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,Koeni
gin-Luise-Str.22,14195Berlin,Germany
Pathologie Biologie58(2010)207–213
A R T I C L E I N F O
Article history:
Received13July2009
Accepted14September2009 Available online5November2009
Keywords:
Maillard reaction
Food industry
Thermal unit operations
Non-thermal pasteurisation
High hydrostatic pressure
Pulsed electricfields
Mots cle´s:
Re´action de Maillard
L’industrie agro-alimentaire Proce´de´s de traitement thermique Pasteurisation a`froid
Haute pression
Champs e´lectriques pulse´s A B S T R A C T
The Maillard reaction between reducing sugars and amino acids is a common reaction in foods which undergo thermal processing.Desired consequences like the formation offlavor and brown color of some cooked foods but also the destruction of essential amino acids and the production of anti-nutritive 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 heat-induced changes in the food quality.Firstly,improved thermal technologies,such a
s ohmic heating, which allows direct heating of the product and overcoming the heat transfer limitations of conventional thermal processing are presented in terms of their applicability to reduce the thermal exposure during food preservation.Secondly,non-thermal technologies such as high hydrostatic pressure and pulsed electricfields and their ability to extend the shelf life of food products without the application of heat, 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.
ß2009Elsevier Masson SAS.All rights reserved. R E´S U M E´
La re´action de Maillard entre des sucres re´ducteurs et des acides amine´s a lieu dans les aliments lors d’un traitement thermique.Des conse´quences de´sirables comme la formation de certains aroˆmes et de couleur brune,mais aussi des conse´quences inde´sirables comme la destruction d’acides amine´s essentiels et la formation de substances non nutritives,ne´cessitent la prise en conside´ration de la re´action de Maillard et de ses me´canismes.Ce travail scientifique a pour but d’expliquer les avance´s re´centes dans le domaine des technologies de fabrication d’aliments concer
nant 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´sente´es,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 dure´e 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 re´action 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´sente´e.Le potentiel de combiner diffe´rents proce´de´s afin de controˆler l’occurrence de la re´action de Maillard dans le domaine de la pre´servation alimentaire sera de´montre´.
ß2009Elsevier Masson SAS.Tous droits re´serve´s.
*Corresponding author.
Adresse e-mail:henry.jaeger@tu-berlin.de(H.Jaeger).
0369-8114/$–see front matterß2009Elsevier Masson SAS.All rights reserved. doi:10.1016/j.patbio.2009.09.016
1.Introduction
The Maillard reaction can be considered as one of the most important chemical reaction taking place during food processing.Its influence on food quality attributes such as color,flavor and nutritional value includes desired as well as unwanted effects and requires the consideration of processing conditions as well as physico-chemical properties of the food material.A multitude of reaction products can be formed in the food matrix and attributed to characteristics such as antioxidative,antimicrobial,mutagenic or cancerogenic [1–3].
Apart from Maillard products generated in food materials during processing and storage,the Maillard reaction and glycosila-tion are also occurring in-vivo with important pathological consequences for biological systems [4].
The paper aims to discuss the impact of food manufacturing processes on the formation of Maillard pr
oducts focusing on alternative thermal processing as well as recent applications of non-thermal technologies in food preservation.2.Maillard reaction during food processing
A series of chemical reactions between reducing sugars and amino compounds occurring during production and storage of foods can be summarized as Maillard reaction.The main variables affecting the extent of the Maillard reaction are temperature and time which depend on processing conditions as well as pH,water activity and type and availability of the reactants which are based on product properties but may be changed as a result of the processing of food and raw materials [5].
Processes such as roasting,baking or frying rely on favorable
effects of the Maillard reaction such as color and flavor formation whereas during drying,pasteurization and sterilization the occurrence of the Maillard reaction is unfavorable.Nutritional losses of essential amino acids that are involved in the reaction as well as the formation of reaction products are among those unwanted effects [6,7].
The main challenge is therefore the specific design,the optimization and the control of the above-mentioned processes for the production of food with the desired quality and stability [8].
Since temperature and time present the most significant processing factors influencing the Maillard reaction,the reduction of the thermal load to which a product is exposed during processing is a key factor to control the extent of the reaction.The below-mentioned concepts point out possible approaches that 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 and heat transfer limitations.
Preservation of food can be achieved not only by thermal inactivation of microorganisms and enzymes but also by non-thermal technologies that are based on alternative inactivation mechanisms.High hydrostatic pressure treatment as well as pulsed electric field processing can be considered as new no
n-thermal preservation methods working at reduced treatment temperature and therefore avoiding the occurrence of heat induced product changes.
The Maillard reaction requires reducing sugars and amino-compounds as reactants.The successful post-harvest removal of these compounds from the food raw material is a promising possibility to reduce the formation of Maillard products during subsequent processing.Enzymatic conversion of amino-acids as well as sugars have been proposed and pulsed electric field pretreatments to improve diffusion processes are promising process combinations.
With the goal to design fresh-like yet shelf stable products comes the need to optimise existing food processing technologies as well as to develop new concepts for gentle food preservation as mentioned above.
3.Improving thermal processing of food
Since the microbial inactivation effect of heat increases faster with increasing temperature than undesired chemical reactions,application of high temperatures for short treatment times (HTST-processes)are favourable.In addition to that,a lethal effect on microorganisms and bacterial spores in particular requires a critical temperature.The process design therefore aims to raise temperature in a v
ery short time to a critical level at which a certain holding time allows sufficient inactivation and apply rapid cooling afterwards.As shown in Fig.1methods applying indirect heat treatment like traditional pasteurisation using plate heat exchangers require longer times for temperature increase and subsequent cooling in comparison to processes based on direct heat treatment such as direct steam injection [10]or ohmic heating [11]which result in an instantaneous and uniform temperature increase.In case of direct steam injection,the thermal energy can also be removed quickly by vacuum-flash cooling whereas the cooling process still remains the limiting factor for the reduction of the thermal load when ohmic heating is applied.
In the following section,ohmic heating is used to exemplify the potential for a rapid heating of foods taking advantage of the specific potentials and opportunities of the technology and food properties due to the direct heating mechanism.
Fig.1.Comparison of the temperature–time profile of direct (e.g.steam injection)and indirect (e.g.plate heat exchanger)heat treatments.
H.Jaeger et al./Pathologie Biologie 58(2010)207–213
208
Ohmic heating uses the electrical resistance of foods to convert electricity to heat[12].The heat is generated within the product and the thermal conductivity of the food is no limiting factor.The process can be used for UHT treatment of foods,especially particulate and high viscous foods[13].In comparison to microwave or radio frequency heating,the penetration depth is no limiting factor but direct contact between electrodes and the food is required for ohmic heating.The food is heated rapidly and evenly and heat transfer coefficients do not limit the rate of heating.Therefore,heat sensitive components are not degraded since no localized over-heating occurs.Ohmic heating allows a high temperature short time process application to solid/liquid food mixtures with a high retention of nutrients and vitamins and the reduction of other heat induced changes.Leizerson and Shimoni[14]studied the impact of ohmic heating on stability and sensory characteristics of orange juice and found a higher retention offlavor compounds due to the rapid and uniform heating process.
Mc Kenna et al.[15]investigated the impact of radio frequency and ohmic heating on meat quality.A shortening of cooking times and the avoidance of quality losses in the outer regions of the product which often occur as a result of a higher heat exposure due to the low rate of heat penetration during steam or hot water cooking could be obtained.Color changes as a result of non-enzymatic browning during hot water sterilization and ohmic heating of pea puree have been compared by Icier et al.[16]. E
nzyme inactivation was found to occur at lower processing times than conventional hot water sterilization and color changes as a result of non-enzymatic browning were less pronounced in the samples treated by ohmic heating.
Since the heat is generated inside the food and no temperature gradient is required for thermal conduction and convection,product contact with hot surfaces such as the plates of heat exchangers can be avoided and wall-overheating with protein and mineral fouling can be limited.This phenomena was studied by Fillaudeau et al.[17]and a comparison of an UHT treatment of milk using ohmic heating and a conventional plate heat exchanger was conducted and processing parameters like 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 conven-tional 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 conven-tional 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 proces-sing 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 of assuring safety and quality attributes in minimally-processed food products[21].High pressure pasteurization is currently the main application in industrial high pressure processing working at pressures from300to600MPa at ambient or refrigerated temperature for2to30min and increasing interest in HP sterilisation processes is also developing[22].
Complex reaction effects like inactivation of enzymes or microorganisms are altered in their reaction rates by pressure as well as by temperature.Proteins are particularly affected by pressure treatments[23].They may unfold and denature, reversibly or irreversibly,depending on the kind of protei
n and the intensity of the treatment but covalent chemical bonds are not affected during HP treatments resulting in minimal modifications in nutritional and sensory quality of foods.
reaction to a book or an articleMicrobial inactivation by HP has been concluded to be the result of a combination of factors.Modifications of the cell membrane permeability and ion exchange capability,changes in cell morphology and biochemical reactions as well as protein denaturation and inhibition of genetic mechanisms can be considered as the relevant mechanisms[24,25].
During traditional thermal treatment and high pressure processing desired reactions such as inactivation of pathogenic microorganisms are the main target.At the same time,unwanted reactions which would lead to quality losses need be to taken into account.Such reactions have different temperature or pressure dependencies and show different rate constants in comparison to the microbial inactivation[26].An illustration of wanted and unwanted reactions and their pressure and temperature depen-dencies enables the p–T diagram(Fig.2).As to be seen in Fig.2, inactivation at high hydrostatic pressure allows the reduction of the treatment temperature resulting in a non-thermal pasteurisa-tion process although a synergism between temperature and pressure occurs.On the other hand,it is possible to decrease the applied pressure by increasing the process temperature but the occurrence of unwanted reactions has to be considered at the same time.
Since chemical reactions are also influenced by pressure according to the principle of Le Chatelier,the Maillard reaction must be taken into account during high pressure processing of foods[27]
.
Fig.2.p–T diagram for the illustration of process parameters during HP treatment. The points represent
the same summarized effect of wanted icrobial inactivation.The dashed lines represent a similar summarized effect of unwanted reactions.After increasing the dwell times from t1to t2,the lines are shifted to the left. (Illustration according to V.Heinz,DIL,2008,personal communication)
H.Jaeger et al./Pathologie Biologie58(2010)207–213209
The influence of high hydrostatic pressure up to600MPa on the Maillard reaction was studied in model systems containing amino acids orß-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, action 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(400MPa,608C)and resulted in increased levels of intermediate and advanced reaction products. Similar results have been reported by Hill et al.[31].
The impact of HP processing on color,texture andflavor of fruit-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 Maillard reaction and the effect of high pressure on lactosylation of ß-lactoglobulin was found to be lower than the one of the applied thermal treatments.
On the other hand,the study of Campus et al.[34]investigated the effects of high-pressure treatment on chemical characteristics of dry cured loin where a reduction of severalflavour compounds deriving from Maillard reactions was observed in comparison to the untreated sample.Thisfinding again underlines the fact,that on the one hand,non-thermal processing retains fresh-like characteristics but that the reduction of the Maillard reaction may also lead to a decrease of the formation of typical color and flavor characteristics.
Nienaber[35]investigated the stabilization of fresh orange juice by HP treatment and conducted a shelf life study showing the potential of HP processing to produce a shelf-stable juice with fresh-like quality for several months when stored under refrigeration.
HP used as a food preservation method is able to reduce deteriorative effects on food quality characteristics occurring during conventional processing.However,the effect of HP in combination with elevated temperatures and occurring shift of pH during treatments with the resulting impact on Maillard reaction pathways requires consideration.
4.2.Pulsed electricfields(PEF)
The application of pulsed electricfield treatment of foods is based on the permeabilization of biological membranes.PEF processing involves the application of short pulses(in the range of m s to ms)of high electricfields.The result of PEF treatment is the disintegration of the cell membrane consisting of a bilayer of phospholipids.Depending on treatment intensity,the generated membrane pores can be permanent or temporary.
In the case of irreversible electroporation the semipermeable 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].
Effective inactivation for most of the spoilage and pathogenic microorganisms has been shown and the potential to achieve sufficient reduction of microbes in various food products like fruit or vegetable juice
s[38–41],model beer[42]or milk[43,44]has been investigated.
In order to avoid detrimental changes in sensory and nutritive properties pulsed electricfield pasteurisation of fruit juices is a promising preservation method.Although conventional heat treatments ensure safety and extend the shelf life of juices,undesirable brown colour development as a result of the Maillard reaction between amino and carbonyl compounds and the subsequent formation of5-hydroxymethylfurfural(HMF)occurs. HMF can be used as an indicator for the freshness and quality of juices since HMF is almost absent in fresh and untreated juices but the concentration is increased after heat-treatment or long-term storage.Aguilo´-Aguayo et al.[45]investigated the non-enzymatic browning after PEF pasteurisation of fruit and vegetable juices. Fig.3shows the HMF contents in strawberry,tomato and watermelon juice after thermal and PEF treatment.The PEF processed juices had a lower HMF concentration than those treated by heat,a fact that can be attributed to the reduced thermal load to which the product is exposed during PEF preservation.However, pulse frequency,pulse width and polarity of the pulse were found to have a significant influence on HMF content in strawberry and tomato juice whereas in watermelon juice changes in HMF concentration were minor by applying PEF treatments.
The effect of PEF on physicochemical characteristics of citrus juices was investigated by Cserhalmi et
al.[46].Non enzymatic browning index(NEBI)and hydroxymethyl furfural content did not change due to the PEF treatments and volatile aroma compounds have been retained.Table1summarizes the values obtained by the authors for different citrus juices.
The impact of PEF preservation on color,browning and hydroxymethylfurfural during storage of orange juice was investigated by Corte´s et al.[47]and compared to conventional pasteurisation.The non-thermal treated orange juice showed less non-enzymatic browning than the pasteurised one after a storage period of6weeks as measured photometrically by the browning index.The HMF content directly after treatment was higher in the pasteurised juice than in the PEF treated one but differences were found to diminish during storage.
Microbial inactivation coupled with quality retention during non-thermal PEF treatment of liquid foods makes it an
appropriate Fig.3.HMF content of unprocessed,heat-treated(908C for60s)and PEF treated (35kV/cm,treatment time1000m s,frequency150Hz,monopolar pulses,pulse width4m s)juices according to Aguilo´-Aguayo et al.[45].
Table1
Non enzymatic browning index(NEBI)and hydroxymethyl furfural(HMF)content (standard deviation in brackets)in unprocessed and PEF-treated(28kV/cm; treatment time100m s)citrus juices according to Cserhalmi et al.[46].
Unprocessed juice PEF processed juice
NEBI HMF(mg/l)NEBI HMF(mg/l) Grapefruit0.105(0.0016)0.49(0.02)0.106(0.0013)0.49(0.02) Lemon0.0884(0.0045)0.19(0.05)0.0957(0.0034)0.25(0.04) Orange0.1091(0.0026)0.25(0.08)0.1094(0.0018)0.22(0.03) Tangerine0.1130(0.0011)0.17(0.03)0.1140(0.0007)0.18(0.06)
H.Jaeger et al./Pathologie Biologie58(2010)207–213 210
process to fulfill the consumer demands for high quality,minimally processed but safe foods with exten
ded 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 and cracker products and by Zyzak et al.[52]for potato products.Whereas the addition of asparaginase and other additives to basic 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 slices or chips and the sufficient diffusion of the components into the tissue remains limited.
Therefore,PEF technology could be applied as a new method for cell disintegration.The occurring increase in membrane permea-bilization by exposure of biological cells to an external electric field positively affects the mass transfer rate.The consequence is that the diffusion of intracellular compone
nts in extracellular liquid is increased,while leaving the product matrix relatively unchanged [53–55].On the other hand,infusion of molecules into the food matrix can be facilitated as well [56].Post-harvest PEF pretreat-ment of food raw materials is therefore considered to be a method able to facilitate mass transfer processes and to assist in removing sugars or amino acids that represent the necessary substrates for the Maillard reaction.
Own experiments using PEF for the disintegration of potato tissue prior to further processing to French fries showed the potential of removing substrates by two ways:
increased release of sugars during blanching;
increased infusion of enzymes (glucose oxidase or asparaginase)for enzymatic conversion of the substrates.
Fig.4shows the reduction of the sugar content of potato slices achieved after PEF pretreatment and blanching in comparison to
non PEF pretreated samples.A 50%increase in glucose reduction in comparison to the untreated potato could be shown for the PEF pretreated sample.
Pore formation in the cell membranes of the potato tissue and disintegration lead to enhanced mass transfer during blanching and an increased release of sugars.The cell disintegration index (percent of electroporated cells)was determined by impedance measure-ment [57]and amounted to 36%after the PEF pretreatment.
In addition to the release of sugar and the removal of substrate by diffusion,the impact of PEF cell disintegration on the infusion of glucoseoxidase was investigated in order to obtain a higher penetration rate and a better distribution of the enzyme in the potato tissue to allow a high glucose oxidation rate.The same principles apply for the utilization of asparaginase.
Fig.5shows the enzymatic decrease of the glucose content in the potato slices.A reduction of 52%was achieved by the application of glucoseoxidase without prior cell disintegration whereas the decrease could be enhanced by the PEF treatment to 65%.The electroporation proved to be an effective method to improve the diffusion of the enzyme as well as the enzyme substrate accessibility due to the cell disintegration.
In addition to the application of PEF induced cell disintegration for subsequent removal of sugars or amino acids,the electropora-tion pretreatment and the enhanced diffusion properties can also 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 pasteur-ization’’of foods as well as for structural modifications allows to overcome the necessity of heating the food matrix and therefore reduces heat induced changes in product quality including nutritional and sensorial
properties.
Fig.4.Difference in sugar content in potato slices after PEF treatment (1.5kV/cm and 20pulses)and hot water blanching (708C for 90s)in comparison to not PEF treated
samples.
Fig.5.Reduction of glucose in potato slices as a result of treatment with glucose oxidase and PEF-pretreatment (1.5kV/cm,20pulses)to enhance enzyme diffusion in the potato tissue.
H.Jaeger et al./Pathologie Biologie 58(2010)207–213211
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