The Maillard Reaction and Its Control During Food Processing. the Potential of Emerging Technologies

Pathologie Biologie 58 (2010) 207–213

The Maillard reaction and its control during food processing. The potential of emerging technologies La re´action de Maillard et son controˆle pendant la fabrication des aliments. Le potentiel des nouvelles technologies

H. Jaeger *, A. Janositz, D. Knorr

Department of Food Biotechnology and Food Process Engineering, Berlin University of Technology, Koenigin-Luise-Str. 22, 14195 Berlin, Germany

ARTICLE INFO ABSTRACT

Article history: The Maillard reaction between reducing sugars and amino acids is a common reaction in foods which Received 13 July 2009 undergo thermal processing. Desired consequences like the formation of flavor and brown color of some Accepted 14 September 2009 cooked foods but also the destruction of essential amino acids and the production of anti-nutritive Available online 5 November 2009 compounds require the consideration of the Maillard reaction and relevant mechanisms for its control. This paper aims to exemplify the recent advances in food processing with regard to the controllability of Keywords: heat-induced changes in the food quality. Firstly, improved thermal technologies, such as ohmic heating, Maillard reaction which allows direct heating of the product and overcoming the heat transfer limitations of conventional Food industry thermal processing are presented in terms of their applicability to reduce the thermal exposure during Thermal unit operations Non-thermal pasteurisation food preservation. Secondly, non-thermal technologies such as high hydrostatic pressure and pulsed High hydrostatic pressure electric fields and their ability to extend the shelf life of food products without the application of heat, Pulsed electric fields thus also preserving the quality attributes of the food, will be discussed. Finally, an innovative method for the removal of Maillard reaction substrates in food raw materials by the application of pulsed electric field cell disintegration and extraction as well as enzymatic conversion is presented in order to demonstrate the potential of the combination of processes to control the occurrence of the Maillard reaction in food processing. ß 2009 Elsevier Masson SAS. All rights reserved. RE´ SUME´

Mots cle´s: La reáction de Maillard entre des sucres re´ducteurs et des acides amine´s a lieu dans les aliments lors d’un Reáction de Maillard traitement thermique. Des conse´quences de´sirables comme la formation de certains aroˆmes et de couleur L’industrie agro-alimentaire brune, mais aussi des conse´quences inde´sirables comme la destruction d’acides amine´s essentiels et la Proce´de´s de traitement thermique formation de substances non nutritives, nećessitent la prise en conside´rationde la reáction de Maillard et de Pasteurisation a` froid ses mećanismes. Ce travail scientifique a pour but d’expliquer les avance´srećentes dans le domaine des Haute pression Champs e´lectriques pulse´s technologies de fabrication d’aliments concernant en particulier des changements controˆlables de qualite´ lors d’un traitement thermique. Premie`rement, des ame´liorations de technologies de traitement thermique, comme le chauffage ohmique, permettent de re´duire le temps du traitement thermique du produit durant sa ste´rilisation. Cette technologie permet de chauffer le produit directement et arrive a` de´passer les limitations de transfert thermique des traitements conventionnels. Dans un second temps, des technologies non thermiques sont pre´senteés, comme par exemple l’utilisation de haute pression hydrostatique et le traitement par un champ e´lectrique pulse´. La capacite´ des ces technologies a` prolonger la dureé de vie de produits alimentaires sans utiliser de traitements thermiques et par conse´quent la pre´servation de certains attributs de qualite´ sera discute´s. Finalement, une me´thode innovatrice permettant d’e´liminer les substrats de la reáction de Maillard des matie`res premie`res par l’application d’un champ e´lectrique pulse´ qui provoque une de´sinte´gration de cellule, suivi par une extraction et une conversion enzymatique sera pre´senteé. Le potentiel de combiner diffe´rents proce´de´s afin de controˆler l’occurrence de la reáction de Maillard dans le domaine de la pre´servation alimentaire sera de´montre´. ß 2009 Elsevier Masson SAS. Tous droits re´serve´s.

* Corresponding author. Adresse e-mail : [email protected] (H. Jaeger).

1. Introduction Preservation of food can be achieved not only by thermal inactivation of microorganisms and enzymes but also by non- The Maillard reaction can be considered as one of the most thermal technologies that are based on alternative inactivation important chemical reaction taking place during food processing. mechanisms. High hydrostatic pressure treatment as well as Its influence on food quality attributes such as color, flavor and pulsed electric field processing can be considered as new non- nutritional value includes desired as well as unwanted effects and thermal preservation methods working at reduced treatment requires the consideration of processing conditions as well as temperature and therefore avoiding the occurrence of heat physico-chemical properties of the food material. A multitude of induced product changes. reaction products can be formed in the food matrix and attributed The Maillard reaction requires reducing sugars and amino- to characteristics such as antioxidative, antimicrobial, mutagenic compounds as reactants. The successful post-harvest removal of or cancerogenic [1–3]. these compounds from the food raw material is a promising Apart from Maillard products generated in food materials possibility to reduce the formation of Maillard products during during processing and storage, the Maillard reaction and glycosila- subsequent processing. Enzymatic conversion of amino-acids as tion are also occurring in-vivo with important pathological well as sugars have been proposed and pulsed electric field consequences for biological systems [4]. pretreatments to improve diffusion processes are promising The paper aims to discuss the impact of food manufacturing process combinations. processes on the formation of Maillard products focusing on With the goal to design fresh-like yet shelf stable products alternative thermal processing as well as recent applications of comes the need to optimise existing food processing technologies non-thermal technologies in food preservation. as well as to develop new concepts for gentle food preservation as mentioned above. 2. Maillard reaction during food processing 3. Improving thermal processing of food A series of chemical reactions between reducing sugars and amino compounds occurring during production and storage of Since the microbial inactivation effect of heat increases faster foods can be summarized as Maillard reaction. The main variables with increasing temperature than undesired chemical reactions, affecting the extent of the Maillard reaction are temperature and application of high temperatures for short treatment times (HTST- time which depend on processing conditions as well as pH, water processes) are favourable. In addition to that, a lethal effect on activity and type and availability of the reactants which are based microorganisms and bacterial spores in particular requires a on product properties but may be changed as a result of the critical temperature. The process design therefore aims to raise processing of food and raw materials [5]. temperature in a very short time to a critical level at which a Processes such as roasting, baking or frying rely on favorable certain holding time allows sufficient inactivation and apply rapid effects of the Maillard reaction such as color and flavor formation cooling afterwards. As shown in Fig. 1 methods applying indirect whereas during drying, pasteurization and sterilization the heat treatment like traditional pasteurisation using plate heat occurrence of the Maillard reaction is unfavorable. Nutritional exchangers require longer times for temperature increase and losses of essential amino acids that are involved in the reaction as subsequent cooling in comparison to processes based on direct well as the formation of reaction products are among those heat treatment such as direct steam injection [10] or ohmic heating unwanted effects [6,7]. [11] which result in an instantaneous and uniform temperature The main challenge is therefore the specific design, the increase. In case of direct steam injection, the thermal energy can optimization and the control of the above-mentioned processes also be removed quickly by vacuum-flash cooling whereas the for the production of food with the desired quality and stability [8]. cooling process still remains the limiting factor for the reduction of Since temperature and time present the most significant the thermal load when ohmic heating is applied. processing factors influencing the Maillard reaction, the reduction In the following section, ohmic heating is used to exemplify the of the thermal load to which a product is exposed during potential for a rapid heating of foods taking advantage of the processing is a key factor to control the extent of the reaction. specific potentials and opportunities of the technology and food The below-mentioned concepts point out possible approaches that properties due to the direct heating mechanism. will be further described in following sections. The occurrence of the Maillard reaction is desired to a certain extent and responsible for the formation of color and flavor. However, these quality attributes mainly occur on the surface of the food (crust of bread or meat after baking and roasting). A reduction of the total thermal process intensity by a controlled thermal treatment of the product surface for color and flavor development can be achieved by the application of emerging technologies such as infrared heating [9]. Thermal preservation processes and the inactivation of pathogenic and spoilage microorganisms and enzymes require a minimal treatment temperature and a corresponding holding time. Processing times for heating and cooling below a certain critical temperature do not contribute to inactivation but may lead to degradation of nutritionally valuable ingredients as well as to the formation of unwanted compounds. The reduction of processing times for heating to a final temperature and cooling is possible by improving the heat transfer and/or by applying alternative thermal technologies such as direct steam injection or ohmic heating with direct heating of the product thus avoiding temperature gradients Fig. 1. Comparison of the temperature–time profile of direct (e.g. steam injection) and heat transfer limitations. and indirect (e.g. plate heat exchanger) heat treatments. H. Jaeger et al. / Pathologie Biologie 58 (2010) 207–213 209

Ohmic heating uses the electrical resistance of foods to convert pressures from 300 to 600 MPa at ambient or refrigerated electricity to heat [12]. The heat is generated within the product temperature for 2 to 30 min and increasing interest in HP and the thermal conductivity of the food is no limiting factor. The sterilisation processes is also developing [22]. process can be used for UHT treatment of foods, especially Complex reaction effects like inactivation of enzymes or particulate and high viscous foods [13]. In comparison to microorganisms are altered in their reaction rates by pressure microwave or radio frequency heating, the penetration depth is as well as by temperature. Proteins are particularly affected by no limiting factor but direct contact between electrodes and the pressure treatments [23]. They may unfold and denature, food is required for ohmic heating. The food is heated rapidly and reversibly or irreversibly, depending on the kind of protein and evenly and heat transfer coefficients do not limit the rate of the intensity of the treatment but covalent chemical bonds are not heating. Therefore, heat sensitive components are not degraded affected during HP treatments resulting in minimal modifications since no localized over-heating occurs. Ohmic heating allows a in nutritional and sensory quality of foods. high temperature short time process application to solid/liquid Microbial inactivation by HP has been concluded to be the food mixtures with a high retention of nutrients and vitamins and result of a combination of factors. Modifications of the cell the reduction of other heat induced changes. Leizerson and membrane permeability and ion exchange capability, changes in Shimoni [14] studied the impact of ohmic heating on stability and cell morphology and biochemical reactions as well as protein sensory characteristics of orange juice and found a higher retention denaturation and inhibition of genetic mechanisms can be of flavor compounds due to the rapid and uniform heating process. considered as the relevant mechanisms [24,25]. Mc Kenna et al. [15] investigated the impact of radio frequency During traditional thermal treatment and high pressure and ohmic heating on meat quality. A shortening of cooking times processing desired reactions such as inactivation of pathogenic and the avoidance of quality losses in the outer regions of the microorganisms are the main target. At the same time, unwanted product which often occur as a result of a higher heat exposure due reactions which would lead to quality losses need be to taken into to the low rate of heat penetration during steam or hot water account. Such reactions have different temperature or pressure cooking could be obtained. Color changes as a result of non- dependencies and show different rate constants in comparison to enzymatic browning during hot water sterilization and ohmic the microbial inactivation [26]. An illustration of wanted and heating of pea puree have been compared by Icier et al. [16]. unwanted reactions and their pressure and temperature depen- Enzyme inactivation was found to occur at lower processing times dencies enables the p–T diagram (Fig. 2). As to be seen in Fig. 2, than conventional hot water sterilization and color changes as a inactivation at high hydrostatic pressure allows the reduction of result of non-enzymatic browning were less pronounced in the the treatment temperature resulting in a non-thermal pasteurisa- samples treated by ohmic heating. tion process although a synergism between temperature and Since the heat is generated inside the food and no pressure occurs. On the other hand, it is possible to decrease the temperature gradient is required for thermal conduction and applied pressure by increasing the process temperature but the convection, product contact with hot surfaces such as the plates occurrence of unwanted reactions has to be considered at the same of heat exchangers can be avoided and wall-overheating with time. protein and mineral fouling can be limited. This phenomena was Since chemical reactions are also influenced by pressure studied by Fillaudeau et al. [17] and a comparison of an UHT according to the principle of Le Chatelier, the Maillard reaction treatment of milk using ohmic heating and a conventional plate must be taken into account during high pressure processing of heat exchanger was conducted and processing parameters like foods [27]. flow characteristics in the treatment chamber were considered to be relevant for the prevention of deposits on the electrode surface during ohmic heating. The relevance of occurring deposits and protein fouling during ohmic heating and conventional heat exchangers was also studied by Avadi et al. [18] who reported similar results. Ohmic heating represents a promising alternative thermal method for the processing of particulate products where conventional heat transfer techniques require over-processing of the liquid phase to ensure the sufficient sterilization of each particulate. Rapid heating rates and uniformity of the temperature increase without limitation by conductive and convective heat transfer can be considered as the main advantages.

4. Application of non-thermal technologies

This chapter shows how two alternative non-thermal processing methods, high hydrostatic pressure (HP) and pulsed electric ﬁeld (PEF) treatment [19,20] contribute to the production of fresh- like and shelf stable foods minimizing the detrimental effect of traditional thermal processing.

4.1. High hydrostatic pressure (HHP)

The application of HHP processing has shown considerable potential as an alternative technology to heat treatments, in terms Fig. 2. p–T diagram for the illustration of process parameters during HP treatment. The points represent the same summarized effect of wanted reactions, e.g. microbial of assuring safety and quality attributes in minimally-processed inactivation. The dashed lines represent a similar summarized effect of unwanted food products [21]. High pressure pasteurization is currently the reactions. After increasing the dwell times from t1 to t2, the lines are shifted to the left. main application in industrial high pressure processing working at (Illustration according to V. Heinz, DIL, 2008, personal communication) 210 H. Jaeger et al. / Pathologie Biologie 58 (2010) 207–213

The influence of high hydrostatic pressure up to 600 MPa on the Maillard reaction was studied in model systems containing amino acids or ß-caein and sugars by Schwarzenbolz et al. [28]. The formation of the amino acid derivate pentosidine was found to be increased by increasing the pressure whereas the formation of pyralline was reduced. Other studies found the acceleration of early Maillard reaction pathways with pressure, e.g. reaction products formed from tryptophan and glucose or xylose, and the slowdown of subsequent reaction steps [29]. High-pressure effects on the Maillard reaction between glucose and lysine were investigated by Moreno et al. [30] and the pressure-induced changes in pH were found to strongly influence the HP effects of different stages of the Maillard reaction. The formation and subsequent degradation of Amadori rearrangement products was accelerated by HP (400 MPa, 60 8C) and resulted in increased levels of intermediate and advanced reaction products. Similar results have been reported by Hill et al. [31]. Fig. 3. HMF content of unprocessed, heat-treated (90 8C for 60 s) and PEF treated The impact of HP processing on color, texture and flavor of fruit- (35 kV/cm, treatment time 1000 ms, frequency 150 Hz, monopolar pulses, pulse width 4 ms) juices according to Aguilo´ -Aguayo et al. [45]. and vegetable-based food products was reviewed by Oey et al. [32]. Rada-Mendoza et al. [33] studied the impact of different denaturation conditions on the susceptibility of proteins to the undesirable brown colour development as a result of the Maillard Maillard reaction and the effect of high pressure on lactosylation of reaction between amino and carbonyl compounds and the ß-lactoglobulin was found to be lower than the one of the applied subsequent formation of 5-hydroxymethylfurfural (HMF) occurs. thermal treatments. HMF can be used as an indicator for the freshness and quality of On the other hand, the study of Campus et al. [34] investigated juices since HMF is almost absent in fresh and untreated juices but the effects of high-pressure treatment on chemical characteristics the concentration is increased after heat-treatment or long-term of dry cured loin where a reduction of several flavour compounds storage. Aguilo´ -Aguayo et al. [45] investigated the non-enzymatic deriving from Maillard reactions was observed in comparison to browning after PEF pasteurisation of fruit and vegetable juices. the untreated sample. This finding again underlines the fact, that Fig. 3 shows the HMF contents in strawberry, tomato and on the one hand, non-thermal processing retains fresh-like watermelon juice after thermal and PEF treatment. The PEF characteristics but that the reduction of the Maillard reaction processed juices had a lower HMF concentration than those treated may also lead to a decrease of the formation of typical color and by heat, a fact that can be attributed to the reduced thermal load to flavor characteristics. which the product is exposed during PEF preservation. However, Nienaber [35] investigated the stabilization of fresh orange juice pulse frequency, pulse width and polarity of the pulse were found by HP treatment and conducted a shelf life study showing the to have a significant influence on HMF content in strawberry and potential of HP processing to produce a shelf-stable juice with fresh- tomato juice whereas in watermelon juice changes in HMF like quality for several months when stored under refrigeration. concentration were minor by applying PEF treatments. HP used as a food preservation method is able to reduce The effect of PEF on physicochemical characteristics of citrus deteriorative effects on food quality characteristics occurring juices was investigated by Cserhalmi et al. [46]. Non enzymatic during conventional processing. However, the effect of HP in browning index (NEBI) and hydroxymethyl furfural content did combination with elevated temperatures and occurring shift of pH not change due to the PEF treatments and volatile aroma during treatments with the resulting impact on Maillard reaction compounds have been retained. Table 1 summarizes the values pathways requires consideration. obtained by the authors for different citrus juices. The impact of PEF preservation on color, browning and 4.2. Pulsed electric fields (PEF) hydroxymethylfurfural during storage of orange juice was investigated by Corte´s et al. [47] and compared to conventional The application of pulsed electric field treatment of foods is pasteurisation. The non-thermal treated orange juice showed less based on the permeabilization of biological membranes. PEF non-enzymatic browning than the pasteurised one after a storage processing involves the application of short pulses (in the range of period of 6 weeks as measured photometrically by the browning ms to ms) of high electric fields. The result of PEF treatment is the index. The HMF content directly after treatment was higher in the disintegration of the cell membrane consisting of a bilayer of pasteurised juice than in the PEF treated one but differences were phospholipids. Depending on treatment intensity, the generated found to diminish during storage. membrane pores can be permanent or temporary. Microbial inactivation coupled with quality retention during In the case of irreversible electroporation the semipermeable non-thermal PEF treatment of liquid foods makes it an appropriate character of the membrane becomes permanently destroyed which results in cell death and can be used for microbial inactivation and the non-thermal pasteurization of liquid foods [36,37]. Table 1 Effective inactivation for most of the spoilage and pathogenic Non enzymatic browning index (NEBI) and hydroxymethyl furfural (HMF) content (standard deviation in brackets) in unprocessed and PEF-treated (28 kV/cm; microorganisms has been shown and the potential to achieve treatment time 100 ms) citrus juices according to Cserhalmi et al. [46]. sufficient reduction of microbes in various food products like fruit or vegetable juices [38–41], model beer [42] or milk [43,44] has Unprocessed juice PEF processed juice been investigated. NEBI HMF (mg/l) NEBI HMF (mg/l) In order to avoid detrimental changes in sensory and nutritive Grapefruit 0.105 (0.0016) 0.49 (0.02) 0.106 (0.0013) 0.49 (0.02) properties pulsed electric field pasteurisation of fruit juices is a Lemon 0.0884 (0.0045) 0.19 (0.05) 0.0957 (0.0034) 0.25 (0.04) promising preservation method. Although conventional heat Orange 0.1091 (0.0026) 0.25 (0.08) 0.1094 (0.0018) 0.22 (0.03) treatments ensure safety and extend the shelf life of juices, Tangerine 0.1130 (0.0011) 0.17 (0.03) 0.1140 (0.0007) 0.18 (0.06) H. Jaeger et al. / Pathologie Biologie 58 (2010) 207–213 211 process to fulfill the consumer demands for high quality, minimally processed but safe foods with extended shelf life.

5. Removal of substrate

Among the harmful Maillard reaction compounds, acrylamid received great attention in the recent years. It is spontaneously formed during heat treatment such as cooking and frying of foods rich in reducing sugars and amino-acids, mainly L-asparagine, as part of the Maillard reaction [48]. Lowering the content of these substrates and the related Maillard reaction products can be either achieved by means of selecting appropriate raw materials or technological process parameters [49]. For the reduction of the acrylamid content, the application of L-asparaginase as well as the addition of glycine as a competitor for the precursor asparagines was suggested by Vass et al. and Capuano et al. [50,51] for bakery Fig. 5. Reduction of glucose in potato slices as a result of treatment with glucose and cracker products and by Zyzak et al. [52] for potato products. oxidase and PEF-pretreatment (1.5 kV/cm, 20 pulses) to enhance enzyme diffusion Whereas the addition of asparaginase and other additives to basic in the potato tissue. formulations of bakery products and subsequent blending allows sufficient dispersion of the components, the application of the above-mentioned strategies for textured raw materials like potato non PEF pretreated samples. A 50% increase in glucose reduction in slices or chips and the sufficient diffusion of the components into comparison to the untreated potato could be shown for the PEF the tissue remains limited. pretreated sample. Therefore, PEF technology could be applied as a new method for Pore formation in the cell membranes of the potato tissue and cell disintegration. The occurring increase in membrane permea- disintegration lead to enhanced mass transfer during blanching and bilization by exposure of biological cells to an external electric field an increased release of sugars. The cell disintegration index (percent positively affects the mass transfer rate. The consequence is that of electroporated cells) was determined by impedance measure- the diffusion of intracellular components in extracellular liquid is ment [57] and amounted to 36% after the PEF pretreatment. increased, while leaving the product matrix relatively unchanged In addition to the release of sugar and the removal of substrate [53–55]. On the other hand, infusion of molecules into the food by diffusion, the impact of PEF cell disintegration on the infusion of matrix can be facilitated as well [56]. Post-harvest PEF pretreat- glucoseoxidase was investigated in order to obtain a higher ment of food raw materials is therefore considered to be a method penetration rate and a better distribution of the enzyme in the able to facilitate mass transfer processes and to assist in removing potato tissue to allow a high glucose oxidation rate. The same sugars or amino acids that represent the necessary substrates for principles apply for the utilization of asparaginase. the Maillard reaction. Fig. 5 shows the enzymatic decrease of the glucose content in Own experiments using PEF for the disintegration of potato the potato slices. A reduction of 52% was achieved by the tissue prior to further processing to French fries showed the application of glucoseoxidase without prior cell disintegration potential of removing substrates by two ways: whereas the decrease could be enhanced by the PEF treatment to 65%. The electroporation proved to be an effective method to increased release of sugars during blanching; improve the diffusion of the enzyme as well as the enzyme increased infusion of enzymes (glucose oxidase or asparaginase) substrate accessibility due to the cell disintegration. for enzymatic conversion of the substrates. In addition to the application of PEF induced cell disintegration for subsequent removal of sugars or amino acids, the electropora- Fig. 4 shows the reduction of the sugar content of potato slices tion pretreatment and the enhanced diffusion properties can also achieved after PEF pretreatment and blanching in comparison to be used to accelerate thermal processes like drying or frying of plant raw materials [58]. Reduced thermal processing times in turn contribute to the avoidance of heat induced quality losses.

6. Conclusion

Non-uniform heating and the occurrence of over-processing during heat treatments of foods can be reduced by the application of improved thermal processing methods like ohmic heating which allows a shortening of processing times and improvement in product quality by avoiding the limitations of heat transfer occurring during indirect heating processes. Nevertheless, thermal processing for preservation or the modiﬁcation of food structure still relies on the effects of the application of thermal energy for microbial and enzymatic inactivation as well as the alteration of the properties of food ingredients. The application of non-thermal technologies for ‘‘cold pasteurization’’ of foods as well as for structural modiﬁcations allows to overcome the necessity of heating the food matrix and therefore Fig. 4. Difference in sugar content in potato slices after PEF treatment (1.5 kV/cm and 20 pulses) and hot water blanching (70 8C for 90 s) in comparison to not PEF reduces heat induced changes in product quality including treated samples. nutritional and sensorial properties. 212 H. Jaeger et al. / Pathologie Biologie 58 (2010) 207–213

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