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Biochemical Reactions That Occur During Food Processing
Transcript of Biochemical Reactions That Occur During Food Processing
Polyphenols – main components in enzymatic browning
Biochemical Reactions That Occur During
Polyphenols can be divided into many different sub categories, such as anthocyans (colours in fruits), flavonoids (catechins, tannins in tea and wine) and non-flavonoids components (gallic acid in tea leaves). Flavonoids are formed in plants from the aromatic amino acids phenylalanine and tyrosine.
During food processing and storage many polyphenols are unstable due to the fact that they undergo chemical and biochemical reactions. The most important is enzymatic oxidation causing browning of vegetables, fruits. This reaction mostly occurs after cutting or other mechanical treatment of product due to breaking cells.
Enzymatic browning is a chemical process which occurs in fruits and vegetables by the enzyme polyphenoloxidase, which results in brown pigments. Enzymatic browning can be observed in fruits (apricots, pears, bananas, grapes), vegetables (potatoes, mushrooms, lettuce) and also in seafood (shrimps, spiny lobsters and crabs)
Enzymatic browning is detrimental to quality, particularly in post-harvest storage of fresh fruits, juices and some shellfish. Enzymatic browning may be responsible for up to 50% of all losses during fruit and vegetables production.
On the other hand enzymatic browning is essential for the colour and taste of tea, coffee and chocolate.
Polyphenoloxidases are a class of enzymes that were first discovered in mushrooms and are widely distributed in nature. They appear to reside in the plastids and chloroplasts of plants, although freely existing in the cytoplasm of senescing or ripening plants. Polyphenoloxidase is thought to play an important role in the resistance of plants to microbial and viral infections and to adverse climatic conditions.
Polyphenoloxidase also occurs in animals and is thought to increase disease resistance in insects and crustaceans.
In the presence of oxygen from air, the enzyme catalyzes the first steps in the biochemical conversion of phenolics to produce quinones, which undergo further polymerization to yield dark, insoluble polymers referred to as melanins.
Polyphenols, also called phenolic compounds, are group of chemical substances present in plants (fruits, vegetables) which play an important role during enzymatic browning, because they are substrates for the browning-enzymes.
Phenolic compounds are responsible for the colour of many plants, such as apples, they are part of the taste and flavour of beverages (apple juice, tea), and are important anti-oxidants in plants.
Polyphenols are normally complex organic substances, which contain more than one phenol group (carbolic acid):
is the process of becoming brown, especially referring to food. Browning foods may be desirable, as in caramelization, or undesirable, as in an apple turning brown after being cut. Foods, including beverages, can turn brown through either enzymatic or non-enzymatic processes.
There are 2 types of browning:
Structure 1: Phenol
The colour of apples is due to polyphenols
These melanins form barriers and have antimicrobial properties which prevent the spread of infection or bruising in plant tissues. Plants, which exhibit comparably high resistance to climatic stress, have been shown to possess relatively higher polyphenoloxidase levels than susceptible varieties.
Polyphenoloxidase catalyses two basic reactions: hydroxylation and oxidation. Both reactions utilize molecular oxygen (air) as a co-substrate. The reaction is not only dependent on the presence of air, but also on the pH (acidity). The reaction does not occur at acid (pH <5) or alkaline (pH >8) conditions.
Contrary to enzymatic or oxidative browning, non-enzymatic browning is a chemical process that produces a brown color in foods without the activity of enzymes. The two main forms of nonenzymatic browning are caramelization and the Maillard reaction. Both vary in reaction rate as a function of water activity.
2 major types of non-enzymatic browning reactions:
is one of the most important types of browning processes in foods, together with Maillard reactions and enzymatic browning. Carmelization leads desirable colour and flavour in bakery's goods, coffee, beverages, beer and peanuts. Undesirable effects of caramelization are for example burned sugar smell and blackening.
Caramelization causes important changes in foods, not only in colour but also in flavour. As no enzymes are involved in the carmelization process, it is a non-enzymatic browning reaction.
Caramelization occurs during dry heating and roasting of foods with a high concentration of carbohydrates (sugars).
The Maillard reaction
is not a single reaction, but a complex series of reactions between amino acids and reducing sugars, usually at increased temperatures. Like caramelization, it is a form of non-enzymatic browning.
In the process, hundreds of different flavour compounds are created. These compounds in turn break down to form yet more new flavour compounds, and so on. Each type of food has a very distinctive set of flavour compounds that are formed during the Maillard reaction.
Maillard reactions are important in baking, frying or otherwise heating of nearly all foods. Maillard reactions are (partly) responsible for the flavour of bread, cookies, cakes, meat, beer, chocolate, popcorn, cooked rice. In many cases, such as in coffee, the flavour is a combination of Maillard reactions and caramelization. However, caramelization only takes place above 120-150 °C, whereas Maillard reactions already occur at room temperature.