chemical changes in proteins produced by thermal processing

7/28/2019 Chemical Changes in Proteins Produced by Thermal Processing

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:cal Changes in Proteinsby ThermalProcessingR. Dutsonof Food Scence and Human Nutrton, Mchgan State Unversty. East Lansing. MI 48824W. Orcuttof Anmal Science, TexasA&M University, COllege Staton. TX 77843

Variousprocessing procedures are applied to food systemsquality improvement, and consumer con

These procedures generally provide food that isnutritious, economical, safe, and , convenient.PTOcessing''S used by much of the food processing

to impart these attributes to its products, and manyproceduies including cooking, blanching, pasteuriion, and sterilization (1) are employed. Thermal processingf' both beneficial and detrimental effects on food systems;the food scientist attempts to optimize the benefi! effects and to minimize the negative effects of heat pro-

(J , 2). _\Heat is alsoone oftlte primary factors in the enhancementp'1latability of most foods, with many of the beneficial

changesrelated to food proteins. These changesflavor and structural components; as most onsumersa3r ee, properly cooked food products are generally morethan those in the uncooked stage or those which

been overcooked. Other factors which must be consid, in addition to those of preservation and palatability, arealterations in nutritional properties of proteins caused byEndogenous -enzymes found in many food systemsproduce 'either beneficial or de leterious effects on com

of a particular food system or on thll type ofproductsted by a part icular enzyme. Heating of foods generallythese enzyines and pre-vents thei r action upon foodnents. \lfecls :>f Thermal Processing on Proteins

MaillardReactionOne of the predominant reactions which oceurs uponheating af protein foods is the Maillard reaction (non-enzymatic browning) which oceurs between free amine andear

bonyl groups within foed systems. Although most free aminesand earbonyl groups have the capacity to -partieipate inMaillard -t.ype reaet.ions, the major food substanees participating in t.hese reactions are the carbonyl groups of reducingsugars ana free amino groups of proteins and other components (3). Although the Maillard reaetion ean occur at ambientand loweitemperatures, inere8sed temperatures have amarked effect upon its activation. Recent reviews fully de-

o, 11C -H + H l N - R . ..I

H-C-OHIsugar amine

R--r--r!!8H1H-C-OH

Schiff hase

scribe characteristics and properties of the Maillard reaetion(3-5); therefore, only the salient features 01' this reaction willbe presented here.

Figure 1 shows the fiTst stnge o tho Maillard !@l:IcLiuJI,which involves the production of a Sehiff base with the earbonyl and amine groups. The Schiff base produeed by thereaetion sequenee in Figure 1 normally undergoes an Amadorirearrangement to produce a ketoseamine. Both the Schiff baseand the Amadori compound then undergo further reactionsto produce a wide viuiety of reaction prodticts which eventually produce the melanoidins responsible for the brown color.A group of these reaction produets is presented in Figure 2.The Amadori eompound arid its products have the capabilityof participating in other Schiff base and aldol condensationreaetions, producing a myriad of products which are difficultto char.acterize in food systems (3,5). Much is known aboutthe chemistry of Maillard react.ion, however, its complexityin foad systems in such that only a fraction of the total numberof reactions is understood. In fact, many of the reaetionproducts resulting from Amadori compo\.lDos can causecross-linking between molecules, resulting in the formationof numerous polymers (5). ,

Although there are many beneficial effects ofthe Maillardreaction in foods, sueh as improved flavor (3,5), the 7-::a(;,;ioncan also produce detrimenL'II effects in foods, primarily L:0 1P.a nutritional st.anclpoin t; hOWeVp.T, j f the Maillard l oaction illfoods is extremely severe, detrimentall1avors \ViII also ensu.One of the possible adverse effects of the Maillard reacticDis the reduction of protein nutritional quality through lossesin availability of specific amino acids, particularly lysine, (Jrthrough development of toxins or mutagens (8).Satterlee and Chang (8) have described the nutritionalchanges induced by the Maillard reaction as follows: (1) decrease in the bioavai labili ty of lysine, as well as several otheressential amino aeids; (2) decrease in protein digestibility; and(3) possible formation of growth inhibitory and/or toxiccompounds. Figure 3 appears to eorroborate these statemeritsby demonstrating a reduction in the growth rate of rats fed a ,browned versus control diet (5). On the other hand, accordingto Hegarty (lO), if dietary intake of protein is high and ifsufficient quantities of the essential aminu acids are consumed, the reduction ii1 protein quality due to Maillard-type

R-NHICH1I

COH1--- R-NHICH,1

C=O; 1l-amino1deoxy2-ketose

enol formIr keto form1Figure 1, Proposed type reactlons Invo!ved In Initlatlon 01 nonenzymatlc brownlng. Adapiee from Paul (6) with permlsslon (John Wiley & Sonso Inc,).


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--,..1 N-subsll luled g l y c o s ~ . I a m i l l e alduhl'xosc + amine

i\H e - N/

1I ""'C --OI ITilO1_r l lOIl (J) .Ilr=N::-_. e - o l lICH1 1l0l! Il C = OIl " ' - 'O(-IH:Ci l ! ( ) H, 111 0 5-hydruxyrncthyl. . /7 ,.,0 2-furalduhydcc=oH 11 ~ ."::,,.'.'"""


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Pr o pertySubunit

ViscosityHydrntionGclution(following

heating)

I-! C/lLlIlg Lplllpe rl l Lur e (OC ); 00 120 1'10 H in..._._--- - __-__------_._.----- - - _ . _ - -- - - ------\- --dis80ciation -unioldinr. _ _._ __ _ _ _____de!(r:ldlll.iondccrease-precipitati (ln _ ___ _____ _ in crease in solllbilil.y

increa1


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Table 2. o-Aspartlc Acld (o-Asp) Con tent of Commerclal Foodsand Food Ingredlents '

Product D-ASpRo-Asp/L-Asp o-Asp + L-AspTexturized soy proteln 0.095 0.09Baby formula (soy-based) 0.108 0.10Simulated bacon (soy: based) 0.143 0.13Corn chips 0.164 0.14Dalry (caseln-based) 0.208 0.17

Relatlve ,atlos " , total aspartlc ~ c l d aro its o- aro L-


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(34,35). The toughening of th e second phase (hc65 ond 75C ) may a lso be affec te d by los8 uf juice on(\form ation ofnew cross -linkages in th e coagulated rn yofi.proLeins (22). The decrease in shear force (improved

which occurs abovc 80 0 e may be associatcd withdegradat ion o f cross-l ink s wiibin th e myofibrill a r prolcins

evidenced by lh e production 01' hydrogcl1 suICide from d igroup s in this temperature range (22) .

Eggs have be en used in many food products oecnuse 01' lh cpropcrtles of holh egg white a nel yolk prol.einsSorne of th e more familiar food procluds \yhich, inmake us e of these functional properties. incorporateg proteins ar e : cllkes, mcrin,;ues, souf!1cs, clIt:;tards, etc.

The beneficial functional properlies ()f egg prol.eins me cluetheir dcnllluration ami coagulal.ioJ1 aL s llt' cil'ic l e l l l p ( ! r H t . u r e

formation of u st.uble malrix upon co ugu lulioll (111).majority of egg proLeins coagulul.e 1H'!WCl'll l; tl :1/ 1.1 ''([ , 0 ( '-39); however, lh e conglllalioll lellljJt'r,1i.1Jre is ;lill,clcd hyj an d salL.collcelltraLiol1. as would 1.1' cXi,, ttcd :, ill(,(' s:lil. .1I1t1a lLer Lhe hydrogc n bonding a d . " r ~ L i , , { t ) r p r"Lei " ".Hydroge n s ulfid e. produ cc d fr olll bul h sult11ydry l {nd di

groups wiLhin t h e p r o L e j n ~ (40), ~ g i n s to be re l " ",(,de{1i products ar e heaLed Lo tcm[Jc raLures el< cce clingThe green color [ormed ()J1 th e surface of th" yo lk 01'

eggs is a resuit o( hydro gl,n u l f i d li ber/l Lionom albumin and it s interact.i o n wiLh iroll i'rolll l he yolk (

chemical changes in proteins produced by thermal processing

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