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Bromelin, a proteolytic enzyme, from pineapple digests proteins hence changes the gelation to compounds that do not form a gel. The enzyme bromelin clots the milk and digests the clot. All fruits contain some organic acids but not always in sufficient concentration to cause the curdling of milk. Destroying of the enzymes before combining the fruit with milk prevents curdling caused by the enzyme action, by blanching or by using canned fruits.
- Effect of phenolic compounds (Tannins) and salts: Fruits and vegetables contain tannins chiefly in green stages. Curdling of milk may occur if tannin containing foods such as potatoes are cooked in the milk. Tannins are also present in brown sugar and cocoa products.
Seeds and stems may contain significant amount of phenolic substances. Among vegetables, the roots, pods, some seeds and woody stems are likely to contain more phenolic compounds than other parts of the plant, although distribution is general throughout the plant. Salts present in milk or food or addition of sodium chloride can curdle the milk.
- Microbial spoilage: Milk is an excellent medium for the growth of micro-organisms. The odour of milk is due to the activity of the micro-organisms. Milk can be contaminated from the udders to the vessels. Good hygeinic conditions are to be used at all levels of milk handling to keep the bacterial count level low.
Freshly drawn milk is cooled immediately to 4°C to arrest microbial growth or heated to boiling to destroy them. If raw milk is placed at room temperature a number of fermentative changes take place. Curdling may occur due to lactic acid produced by bacteria. Some bacteria produce in addition to acid some gaseous products while others bring about proteolysis of casein resulting in off flavour and unpleasant odour. There are also bacteria which bring about ropiness in milk.
Steps involved in spoilage of milk
Milk undergoes predictable changes during spoilage.
When acid is increased in milk, moulds and yeasts present in milk flourish. They utilise the acid and bring about chemical changes producing an alkaline condition. Tuberculosis, typhoid fever, sore throat, gastroenteritis and diphtheria organisms may be present in contaminated milk.
Table __ gives the microorganism that may occur in milk, their sources and the changes they produce.
Some micro-organisms bring about desirable changes by forming volatile acid like acetic acids and control the flavour and aroma. These are used in making butter, curds and other fermented products.
Flow chart: Steps involved in spoilage of milk
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