; however, in systems where low aw is not associated with a large increase in viscosity, e.g. in an ethylene- glycol based model26, no maximum appears and the reaction rate steadily decreases from low to high aw, indicating a kinetic-dependent effect in the reaction4. The bell shape of the curve arises from two opposite driving forces (Fig. 2). At low aw the concentration of reactants is counteracted by viscosity and diffusion resistance, which lowers molecular 27, 28 mobility and limits reaction rate ; at high aw the increase in mobility, which would account for increased reaction rates, is actually accompanied by Fig. 2Schematic representation of driving forces involved in a the dilution of the reactants. The browning reaction non enzymatic (Maillard) browning reaction vs. aw. rate is finally limited by two independent variables: reactant concentration (the reaction rate increases as aw decreases) and reactant diffusivity (the reaction rate decreases as aw decreases and tends towards zero for viscosity above ∼1012 Pas).

Accordingly, Venir and Maltini (unpublished data) have evaluated the browning rate of freeze dried apple tissue after rehydration at 25 °C for 20 days (Fig. 3). Cubes were subjected or not to blanching (90 °C × 1 min) in water or sucrose solution (60% w/w) prior to freeze-drying. As expected, the apple cubes subjected to blanching in water solution did not exhibit significant browning, since the enzymes responsible for oxidation were inactivated by the thermal treatment (Fig. 3a).

The apple cubes blanched in sucrose solution exhibited a characteristic bell shaped curve as a function of aw, with a maximum at aw ≅ 0.6 (Fig. 3b). Browning was not expected to be enzymatic, since enzymes were inactivated by thermal treatment, but rather the result of Maillard reaction. Indeed, sucrose from the solution used for blanching which had been Fig. 3Browning (redness a*, CIElab Hunter scale parameter) of freeze dried apples after rehydration at different relative humidity heated to a high temperature for the experimental for 20 days at 25 °C. Pretreatments: (a) blanching in water time, could have undergone inversion and thus (90 °C × 1 min); (b) osmo-blanching in sucrose solution (60% w/w; promoted the Maillard reaction. The bell shaped curve 90 °C × 1 min); and (c) un-treated apple cubes. VENIR & MALTINI: PHYSICAL PROPERTIES IN STABILITY OF PLANT-BASED FOOD PRODUCTS 899

is reported to increase as a function of aw, as described involved in cereal systems and cereal processing, food in the “food stability map”23. Nevertheless, the curve freezing and frozen foods, confectionary and candies, observed for this reaction deviated from the expected dehydration processes and dehydrated foods, behaviour. In fact, the mechanism of enzymatic extrusion and extruded foods, among others. The browning consists of two steps: the oxidation of glass-forming compounds in foods are carbohydrates phenols and the polymerization of oxidized phenols. and, to a lesser extent, proteins35,36, whereas fats do Oxidized phenols are reported to be only slightly not exhibit glass transition. coloured, whereas their polymerization leads to the The broader implications of non-equilibrium glassy − development of significant browning29 32. Excluding and rubbery states, and of the glass transition Maillard reaction involvement−supported by the fact temperature (Tg), for the quality, safety and storage that no browning occurred in the samples blanched in stability of a wide range of glass-forming aqueous water, where the thermal treatment should have food systems was firstly recognized by White and triggered the reaction - the bell shaped curve requires Cakebread37. Further evolution led to the theory a different explanation. It can be assumed that known as “Food Polymer Science” (FPS), which enzymatic reactions were responsible for the emphasizes the fundamental and generic similarities oxidation of the phenols, which, however, were not between synthetic polymers and food molecules, allowed to polymerize at low aw (high viscosity and providing a theoretical and experimental framework hindered diffusivity); at high aw, both the reduced for the study of kinetically constrained foods systems, oxygen diffusion through the collapsed matrix33, 34 and such as low moisture, amorphous, rubbery or glassy dilution of the reactants could have been involved in systems2,38−41. The amorphous state of a material the lesser degree of browning. Again, the two refers to its random, disordered molecular structure, opposite effects determined maximum browning at where the glass transition is one of the most important intermediate aw. Samples hydrated at higher aw’s physicochemical characteristics. In these systems, the exhibited lower browning than those at intermediate glass transition temperature (Tg) and the state diagram 3 aw, even if they passed through these intermediate are useful for the prediction of degradative changes . values, suggesting that the kinetics of hydration was Rates of change in amorphous materials (collapse, much faster than those of the Maillard and phenol shrinkage, sticking, caking, agglomeration, polymerization reactions. crystallization, loss of volatiles, etc.) are time

Mobility and glass transition temperature (kinetic dependent and controlled by the ability of molecules property) within the material to respond to changes in their surroundings. Many food systems are far from equilibrium, hence treatments based on the equilibrium thermodynamics Glass transition occurs over a temperature range in of very dilute solutions (namely water activity and the cooling of a liquid-like material, leading to sorption isotherms) fail. In these systems one must solidification to a glassy substance (vitrification). Tg always deal with kinetics2 and mobility/diffusivity are is the temperature where a polymer goes from a hard, 12 the main factors controlling reaction rates. Food glassy state (viscosity = 10 Pa⋅s) to a rubbery state dehydration and freezing lead to high levels of non- (viscosity = 108 Pa⋅s) and occurs at about 100–150 °C crystalline solids and amorphous regions in the below the equilibrium melting temperature of a pure 42 dehydrated/unfrozen structures. Amorphous, non substance . Tg can be measured by Differential crystalline solids are typical of low water content and Scanning Calorimetry (DSC) as a change in heat frozen foods, where non-equilibrium, glass-like capacity and it appears as a step transition in the structures are formed. Processed fruits and vegetables thermal trace. The glass transition temperatures of are often low moisture, sugar-rich foods, food components can range between below room characterized by colour, flavour and structural temperature−such as fructose−up to the properties and they can undergo physical structural decomposition temperatures of biopolymers, such as 43-45 46 changes which are strongly related to the glass starch and gluten proteins . Values of Tg of some transition temperature, Tg. The kinds of processing dry solutes, organic acids, fruits, and vegetables are which require an understanding of the amorphous reported (Table 1). state and glass transition of food systems are those Food solids are plasticized by water molecules47 occurring at low moisture content, such as those and other low molecular weight hydrophilic solvent 900 INDIAN J EXP BIOL, NOVEMBER 2013

Table 1—Tg of some dry plant food components, fruit and vegetables, (data from Maltini et al.4)

Food components Tg (°C) Sucrose 67 Glucose 31 Solutes Fructose 5 Citric acid 6 Malic acid -21 Orange 45 Raspberry 41 Strawberry 29

Blackberry 22 Fig. 4Schematic state diagram illustrating the major curves and Fruits Peach 20 the Tg related changes in low moisture foods. Apricot 18 Apple 18 hindered diffusion. Frozen food systems would lie to Blueberry 15 the left of T’g, whereas intermediate- or low-moisture Lemon 11 food systems would be to the right of T’g. Potatoes 71 Celery 58 Below the Tg curve the physical changes are Vegetables Carrot 57 avoided, since the Tg line represents an upper limit Cabbage 43 below which the system is in a glassy state with ≥ 12 ⋅ Pear 5 viscosity 10 Pa s, and where mobility is highly limited. components. Water is a mobility enhancer since its The position of the Tg curve is largely determined low molecular weight leads to a large increase in by the average molecular weight of the solutes in the mobility, due to increased free volume and decreased system. Tg of the different sugars mainly available in 2 local viscosity . Plasticization causes a decrease in Tg fruit and vegetable matrix are reported (Table 1). value, which is related to the moisture content through Glass transition of plant food ingredients, namely that the glass transition curve. Tg vs. the solute mass of fruit and fruit derivatives, occurs at temperatures fraction is described by the well known Gordon and reflecting the sugar components of the materials5, 49−56 48 Taylor equation , and is plotted in the state diagram. or well above 100 °C for dry, high molecular weight The state diagram represents a combination of both systems, such as dry cereals and high protein equilibrium and metastable (or kinetic) events, and as foods38, 57−60. a matter of fact the glass transition curve, the melting When the temperature is above Tg an amorphous curve, together with the temperatures of glass solid exits in a rubbery state. In this state, the transition (T’g) and melting (T’m) of the maximally molecular mobility of the reactants is accelerated, cryo-concentrated solution and its concentration (C’g) resulting in increased rates of physicochemical are reported (Fig. 4). changes in dried products. Physical and structural In food systems the glass transition curve stretches changes, namely collapse during drying, sticking and from the Tg of pure water ∼ -137.15 to the Tg of the caking of dry powders, agglomeration, crystallization, dry solute(s). The Tm curve starts at the freezing point loss of volatiles, loss of crispness and other forms of of pure water and decreases (owing to colligative texture degradation, are closely related to the glass effects) to the value of T’m. T’g is the temperature transition temperature and may occur when Tg drops through which a freeze concentrated solute will pass below the processing or storage temperature61,62. All during cooling and represents the Tg of the maximally amorphous components are metastable (out of freeze-concentrated solution. As a diluted solution is thermodynamic equilibrium) and tend to crystallize cooled, water will crystallize at Tm and with further over time during storage. The amorphous form of low cooling, more and more ice is frozen out, until the molecular weight carbohydrates and protein concentrated solution becomes so viscous (system has hydrolyzates are very hygroscopic63. If a local portion become a mechanical solid with extraordinarily high of the product in a packaging gains moisture, the Tg is viscosity) that no more ice will crystallize owing to locally depressed and the crystallisation rate will be VENIR & MALTINI: PHYSICAL PROPERTIES IN STABILITY OF PLANT-BASED FOOD PRODUCTS 901

accelerated. The ejected moisture is adsorbed at the non-enzymic reactions were found in dried vegetables 76,77 surface of neighbouring particles creating stored above Tg . The rate of oxidation of sensitive 64 interparticulate liquid bridges which lead to caking . compounds in a dry matrix is also enhanced above Tg, Surrounding particles, which absorb moisture, will especially due to crystallisation which releases also be crystallised and the crystallization can proceed encapsulated materials from the system to the surface. as a chain phenomenon. The rate of change depends on the (T-Tg) The viscous flow of the semisolid network which difference. The direct plasticizing effect of increasing forms in the initial stages of drying processes leads to moisture content at constant temperature is equivalent collapse. Collapse determines the loss of a porous to the effect of increasing temperature at constant 2 structure which is associated with an irreversible moisture and the (T-Tg) difference includes both the occlusion of the pores and with the formation of a thick effects of the storage/processing temperature (through 4,65−66 viscous body . Among the main consequences, a T), and the moisture content (through Tg). The higher reduction in drying rates, loss of volatiles, high residual the (T-Tg) difference, the farther the system is from moisture and poor storage and rehydration properties the glassy state and kinetic stability and, in general, are reported. the faster is the rate of reactions. Collapse, caking and stickiness are related The physical stability of amorphous food materials phenomena and are among the most deleterious has been effectively investigated, by means of the changes, which irreversibly affect spray dried and food polymer science approach, through the glass freeze dried fruit powders67. The caking of an transition temperature (kinetic property), whereas amorphous powder is a time dependent phenomenon. If moisture sorption isotherms and water activity the dried product is warmed to above its glass (thermodynamic property) provide information about transition, its surface becomes viscous and can form the physical properties of food, as the plasticizer level bridges between adjacent particles which result in an is adjusted and temperature is held constant. The agglomeration68 such as in the storage of sugar-rich influence of temperature and water activity on glass dried fruit products61. transition can be investigated by both thermal and moisture sorption methods78. A critical moisture Relatively high Tg dry values characterize vegetable content (critical m), that results in a glass transition at matrices, whereas the Tg temperatures of fruit juices are closely related to their sugar/acid composition a given storage or processing temperature, can be found and the concept of critical aw may be a link (Table 1). The Tg of dry juices is often lower than the 3,4 storage temperature, and caking cannot be avoided, between thermodynamic and kinetic approaches . unless the storage temperature is further reduced, or the The critical aw can be identified as an operative parameter relating the sorption isotherm to the glass juice Tg is increased through the addition of high Tg components, such as sucrose or maltodextrins. transition curve (Fig. 5). Based on the above considerations, chemical and Glass transition is also related to volatile release. physical changes in foods are usually regarded in Volatile compounds can be entrapped within terms of water activity and thermodynamics, or glass amorphous microregions69−71 during drying of sugar rich systems (such as fruit and vegetable juices). The retention and release of volatiles are considered to be kinetically-governed phenomena. The entrapped volatiles are released when the storage or processing temperature exceeds the glass transition temperature3, 33,72, due to both temperature raise and moisture gain, also leading to structural changes65,66,70,73. In a glassy system the released volatile components have very limited mobility, with diffusion primarily occurring through the pores of the matrix74. When the temperature exceeds Tg volatile diffusivity is greatly increased45,75.

Various chemical reactions are also accelerated in Fig. 5Critical aw above which the Tg of the product drops below dried products if stored at above Tg. Increased rates of storage or operative temperature. 902 INDIAN J EXP BIOL, NOVEMBER 2013

transition and kinetics, respectively for high or low well represented in fruits and many fruit derivatives. water content. Nevertheless, further aspects, other Structure-related changes during the moistening of than aw and Tg, make plant food systems more freeze-dried apple tissue, which was taken to be a basic complex. In heterogeneous, multidomain, multiphase plant food structure, were reported5,79. The authors and more structured food systems (e.g. dehydrated evaluated some physical changes (collapse, shrinkage, fruit or vegetable cubes, slices or rings) the consistency, colour, volatile release) in freeze dried soluble/insoluble high molecular weight compounds, apple cubes and cell walls, after hydration at different namely hydrocolloids, such as pectins and cell wall relative humidity, with respect to both thermodynamic components, having only a negligible effect on Tg and and kinetic factors. Changes affecting consistency, aw, can significantly increase the macro-viscosity of volume, shape, architectural and diffusional properties the system, and thereby affect the occurrence of occurred at a much higher moisture content than the physical changes. expected critical aw and to a much larger (T-Tg). These changes were more evident at a in the range 0.40– Physical changes in complex, multicomponent, w 0.50, where the (T-Tg) difference was 50–60 °C. In multiphase plant food tissue similar cases, characterized by complex phase- Structured and undamaged, biological materials separated systems with insoluble polymeric phase, the exhibit a structural hierarchy. The structure and dependence of kinetically controlled changes on the properties manifested at each successive level are glass transition temperature may be modified to a dependent on the attributes of elements in the considerable extent. preceding level, the elements’ relative concentration, the physical forces involved in their interaction, and the Conclusion manner in which the elements are spatially arranged67. Plant food stability is influenced by chemical, Hence, food elements (solid, liquid and gaseous) and physical and enzymatic reactions, and is dependent on architecture should be considered in a dynamic and the system's physical state. Relations between the interrelated condition. Maltini et al.4 provided a stability of plant foods and the system’s physical convenient description of the general constitution of properties are usually regarded in terms of water plant foods. Vegetable and fruit tissue can be described activity at high water content, and in terms of glass as a watery solution of low molecular weight species, transition at low water content. The thermodynamic mainly sugars, salts and organic acids and of high and kinetic approaches can be unified through the molecular weight hydrocolloids, contained in a water- concept of critical aw, which is the aw that corresponds insoluble cellular matrix of macromolecules, mostly to the Tg relevant to a given change, namely where Tg carbohydrates, which also include insoluble pectic decreases below the storage/operating temperature. substances, hemicelluloses, proteins and, sometimes, These approaches have predictive capacity for simple, lignins. Intracellular air spaces are present in homogeneous systems such as concentrated juices, parenchymous tissue and these may be considered as purees for high moisture products, and dry powders true structural elements, in that they have a very for low moisture dry foods. Conversely, in complex, characteristic influence on the perceived texture. All of multidomain, multiphase, structured systems such as these constituents are able to interact with water and vegetable and fruit pieces, rings, cubes, etc., the have the ability to lower its vapour pressure (aw). The insoluble polymeric phase may affect the relationship small molecules depress aw mainly through polar between Tg/aw and the dynamics of physical changes. binding, whereas large biopolymers operate through Nevertheless, it is still possible to correlate the kinetic surface interactions and capillary effects. Similarly, and thermodynamic parameters by means of a concept some constituents will strongly affect glass transition which can be called effective critical aw, and which temperatures (Tg), while others will not. For example, corresponds to the aw at which the Tg-dependent the Tg of fruits and vegetable tissues is associated to changes actually occur, provided it is that of the juice and no Tg appears for the cellular measured/determined for each specific system. matrix alone5,56. In addition, hydrocolloids, such as soluble pectins, will have only a negligible effect on T References g and a , but can strongly increase the macro viscosity of 1 Roos Y H, Water activity and plasticization, in Food shelf w life stability. Chemical biochemical and microbiological the soluble fraction and reduce the tendency of fruit changes, edited by N A Eskin & D S Robinson (CRC Press and vegetable extracts to collapse. Such complexity is LLC, Florida, USA) 2000, 3. VENIR & MALTINI: PHYSICAL PROPERTIES IN STABILITY OF PLANT-BASED FOOD PRODUCTS 903

2 Slade L & Levine H, Beyond water activity: recent advances 23 Labuza T P, McNally L, Gallagher D, Hawkes J & Hurtado based on an alternative approach to the assessment of food F, Stability of intermediate moisture foods, J Food Sci, 37 quality and safety, Crit Rev Food Sci Nutr, 30 (1991) 115. (1972) 154. 3 Roos Y H, Phase transitions in foods (Academic Press, 24 Salwin H, The role of moisture in deteriorative reactions of California, USA) 1995. dehydrated foods, in Freeze-drying of foods, edited by F R 4 Maltini E, Torreggiani D, Venir E & Bertolo G, Water Fisher (National Academy of Science—National Research activity and the preservation of plant foods, Food Chem, 82 Council, Washington, DC) 1962, 58. (2003) 79. 25 Labuza T P, Oxidative changes in foods at low and 5 Venir E, Munari M, Tonizzo A & Maltini E, Structure intermediate moisture levels, in Water relations of foods, related changes during moistening of freeze dried apple edited by R B Duckworth (Academic Press, London) 1975, tissue, J Food Eng, 81 (2007) 27. 455. 6 Guillard V, Broyart B, Bonazzi C, Guilbert S & Gontard N, 26 Loncin M, Basic principles of moisture equilibria, in Freeze- Modelling of moisture transfer in a composite food: dynamic drying and advanced food technology, edited by S A Goldblith, L Rey & W W Rothmayr (Academic Press, water properties in an intermediate aw porous product in London) 1975, 599. contact with high aw filling, Chem Eng Res Des Trans IChem E,81 (2003) 1090. 27 Labuza T P, Tannenbaum S R & Karel M, Water content and 7 Roca E, Guillard V, Guilbert S & Gontard N, Moisture stability of low moisture and intermediate-moisture foods, migration in a cereal composite food at high water activity: Food Technol, 24 (1970) 543. Effects of initial porosity and fat content, J Cereal Sci, 43 28 Labuza T P & Saltmarch M, The nonenzymayic browning (2006) 144. reaction as affected by water in foods, in Water activity: 8 Bourlieu C, Guillard V, Powell H, Vallès-Pàmies B, Guilbert influences on food quality, edited by L B Rockland & G F S & Gontard N, Modelling and control of moisture transfers Stewart (Academic Press, New York) 1981, 605. 29 Mathew A G & Parpia H A B, Food Browning as a in high, intermediate and low aw composite food, Food Chem, 106 (2008) 1350. polyphenol reaction, in Advances in food research, edited by 9 Labuza T P & Hyman C R, Moisture migration and control E M Mark & G F Stewart (Academic Press, New York) in multi-domain foods, Trends Food Sci Tech, 9 (1998) 47. 1971, 75. 30 Vamos Vigiazo L, Polyphenolic oxidase and peroxidase in 10 Scott W J, Water relations of Staphylococcus aureus at fruits in vegetables, CRC Cr Rev Food Sci, 15 (1981) 49. 30 °C, Aust J Biol Sci, 6 (1953) 549. 31 Ju Z G, Yuan Y B, Liu C L, Zhan S M & Wang M X, 11 Lewicki P P, Water as the determinant of food engineering Relationships among simple phenol, flavonoid and properties, A review, J Food Eng, 61 (2004) 483. anthocyanin in apple fruit peel at harvest and scalp Food Technol 12 Labuza T P, Sorption phenomena in foods, , 22 susceptibility, Postharvest Biol Tec, 8 (1996) 83. (1968) 263. 32 Robards K, Prenzler P D, Tucker G, Swatsitang P & Glover

13 Brunauer S, Emmett P H & Teller E, Adsorption of gases in W, Phenolic compounds and their role in oxidative processes multimolecular layers. J Am Chem Soc, 60 (1938) 309. in fruits, Food Chem, 66 (1999) 401.

14 Sun W Q, Methods for the study of water relations under 33 Levi G & Karel M, Volumetric shrinkage (collapse) in desiccation stress, Desiccation and survival in plants: Drying freeze-dried carbohydrates above their glass transition without dying, edited by M Black & H W Pritchard (CABI temperature, Food Res Int, 28 (1995) 145. Publishing, New York) 2002, 47-91. 34 Karathanos V T, Kanellopulos N K & Belessiotis V G,

15 Matholuthi M, Water content, water activity, water structure Development of porous structure during air drying of and the stability of foodstuffs, Food Control, 12 (2001) 409. agricultural plant products, J Food Eng, 29 (1996) 167. 16 Iglesias H A & Chirife J, Bet monolayer values in 35 Jouppila K & Roos Y H, The physical state of amorphous dehydrated foods and food components, Lebensm Wiss corn starch and its impact on crystallization, Carbohydr Technol, 9 (1967) 107. Polym, 32 (1997) 95. 17 Lewicki P P, The applicability of the gab model to food 36 Haque M D K & Roos Y H, Water sorption and water sorption isotherms, Int J Food Sci Tech, 32 (1997) 553. plasticization behavior of spray-dried lactose/protein 18 De Boer J H, The dynamic character of adsorption (Oxford mixtures, J Food Sci, 69 (2004) E384. Clarendon Press) 1953, 61. 37 White G W & Cakebread S H, The glassy state in certain 19 Guggenheim E A, Application of statistical mechanics sugar-containing food products, J Food Technol, 1 (1966) (Oxford Clarendon Press, Oxford) 1966, 186. 73. 20 Wolf W, Spiess W E L, Jung G, Weisser H, Bizot H & 38 Slade L & Levine H, A food polymer science approach to Duckworth R B, The water-vapour sorption isotherms of structure-property relationships in aqueous food systems: microcrystalline cellulose and purified potato starch. Results Non-equilibrium behavior of carbohydrate-water systems, of collaborative study, J Food Eng 3 (1984) 51. Adv Exp Med Biol, 302 (1991b) 29. 21 Wolf W, Spiess W E L & Jung G, Standardization of 39 Slade L & Levine H, Water and the glass transition - isotherm measurements, in Properties of water in foods, dependence of the glass transition on composition and edited by Simatos D & Multon J L (Martinus Nijhoff chemical structure: Special implications for flour Publishers, Dordrecht, The Netherlands) 1985, 661. functionality in cookie baking, J Food Eng, 24 (1995) 431. 22 Timmermann E O, Multilayer sorption parameters: BET or 40 Levine H & Slade L, A polymer physico-chemical approach GAB values?, Colloids and surfaces A: Phyicochem Eng to the study of commercial starch hydrolysis products Aspects, 220 (2003) 235. (SHPs), Carbohydr Polym, 6 (1986) 213. 904 INDIAN J EXP BIOL, NOVEMBER 2013

41 Levine H & Slade L, A food polymer science approach to the 62 Roos Y H, Thermal analysis, state transitions and food practice of cryostabilization technology. Comments, Agric quality. J Therm Anal Calorim, 71 (2003) 197–203. Food Chem, 1 (1989) 315. 63 Aguilera J M, del Valle J M & Karel M, Caking phenomenon 42 Sperling L H, Introduction to physical polymer science in amorphous food powders, Trends Food Sci Tech, 6 (1995) (Wiley, New York) 2006, -880. 149. 43 Zeleznak K J & Hoseney R C, The glass transition in starch, 64 Peleg M & Hollenbach A M, Flow conditioners and Cereal Chem, 64 (1987) 121. anticaking agents, Food Technol, 38 (1984) 93. 44 Roos Y & Karel M, Water and molecular weight effects on 65 To E C & Fink J M, ‘‘Collapse’’, a structural transition in glass transitions in amorphous carbohydrates and freeze dried carbohydrates. II. Effect of solute composition, J carbohydrate solutions, J Food Sci, 56 (1991) 1676. Food Technol, 13 (1978) 567. 45 Roos Y H & Karel M, Phase transitions of mixtures of 66 To E C & Fink J M, ‘‘Collapse’’, a structural transition in amorphous polysaccharides and sugars, Biotechnol Progr, 7 freeze dried carbohydrates. III. Prerequisite of (1991) 49. recrystallization, J Food Technol, 13 (1978) 583. 46 Noel T R, Parker R, Ring S G & Tatham A S, The glass- 67 Aguilera J M & Stanley D W, Microstructural principles of transition behaviour of wheat gluten proteins, Int J Biol food processing and engineering, 2nd edn, (Aspen Publishers, Macromol, 17 (1995) 81. Gaithersburg, Maryland, USA) 1999, 432. 47 Levine H & Slade L, Water as a plasticizer: physico- 68 Peleg M, Glass transitions and the physical stability of food chemical aspects of low-moisture polymeric systems, in powders, in The glassy state in foods, edited by M V Water science reviewers, Vol III, edited by F Franks Blanshard & P J Lillford (Nottingham University Press, (Cambridge University Press, Cambridge) 1988, 79. Leicestershire) 1993, 435. 48 Gordon M & Taylor J S, Ideal copolymers and the second- 69 Flink J M & Karel M, Effect of process variables on order transitions of synthetic rubbers. I. Noncrystalline retention of volatiles in freeze drying, J Food Sci, 35 (1970) copolymers, J Appl Chem, 2 (1952) 493. 444. 49 Roos Y H, Effect of moisture on the thermal behavior of 70 Flink J M & Karel M, Mechanisms of retention of organic strawberries studied using differential scanning calorimetry, volatiles in freeze-dried systems, J Food Technol, 7 J Food Sci, 52 (1987) 146. (1972) 199. 50 Sá M M, Figueiredo A M & Sereno A M, Glass transitions 71 Chirife J & Karel M, Volatile retention during freeze drying and state diagrams for fresh and processed apple, of aqueous suspensions of cellulose and starch, J Agr Food Thermochim Acta, 329 (1999) 31. Chem, 21 (1973) 936. 51 Bai Y, Rahman M S, Perera C O, Smith B & Melton L D, 72 Goubet I, Le Quere J L & Voilley A, Retention of aroma State diagram of apple slices: Glass transition and freezing compounds by carbohydrates: influence of their curves. Food Res Int, 34 (2001) 89. physicochemical characteristics and of their physical state, J 52 Telis V R N & Sobral P J A, Glass transitions and state Agric Food Chem, 48 (1998) 1981. diagram for freeze-dried pineapple, Lebensm WissTechnol, 73 Chirife J & Karel M, Effect of structure disrupting treatments 34 (2001) 199. on volatile release from freeze-dried maltose, J Food 53 Telis V R N & Sobral P J A, Glass transitions for freeze- Technol, 9 (1974) 13. dried and air-dried tomato, Food Res Int, 35 (2002) 435. 74 Whorton C, Factors influencing volatiles release from 54 Moraga G, Martínez-Navarrete N & Chiralt A, Water encapsulation matrices, in Encapsulation and Controlled sorption isotherms and phase transitions in kiwifruit, J Food Release of Food Ingredients, edited by S J Risch & G A Eng, 72 (2006) 147. Reineccius (ACS Symp Ser 590, American Chemical 55 Syamaladevi R M, Sablani S S, Tang J, Powers J & Swanson Society, Washington, DC) 1995, 134. B G, State diagram and water adsorption isotherm of 75 Whorton C & Reineccius G A, Evaluation of the mechanisms raspberry (Rubus idaeus), J Food Eng, 91 (2009) 460. associated with the release of encapsulated flavor materials 56 Aguilera J M, Cuadros T R & del Valle J M, Differential from maltodextrin matrices, in Encapsulation and controlled scanning calorimetry of low-moisture apple products, release of food ingredients, edited by S J Risch & G A Carbohyd Polym, 37 (1998) 79. Reineccius (ACS Symp Ser 590, American Chemical 57 Nikolaidis A & Labuza T P, Glass transition state diagram of Society, Washington, DC) 1995, 142. a baked cracker and its relationship to gluten, J Food Sci, 61 76 Karmas R, Buera M P & Karel M, Effect of glass transition (1996) 803. on rates of nonenzymatic browning in food systems, J Agr 58 Roos Y H, Glass transition temperature and its relevance in Food Chem, 40 (1992) 873. food processing, Annu Rev Food Sci Technol, 1 (2010) 469. 77 Buera M P & Karel M, Application of the WLF equation to 59 Bengoechea C, Arrachid A, Guerreroa A, Hill S E & describe the combined effects of moisture and temperature Mitchell J R, Relationship between the glass transition on nonenzymatic browning rates in food systems, J Food temperature and the melt flow behavior for gluten, casein and Process Pres, 17 (1993) 31. soya, J Cereal Sci, 45 (2007) 275. 78 Carter B P & Schmidt S J, Developments in glass transition 60 Pouplin M, Redl A & Gontard N, Glass transition of wheat determination in foods using moisture sorption isotherms, gluten plasticized with water, glycerol, or sorbitol, J Agric Food Chem, 132 (2012) 1693. Food Chem, 47 (1999) 538. 79 Venir E, Flavors of Dried Apples, in Handbook of fruit and 61 Bhandari B R & Roos Y H, Dissolution of sucrose crystals in vegetable flavors, edited by Y H Hui (John Wiley & Sons the anhydrous sorbitol melt, Carbohydr Res, 338 (2003) 361. Inc, Hoboken, New Jersey) 2010, 515.