FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 28 : Processing and Preservation of Fruits- II

Prevention of enzymatic browning

Enzymatic browning can be prevented either by inactivating the enzyme or cutting off the oxygen.

1. Temperature: The most commonly used method is blanching. This method has several limitations. The enzyme is fairly heat stable and requires to be heated at 100oC for 2-10 minutes for complete inactivation. This may not be possible in practice as cooking for long periods will affect the flavour and texture of the fruits. The optimum temperature for browning is 43oC-50oC. If food is kept at higher or lower temperature browning is reduced. Coagulation of protein occurs during blanching, thereby inactivating the enzyme. When fruit is canned or made into jams or jellies, the browning reaction stops as soon as the fruit is heated sufficiently to denature the enzyme.


2. Change in pH: The optimum pH for polyphenolase activity is between 6.0 and 7.0, lowering of the pH to 4.0 by the addition of citric acid inhibits the phenolase activity. It is also possible citric acid reacts with the copper present in the enzyme. Malic acid also has been found to be effective. Lemon juice contains both citric acid and ascorbic acid and both are effective.


3. Use of antioxidants: Chemical additives like sulphur dioxide and vitamin C serve to inhibit enzymes, alter enzyme substrates or limit entrance of oxygen. Sulphur dioxide, sulphites and bisulphites inhibit effectively browning. A sulphite concentration sufficient to maintain a free sulphur dioxide concentration of 10 parts/million will completely inhibit phenolase. Being a reducing agent sulphite has an additional benefit of preserving the ascorbic acid level. Pineapple juice has a relatively high percentage of sulphydryl compounds which are active antioxidants. Fruits can be dipped in dilute solution of 0.025 percent sodium metabisulphite for 45 seconds. If the sliced fruit is then immersed for a short time for 5 minutes in a solution of 0.2 percent dipotassium phosphate, the odour of sulphur is less pronounced and the fruit remains crisp and does not brown subsequently.
   
   Ascorbic acid retards browning by virtue of its reducing power. It is used along with citric acid to reduce browning. A solution of ascorbic acid is effective in preventing browning as it acts as a reducing agent for the oxidised intermediate of a phenolic compound and thus prevent polymerisation and subsequent oxidation of the substrate. Sometimes, it acts directly on some functional groups in the enzyme.


4. Prevention of contact with oxygen: Contact with oxygen can be reduced by immersing the fruits in water, or liquids like milk, curd, fruit juice or honey or by covering with a wet cloth after cutting.
   
   Addition of sodium chloride or sugar to the cut fruit prevents browning. Due to osmosis, fruit is covered by leached solution and prevents contact with oxygen.
   In a large scale, this is technically difficult to implement and costly. This used when other methods are not applicable or effective in controlling browning. A typical example is browning in avocado products. Packaging the product under nitrogen prevents surface browning effectively.
   
   Deoxygenation and vacuum closing are also used to diminish oxidation. High concentration of sodium chloride or sugar solution depresses the activity of plant oxidases, including phenolases.