Browning Reactions reading this chopter, you should be able to tand the different types of browning reactions seen in food the need for controlling the extent of these reactions in different food products insight into factors which accelerate and retard these reactions sto how these reactions tke place derstand other changes which accompany these reactions and ther effect on nutrtve value Y desirable and undesirable browning reactions. < INTRODUCTION Browning is 2 common colour change seen in food during pre-preparation, processing, or storage of food. Ic occurs in varying degrees in some food material. The colours produced range from cream or pale yellow to dark brown or black, depending on the food item and the extent ofthe reaction. Browning reactions may be desirable or they may be undesirable. In some foods, the brown colour and flavour developed during browning is highly desirable and is associated with a delicious, highly acceptable, and quality product. Browning reactions contribute to the aroma, favour, and colour of the product such as the brown crust of bread and al baked goods, potato chips, roasted nuts, roasted coffee beans, caramel, peanut brittle, and many other processed foods “The undesirable effects of browning reactions are seen in dehydrated food, such as milk, eggs, dry fruits, in cut fruic and citrus fruit juices and juice concentrates, in canned milk, and in coconut. The colour varies from light cream to black while coconut develops a saffron colout. The off-colour and offcodour developed in foods depend on the extent to which the browning reaction has progressed Off flavours may vary from mild favour changes to stale and very bitter. Controlled browning is necessary even in foods where browningis desired because excessive browning "can produce an undesirable product. Changes in odour and flavour, which may also develop along, with browning, may be characteristic ofa food and desirable or may be biter, making the food unpalatable, TYPES OF BROWNING REACTIONS Browning reactions observed in food may be classified as enzymatic browning and nonenzymatic browning. ‘Non-enzymatic browning may be further classified as shown in Fig, 8.1,120 Food Science and Nutrition Non-enzymatic browning (cee oe eee eee Mailard reaction Cararnelzation _Lipid browning Fig: 8.1. Classification of non-enzymatic browning reactions Enzymatic Browning Some light-coloured fresh fruits and vegetables darken when exposed to air as a result of the presng of oxidative enzymes. Enzymatic browning occurs in these fruit and vegetable tissues when celle organization is disrupted by cucting, bruising, or other injury to the tissues. This i due to the action cnidative enzymes on the phenolic substances present inthe fruit or vegetable tissues ‘Apples, bnans, peas, brinjals, and potatoes undergo enzymatic browning, The cll contents come in contact with each bther resulting in browning of uncooked fruits and vegetables which are cut oF bruised. and vegetables which contain phenolic compounds Enzymatic browning takes place only in fruits “These phenolic compounds act as che substrate, and in the presence of oxygen and by che acon o enzymes, the following oxidative reaction is observed (Fig. 8.2). one ‘Quinane (Compound which is Phenaie compound) — (romarorintercelular spaces) | nog dark coloured but ready (Gubstrate) ey SG ~ polymerizes to dark Phenol oxidase (enzyme) Te Fig. 8.2. Enzymatic browning reaction “The formation of O-quinone isthe first step in brown colour formation. Quinones are polymers to form complex compounds which darken in colour. “The deals of cherncal changes which ake place in conversion of colourless substrate in the inst | fruit to the brown-coloured reaction product in the damaged tissues are not known. First quinos are formed, which change to quinones. Polymerized quinones are oxidized non-enzymaticaly and a responsible for brown colour formation, i Phenol oxidase enzymes are specific for certain substrates and are present in many vegetables such as apples, bananas, pears, peaches, potatoes, and brinjals. Phenolic compounds present, such as tannins, and related phenolic substances, such as leucoanth” cyanin, contribute towards the astringency of food. Prevention of Enzymatic Browning Enzymatic browning can be prevented by the following methods. fruits and By inactivating enzymes Enzymes can be inactivated by any of the following measures. Appliaton of beat Blcngo cooking fss and vegetables which ate prone o browning preven ae inzymes are protein in nature and heat denatures proteins there by inactivating Addition of salt Vegetables may be immersed in a solution of sodium chloride to retard enzymat browning ‘The chloride ion in NaCl inhibits enzyme activity. However, this is a temporary meas rerard browning, as the amount of salt required to prevent browning would make the food unpalat* aBrowning Reactions 121 Lowering the pH Enzymatic browning is prevented by k ' ene ering the to 2.5-2. addition acid. Aad slo prevent browning areacoric acd males eo ead ince, Abeta viamin © ats a antioxidant and retards enzymatic browning The quinones formed by the action of enzymes are reduced to their dihy droxyl state by vie, : nes for h dchydroascorbic acid. 'y droxyl state by vitamin C and in turn vitamin C is oxidized to Chiling a food below temperatures optimum for enzyme activity Optimum temperature for enzymes tO actis 43°C (109°F). In cold storage, the browning reaction s i ‘rorage brown unless they are treated, ig Feaction slows down, but even fruits stored in frozen By avoiding contact with oxygen Oxygen should not come in i trate. This can b cel palit come in contact with the substrate, This can be Coating fruit with sugar Coating fruit with sugar or covering it with syrup keeps atmospheric oxygen away from the surface, Intercellular oxygen is present in fruits, but sugar reduces the concentration of ‘oxygen dissolved in the syrup and at the same time suppresses enzyme activity. Inmersing vegetables in water Contact with atmospheric oxygen can be avoided by immersing cut vegetables in water. Since water contains dissolved oxygen, itis more effective if itis First boiled to remove dissolved air. Vacuum packing Protecting food from contact with oxygen as in vacuum packing prevents enzymatic browning since it is an oxidation reaction. By elimination of substrate This method of prevention is not practical. A variety of peach called ‘Sunbeam is an exception because i is deficient in substrate. Sulphuring of fruits prior to dehydration Sulphur prevents oxidative browning due to enzyme activity. Fruic is treated to sulphur fumes prior to drying. Treatment with sulphur dioxide (SO,) gas or sulphurous acid solution (H,SO,) or 0.25 per cent sodium sulphite for 45 seconds is adequate to prevent browning, Bisulphites and metabisulphites are also used. Sulphurous acid is a strong reducing. agent and prevents discolouration. Non-enzymatic Browning Maillard Reaction Maillard was the first to describe the development ofa brown sensation of amino group of protein colour in mixtures containing amino acids and reducing : sugars, The reaction between certain free groups of amino Face a acids, such as the NH, group and a carbohydrate, affects wl group of sugar the product in many ways in addition to the colour change. Seen ‘The aroma and flavour of the ready product i also affeced. See ‘The reaction is also known as carbonylamine reaction or Gaiman protein sugar reaction. Figure 8.3 shows Maillard reaction. f ‘The brown pigment formed contributes to the aroma, Brown pigment flavour, and colour of many ready-to-eat cereals, coffees, Fig. 83. Mallard reaction malted barley, and bakery products such as bread, cakes,|22_ Food Science and Nutrition e colour of the crust is light. The m me then chi IF the dough contains less reducing suse” © ; a tee rage ie ‘sugar, the darker the crust in bakery re lee rolls, zi : i umine ke “The characteristic flavour developed differs in different products Deca 0 acid invohy in the reaction. For example, Food product ‘Amino acid involved © Beer — Glycine evrah baad) oe, 2 Lenses Sy viple srup =. = Aminoburrne acid Conditions which Favour Maillard Reaction High temperature The rate of browning incres ‘Moinure Certain amount of moisture, approximatel or very low wate levels wll decease the rate ofthe reaction: pH value Browning is accelerated by an increase in alkalinity, i.e., a pH above 7.8. Presence of phosphate, citrate, and acetate buffer salts acelerate the rate of browning. Concentration and ype ofamino acid and suger present Maximum browning effect is observed with amin acid lysine and sugar glucose. Presence ofcatast Browning reaction is generally catalyzed and iron. Desirable changes Many favourable changes coffee beans and nuts and improvement in qu: “This reaction is also partly responsible for the favour of meat extracts, bakery items. Undesirable changes Maillard reaction is also responsible for certain undesirable effect foods such as * Off-odour © Off avour—mild, stale, or biter © Off-colour—ranging form mild cream to nearly black a pre ales a conor milk darken on storage for a long time. Coconut rus safron ai i i deieadaaage Ree be prevented by low pH and low temperacures duis “There is a oles Hae a Lae the presence of nutrients involved in the react® -ids, especially lysine and methic 7 is formed cannot be hydrolyzed by digestive enzymes. ionine, oe reaction and the substance Saal tel ere Geen eae ee Pe tee 3 are unavailable tothe body. quite sigificane because amino acids involved in the 26°? | Bread loses 10-15 % k : lysine during bakin Piste ‘ Te WL ge eT ne Saline ands Loins roasts ses rapidly with a rise in temperature. iy 13 per cent will favour the reaction. Very hh by the presence of metals such as cope | in colour, flavour, and aroma are seen during roasting! ality of bread during baking. breakfast cereals, and a 5s seen in dried | aBrowning Reactions 123 ccaramelization Suga are caramelized at 163 to 170°C or 325 to 338°F because of action of heat. Darkening of: ; a mn of heat. 1 of syrups, Brown clout of ae Sea taffy, and brittle are because of earamelization reaction. rs nwning teaction occurs with Scie ada eal “gumber of compounds because of intense heat. ‘curs with sugar alone. Sugar is broken down into a Sucrose Se Sees a SST Caramel + acid | ‘etydratin, and polymerization soe fps cookery When sugar is oie, i pases through regular sags il i gets cameized a1 163°C. ‘The various stages of sugar cookery are listed in Table 8.1. ‘The rate and extent of browning in carbohydrate solutions are influenced by the following factors. Heat For caramelization to take place, high temperature is necessary. Temperature can be lowered by the presence of a catalyst. Effect of pH Both acids and alkali increases browning, but alkalis are more effective. ‘Typeofsugar Fructose shows the greatest degree of browning, followed by sucrose. Glucose shows least browning. Presence of catalysts Metals such as iron and copper aeclerate the rate of reaction. 103 Caramel is used to flavour and colour alcoholic beverages, 106 soups, gravis, sauces, soup cubes, cake mixes, coffee products, i canned meat products, tc, Ascorbic Acid Browning Kokum and strawberry preserves undergo a change in colour during storage. The original bright red/crimson of ‘the anthocyanin pigment in the fresh fruic becomes dull and develops a rusty brown colour. A similar change in colour is also seen in citrus fruit juices and squashes. its undergoes oxidation with the formation of a compound ‘causes discolouration. The oxidized ascorbic acid hastens the cyanin giving the product a dull brownish colour. Factorsx 124 Food Science and Nutrition | Lipid Browning : This typeof browning i seen in fats and fay phases of food. The amino groups of phospholpg, lipoproteins can react with aldehydes and reducing sugars resulting in browning, Lipid browning is quite uncommon and may be observed in fats stored for long periods. undesirable reaction. ROLE OF BROWNING IN FOOD PREPARATION Browning reactions may be desirable or undesirable affecting the appearance, flavour, and arom, food. All browning reactions can be controlled and hence it is essential that the caterer is aware of factors which influence these reactions. ; Enzymatic browning is considered unsightly and undesirable, especially while preparing salads fruit salads, Natural browning is usually undesirable but normal cooking processes, such 2s rasing grilling, or baking, bring about brown colouration. This is expected and desirable and contribuesy, | the acceptance and palatability of food. Cooking brings about chemical breakdown which along wit colour, adds favour to food. The aroma of freshly baked bread and roasted coffee beans is due to dai. able Maillard reaction. Food components interact chemically and give rise o a brown colour accompanied by modifications in flavour, aroma, and taste. Because food components are involved in che formation of the brow. | colour, there is some loss of nutritive value. | DETRIMENTAL EFFECTS OF BROWNING } Enzymatic browning Enzymatic browning of apples and bananas makes fruit salads unappetizing so! cannot be served to customers. Maillard reaction In baked and toasted products and in milk, some lysine and methionine ate st. Ascorbic acid browning Some ascorbic acid is lost from citrus juices and strawberries because of auto oxidation of vitamin C. In desirable browning reactions, icis necessary to control cooking temperatures to prevent blackening or burning of the product Z Enzymatic activity can be retarded by blanching vegetables and fruit to inactiva cr Cooke food isnot disealured by enzymes, en ns tf iaotvate the ens Browning plays an important role in the aesthetic value of food. A b: an 1e aesthetic v: |. A baked product, e.g.» cake ot breed, would lose its acceptability if the golden brown crust and the eaapaniag ‘aroma is absent The Kite staf should understand the various factors which can prevent undesirable browning control them, SUMMARY nd bring abou et YON wily in food products brite. It may be undesirable in some fresh ruts and ng shout changes n colour ranging rom cream veges sich as banaas and sees hea Oe) Tce ree ts ae larg? cut or bruised or the darkening of milk powder 4 ons maybe desea nthe brown Sy cratl hyo rea Moun Seabees,Browning reactions need to be controlled as exc- ‘essve browning may make the product unpalatable, ‘Changes in odour and flavour may also accompany the reaction. They are broadly clasified as enzymatic browning and non-enzymatic browning. Enzymatic browning is seen in fresh, uncooked fruits and vegetables which contain phenolic compounds. In the presence of oxygen, specic enzymes bring about discolouration of phenolic subsvate. This type of browning can be controlled by inactivating the enzymes. ‘Non-enzymatic browning s accelerated by tempera ture and is further classified as |. Mailard reaction Carbonyl group Free group of reducing sugars. Enzyme Catalyst made up of protein and responsible for most reactions in living tissues, Destroyed by heat as protein gets coagulated. Maillard reaction A reaction between proteins or amino acids and sugars resulting in a brown colour, often accompanied by aroma and flavour when food is cooked Oxidation A reaction in which oxygen s gained or hydrogen is lot or loss of electrons. Phenol oxidases Enzymes that oxidize phenolic Browning Reactions 125 2. Caramelization 3. Ascorbic aed browning 4. Lipid browning While most Maillard reactions and caramelization are desirable reactions, ascorbic acid browning and lipid browning are undesirable. Since food components are involved, the nutritive values reduced toa small extent in some reactions. Browning plays an important role in food preparation and it needs tobe controled to ensure high acceptability of the product. The development of colour in baked ‘goods along with enhanced flavour and aroma increases the acceptability and quality of food. KEY TERMS ‘compounds to quinones causing browning in some fruits, vegetables, and mushrooms in which they are present. Phenolic compounds Group of aromatic ‘compounds (with a ring structure) that undergo ‘oxidative enzymatic browning, e., tyrosine, catechins which are colourless pigments in plants. Reducing sugars Sugars that contain the aldehydie ‘or ketonic reducing group, eg. glucose, fructose, lactose, and pentoses (ve carbon sugars) REVIEW EXERCISES. |. Classy the different types of browning reactions seen in food. 2. You have been asked to prepare fresh fruit salad. What precautions would you take to prevent enzymatic browning? 3. Explain briefly: (@) Uses of caramel (b) Factors affecting Maillard reaction (©) Measures to retain red colour in strawberry preserves 4, Listany five examples of desirable browning and the five examples of undesirable browning, mentioning ‘type of browning reaction.