Enzymatic and Non-Enzymatic Browning - Essay Example

Running Header: Enzymatic and non-enzymatic Browning Student’s Name: Instructor’s Name: Course Code: Date of Submission: Enzymatic and non-enzymatic Browning Introduction The process of changing into brown and in particular foodstuff is called browning. In some instances, browning of food may be desirable in such cases like caramelization or undesirable in situation when an apple turns brown after cutting it or undesirable discoloration that takes place on fresh vegetables and fruits as illustrated by Alan (2005). Various foods such as beverages and fruits can change into brown. They are two types of browning that include enzymatic and non-enzymatic browning. They are similarities and differences in enzymatic and non-enzymatic browning of foods in terms of chemical reactions involved. The major difference in the two types of browning is that enzymatic browning requires an enzyme while non-enzymatic browning does not require an enzyme. It is also significant to appreciate different reaction processes that the reaction undergoes. The reactions vary as illustrated within the essay. Enzyme browning reaction has differing reaction to that of non-enzyme browning reaction. However, it is worth to note that they are two major reactions in non-enzyme browning as will be clearly illustrated in the essay. Enzymatic browning is a chemical process that takes place in vegetables and fruits by enzyme polyphenoloxidase resulting in brown pigments. They are two major components used in enzymatic browning that include polyphenols and polyphenoloxidase. Enzymatic browning is usually observed in vegetables (potatoes, lettuce, mushrooms), seafood (crabs, spiny lobsters, and shrimps), and fruits (grapes, pears, bananas, and apricots). Enzymatic browning is harmful to quality and in particular post-harvest storage of juices, vegetables, fresh fruit, and shellfish. In some instances, enzymatic browning is important for the taste and color of chocolate, tea, and coffee. Enzymatic browning as a chemical process involves catechol oxidase, polyphenol oxidase, among other enzymes that create benzoquinone and melanin leading to brown color. Polyphenoloxidase, catechol oxidase catalyze phenols oxidation in fruits leading to formation of quinines compounds that polymerize to form melanin responsible of brown pigments. Polyphenols are usually unstable and they usually undergo oxidation when exposed to air due to a series of biochemical reactions that converts it from one substrate to another as described above. Exposure to oxygen is a major factor contributing to enzymatic browning. Polyphenoloxidase catalyze two basic reactions including oxidation and hydroxylation that make use of molecular oxygen as a co-substrate according to Alan (2005). The reaction depends on the air presence, ph or acidity. It is important to note that reaction does not occur when pH is less than 5 or alkaline is greater than pH 8 conditions. A chemical process that results to color brown in foods with no involvement of enzymes activities is referred to as Non-enzymatic browning. The two major non-enzymatic browning include Maillard and caramelization reactions. However, both vary in rate of reaction as a function of water. The Maillard reactions are a chemical reaction between (the carbonyl group) a reducing sugar and free amino acid of a protein or an amino acid. Heat usually accelerates the rate of maillard reaction although it can proceed at lower temperatures but a rate that is slower. Caramelization is the water removal process from a sugar including glucose or sucrose followed by steps like isomerization and polymerization. It is a chain of complicated chemical reactions. Non-enzymatic browning is responsible for several desirable and pleasant flavors and aromas. However, they are still some undesirable effects of non-enzymatic browning (Yiu & Jozsef, 2006). In enzymatic browning, the enzymes catalyze the initial steps in the biochemical conversion of phenolics to quinines in presence of oxygen from the air. This polymerizes further to dark insoluble polymers that are called melanins. A phenolic compound called tyrosine forms a substrate for the phenoloxidase enzyme. This is converted to Dopa in presence of oxygen molecules by phenoloxidase. This further converts to Dopa Quinone in a faster process to dopa quinine in presence of oxygen molecule as well. Dopa Quinone is then changed to Leuco compound in a fast process. This is then converted to Dopachrome in presence of oxygen molecule in a fast process. This further changes slowly to 5,6-Dihydroxyindole. 5,6-Dihydroxyindole is further converted to Indole 5,6-Quinone in presence of oxygen in a fast reaction process. Indole 5,6-Quinone finally is converted to Melanin in presence of oxygen molecule in a relatively slow process (Yiu & Jozsef, 2006). Figure 1 The diagram shows various enzymatic browning reactions (Yiu & Jozsef, 2006) The reaction rate is dependent of the air presence and pH levels of the fruits. Enzymatic browning reaction is usually favored by Acidic pH. This is unlike non-enzymatic browning that requires heat for the process to be effective. Caramelization as a non-enzymatic reaction involves several complicated chemical reactions. Caramelization leads to desirable flavor and color in beer, coffee, beverages, and bakery goods. Caramelization takes place during roasting and dry heating of foods with high levels of sugars. In simple terms, it involves removal of water from sugar and then isomerization and polymerization process. Caramelization process starts with melting of sugar at high temperatures (1600c) and then boiling or foaming. At this stage, sugar decomposes into fructose and glucose forming anhydro sugars. Condensation step then follows where individual sugars lose water and then react with one another forming difructose-anhydride. The next step is isomerization of aldoses to ketoses with additional dehydration reactions. Fragmentation reactions (flavor production) and polymerization reactions (color production) are the last series of reactions. There is formation of several intermediate compounds that form according to time and temperature. Controlling reactions at varying stages can lead to production of different colors ranging from light yellow to dark brown like in the case of beverage colors including beer. It also leads to varying flavors compounds such as caramel aroma in coffee (Yiu & Jozsef, 2006). Maillard is another non-enzymatic browning reaction that involves a complex of reactions between amino acids and reducing sugars at a higher temperature. The sugar’s carbonyl group that is reactive usually reacts with the amino acids nucleophilic amino group. This leads to formation of a complex mixture of poorly characterized molecules that are in charge of varying flavors and odors. Alkaline environment accelerates the process as deprotonation of the amino group takes place thus have an increased nucleophilicity. The kind of amino acid decides the resultant flavor. Just like enzymatic enzymes, maillard reaction involves steps like condensation, rearrangement, fragmentation, strecker degradation, and polymerization. However, no enzymes are involved in the entire process. There are 3 main steps of the reaction. The initial step consists of glycosylamine formation and rearrangement of N-substituted-1-amino-I-deoxy-2-ketose (Amadori compound). This involves the reaction of reducing sugar with amino acid. The second stage involves amine loss to form intermediates of carbonyl. These form highly reactive carbonyl compounds after fission or dehydration through several pathways. This step depends on isomer of the Amadori compound. The third phase taking place upon subsequent heating entails the interaction of the compounds of carbonyl flavor with other components to form melanoidians (brown nitrogen-containing pigments). This reaction is also called Amadori-rearrangement, it is the starting point of the main reactions of browning. Different flavor compounds are created in Maillard reaction including furfural and hydroxymethylfurfural. Hydroxymethylfurfural is because of hexose reaction while Furfural is as a result of pentose sugar reaction. Maillard reactions are crucial in frying, baking, and heating of all types of food. Maillard reactions are usually responsible for bread, chocolate, popcorn, meat, beer, cookies, and cooked rice. Maillard reaction usually involves several factors for it to produce desirable aroma and color. Such include temperature, time, types of amino acids and sugar, water activity, and other components of food (Yiu & Jozsef, 2006). It is believed that the polyfuryl compounds (melanoidins) provide brown colouration as well as being responsible for Maillard Reaction complexes that exhibit oxygen scavenger ability as argued by Douglas, (2002). Therefore, it acts as natural antioxidants. Maillard reactions affect bioavailability of proteins through the derivatizing protein-bound, dietary limiting amino acids including histidine, lysine, and arginine. Other effects exhibited by Maillard reaction products include antinutritive effects by mechanism involving complexation with micronutrients, vitamins destruction, and by acting as digestive enzymes inhibitors. During heat treatment in milks, lactose and lysine combination leads to formation of Maillard reaction products. In comparing enzymatic and non-enzymatic reactions, both reactions involve more than one stage of reaction. Enzymatic reaction has several processes as illustrated in diagram 1. Caramelization steps include melting of sugars, condensation step, isomerization, fragmentation, and polymerization steps. Maillard reaction involves condensation, rearrangement, fragmentation, strecker degradation, and polymerization. Both enzymatic and non-enzymatic reactions involve polymerization process at their last stages. Polymerization usually involves formation of brown nitrogen containing pigments (melanoidins) (Douglas, 2002). In an enzymatic browning reaction, polymerization takes place on quinines into insoluble polymers referred to as melanin. It is worth noting that they are both reactions are chemical processes that involves change of one state to another through a complex process. However, they are variation in both types of reaction that include varying temperature, pH, and moisture levels. Moisture conditions are always required for enzymes to function actively. Lack of enough moisture conditions leads to inhabitation of enzymes but not their destruction. A non-enzymatic Maillard reaction increases with increase in water activity. However, the reaction produces its own water hence no need for a lot of water as it can inhibit Maillard reactions. On the other hand, caramelization involves the removal of water from sugars through isomerization and polymerization hence differing with enzymatic reaction. Another major difference between enzymatic and non-enzymatic reactions is that, enzymatic reaction requires acidic conditions in order for it to be effective. On the other hand, non-enzymatic reaction requires an alkaline condition. Enzymatic reactions require lower temperatures compared to non-enzymatic reactions. High temperatures deactivates enzymes, temperatures of up to 7000 in most instances deactivates the enzymes reactions. High temperatures in non-enzymatic reaction are useful towards the activation of energy for the reaction to take place. This requires high temperatures for it to be effective. However, enzymatic reaction does not require high temperatures as they can deactivate enzymes or very low temperature as they can inhibit the working of enzymes as illustrated by Douglas, (2002). Conclusion In conclusion, both enzymatic and non-enzymatic browning reactions have beneficial and non-beneficial results. However, both reactions lead to browning effect of various food products including beer and breads among others. As stated earlier, use of enzymes makes the major difference between the two types of browning reactions. The main chemistry involved in enzymatic browning reaction is when Polyphenoloxidase and catechol oxidase catalyze phenols oxidation in fruits leading to formation of quinines compounds that polymerize to form melanin responsible of brown pigments. Non-enzymatic browning does not use enzymes in its browning process. However, they are two types of non-enzymatic browning that include Maillard and Caramelization reactions. All these reaction processes are facilitated by various factors that include ph levels, temperature, and water levels. The Maillard browning reactions are a chemical reaction between (the carbonyl group) a reducing sugar and free amino acid of a protein or an amino acid. Caramelization is the water removal process from a sugar including glucose or sucrose followed by steps like isomerization and polymerization. It is worth to note that all browning reactions have polymerization process at their final stage of reaction. Polymerization is the color production process where brown pigments are produced. In enzymatic browning reactions, polymerization involves change of quinines to insoluble polymers called melanin. In all cases, melanin is the compound responsible for the brown color of foods or fruits. In enzymatic browning, the final product melanin has antimicrobial properties that prevent inflammation and infection to the fruits. In some instances, melanin has antioxidant, anticancer, and antioxidant properties. However, browning causes losses of fruits and vegetables hence decreasing the quality of vegetables and fruits in case of enzymatic browning reaction decreasing flavor and nutritional value of vegetables and fruits. In non-enzymatic browning, changes contribute to desirable development of flavor on cooking. The Maillard reaction plays a key role in many colors and flavors in foods including browning of various meats such as steak, toasted bread, roasted meat, and fried onions among others. Caramelization is a different process from Maillard browning. However, the results of them are similar with naked eyes and taste buds. Caramelization cause browning in the same food that Maillard reaction takes place. Heating promotes both processes. However, Maillard browning reaction involves amino acids whereas caramelization is pyrolisis of some sugars. References Alan W 2005, ‘The Maillard Reaction in Eye Diseases’. Annals of the New York Academy of Science, vol. 1043, no. 5, pp. 582–597. Douglas, B 2002, Colour in food: improving quality, Woodhead Publishing Limited, Cambridge. Yiu, H & Jozsef, B 2006, Handbook of fruits and fruit processing, Blackwell Publishing, Oxford. Read More