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.

• Polyphenols
• Polyphenoloxidase
• Prevention

Polyphenols – main components in enzymatic browning

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):

Structure 1: Phenol

Structure 2: Theaflavin, a polyphenol in tea

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.

The colour of apples is due to polyphenols

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.

Table 1 : An overview of known polyphenols involved in browning

Polyphenoloxidase (PPO, phenolase)

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.

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.

An example of the formation of melanins from a simple polyphenol, tyrosine, is shown in the figure below:

Structure 3 : Formation of melanins from tyrosine

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.

Prevention of enzymatic browning

The control of browning is one of the most important issues in thefood industry, as colour is a significant attribute of food which influences consumer decision and brown foods (especially fruits) are seen as spoiled.

Several methods can be applied to avoid enzymatic browning, based on inactivating the enzyme (heat) or by removing essential components (most often oxygen) from the product.

Blanching

Blanching is a short heat treatment to destroy or inactivate enzymes before freezing of products (mainly vegetables). Enzyme activity may discolour or toughen vegetables during freezing, which results in quality loss. Blanching brightens the colour, softens the texture, but has little effect on nutrient content or flavour as it is a relatively short process.

The blanching temperature depends on the type of enzyme which occurs in the product, but is generally between 70 and 100 °C, sometimes higher when more resistant enzymes are to be inactivated. Table 2 below gives an indication of the temperature needed to inactivate some important enzymes.

Table 2 : Inactivation temperatures of some enzymes

Types of blanching:

• blanching in steam/boiling water;

  Steam or boiling water blanching is a type of heat treatment for controlling enzymatic browning in canned or frozen fruits and vegetables. It is scalding the vegetables or food in water or steam for a short period of time. The steam blanching is 1.5 times longer than boiling water blanching.

• microwave blanching;

  Microwave blanching may not be effective, since research shows that some enzymes may not be inactivated. This could result in off-flavours and loss of texture and colour.

Refrigeration

Refrigeration and chilling are used to prevent spoilage of vegetables and fruits during distribution and retailing. Chilling is applied often for broccoli, berries, spinach, peas, bananas, mangoes, avocados, tomatoes. At temperatures below 7 °C the polyphenoloxidase enzyme activity is inhibited, but the enzyme is not inactivated. Therefore the temperature should be well controlled.

Freezing

Like refrigeration, freezing inhibits, but not inactivates the enzyme. After thawing, the enzyme activity will resume.

Change pH

The enzyme activity is pH dependent. Lowering of the pH to 4.0 by the addition of citric, ascorbic or other acids inhibits the enzyme activity. During home-preparation of vegetables or fruits lemon juice or vinegar is often sprinkled on the fruit to prevent browning.

Dehydratation

Dehydratation is caused by the removing water molecules from the product. The PPO enzyme needs sufficient water to be active. By drying the enzyme is inhibited, but not destroyed.

To avoid flavour and quality loss, dehydration should not involve heat.

Common methods for dehydration are:

• Freezing-drying when moisture is removed by sublimation (the change from solid to gas). Products are frozen and slowly dehydrated under vacuum.
• Lowering water activity by adding water-binding chemicals. The most commonly used substances are salt (sodium chloride), sucrose, and other sugars, glycerol, propylene glycol and syrups or honey.

Irradiation

Irradiation, or as it is sometimes called "cold pasteurization", is a process in which food is submitted to ionized radiation in order to kill bacteria and reduce the enzyme activity. Irradiation is often applied in meats, seafood, fruits, vegetables, and cereal grains for long-term preservation.

Several types of irradiation methods are used in food processing: gamma rays, X-rays and accelerated electrons (electron beams).

Disadvantages of radiation are loss of nutrients and (low) consumer acceptance. Irradiation is thus rarely used.

High pressure treatment

High pressure treatment also called High Pressure Processing (HPP) is a technique of food processing where food is subjected to elevated pressures (500-700 atmosphere) to achieve microbial and enzyme inactivation.

High pressure processing causes minimal changes in foods. Compared to thermal processing, HPP results in foods with fresher taste, and better appearance, texture and nutrition. High pressure processing without heat eliminates thermally induced cooked off-flavours. The technology is especially beneficial for heat-sensitive products, but still very expensive.

Addition of inhibitors

Inhibitions can act in three ways:

1. Inactivation towards the enzyme (acting directly on the enzyme)
2. Inactivation towards substrate (removing the substrate like oxygen or phenolic compounds)
3. Inactivation towards the product (changing the product composition)

Large amount of inhibitors are applied in food processing depending on the type of product and process. The most important inhibitors are shown in table 3.

Table 3 : Inhibitors of enzymatic browning

Ultrafiltration

Ultrafiltration is a membrane separation process, driven by a pressure gradient. The membrane separates liquid components according to their size and structure. In the food industry this technique is for example applied for white wine and fruit juices. Ultrafiltration is able to remove larger molecules like polyphenoloxidase, but not lower-molecular-weight components like polyphenols.

Ultrasonication

Ultrasonication is an advanced method to inactivate enzymes. Ultrasonic sound waves are able to destroy large molecules by liberating highly reactive radicals from water. It is not yet applied on a large scale.

Treatment with supercritical carbon dioxide (SC-CO₂)

Supercritical carbon dioxide (fluid carbon dioxide at high pressure) treatment is mostly applied to destroying micro-organisms but can also be applied for enzyme inactivation, especially for inactivation of PPO in shrimps, lobsters and potatoes. Inactivation of the enzyme is a result of a decrease in pH caused by production of carbonic acid from carbon dioxide.

Main source : http://www.fao.org/AG/ags/agsi/ENZYMEFINAL/Enzymatic%20Browning.htm