Defects Their Causes and Prevention

Condensed milk and evaporated milk are the products suitable for prolonged storage.They have typical properties after production by which they are identified. These properties must be such that the product is fit for sale immediately after production and does not alter during reasonable period of storage. They should, therefore be physically, chemically and bacteriologically fit for human consumption by the end of storage period. Routine examination of the product soon after manufacture as well as during storage may be carried out to judge the quality of the product. If any defect is noticed, proper care should be taken to climate the defect in the subsequent batches. It is, therefore, important to know what type of defects may occur in the product, the reasons for their occurrence and the preventive measures to be used to avoid these defects.The defects which may occur in condensed milk may be divided into two categories as follows:

i.    Microbial Defects

These defects are due to presence of abnormal bacteria in the milk or in products.

The common defects are as follows:

a) Gassy fermentation/Bloats: Formation of gas takes place in cans and barrels of condensed milk causing bulging or bursting of containers. Gas producing yeasts are the cause of many types of gaseous fermentation. The source of contamination may be the raw milk, or inferior quality sugar or unhygienic factory conditions especially not properly washed and sanitized equipments

and filling machine.

To avoid this defect good quality raw milk and properly preheating temperature should be used. Only good quality sugar without any yeast contamination should be selected. Proper sanitary conditions should be maintained during manufacture and packaging of condensed milk. The containers should be filled fully with little space for air or oxygen.

(b) Bacterial thickening: Condensed milk gets thickened progressively during storage. This is due to microorganisms which produce rennin like enzyme.These organisms are easily destroyed during preheating process. Optimum sugar ratio (64.5) may inhibit the growth of microorganism. Low temperature storage also helps in reducing the bacterial thickening.

(c) Mould buttons: This defect occurs during storage due to mould contamination. Small reddish brown pieces of curd about ¼” to ¾” in diameter are formed on the surface causing localized coagulation. This defect occurs after storing the product for some time. The causative mould “Aspergillus repeno” produces a“clotting enzyme” which causes localized clotting. High temperature storage also helps in the growth of the organism. The milk may be infected with the

organism during concentration process.

To avoid this defect, scrupulous cleaning and care of dairy equipments is essential.Since this organism does not grow at low temperature storing of condensed milk at lower temperature will prevent this defect.

ii.   Non Microbial Defects

The non-microbial defects are of chemical or physical origin. These are listed below.

(a) Sandiness: Good quality condensed milk should possess a smooth homogenous texture and be pleasant to palate. Some times however the milk may be gritty containing large number large sized hard lactose. The solid particles are of such size that the product lacks smoothness and grittiness is noticeable, as the sample is being tasted. This defect is readily detected by an average consumer.Sandy, rough grainy, granular, and gritty are the term used to describe this defect.

The sandiness may be due to presence of relatively large size crystals of lactose.Also if excess amount of sugar is used in manufacture of condensed milk, sugar particles may also crystallize out and cause sandiness. If manufacturing conditions are not conducive to the formation of small lactose crystals, large and coarse crystals are formed. Cooling of condensed milk must be carried out in such a manner that smooth texture is obtained by the formation of large number of minute sugar crystals.High viscosity also delays the crystal formation. It is, therefore, necessary to rapidly cool the condensed milk in the initial stages.Correct cooling and induced rapid crystallization with correct amount of seed lactose will help to avoid this defect. Optimum storage temperature is also essential to avoid this defect.

(b) Age thickening: Thickened condensed milk is the most common defects seen in the sweetened condensed milk. This defect varies markedly in its intensity from slight jelly to a firm and consistency. The defect becomes progressively more intensive upon storage, especially at room temperature or above.Preheating temperature of milk and degree to which the milk is concentrated have been observed to have profound effect on age thickening. With high prepackaging ,heating temperatures, there is a greater tendency upon the product to thicken early. With increasing concentration of milk solids the thickening tendency becomes more marked. In order to avoid early thickening of condensed milk,optimum pre heating temperature should be maintained. Sugar should only be added at the end of condensing of milk. The product should preferably be stored at temperature below 15°C with the addition of proper type of stabilizers age thickening may be decreased to a great extent.

(c) Brown colour: usually brown colour discolouration is associated with age thickening, both of which become progressively more intense on storage. The acidity and temperature of storage are factors, which determine the rapidity of change. This defect may be avoided if the condensed milk cans are stored at reasonably low temperatures. Other defects in condensed milk include some flavour defects such as rancid, tallowy, metallic etc. Following good manufacturing practices, good quality raw material, good quality milk, good packaging and storing the product at low temperature may help to avoid the defect.

(d) Fat separation in condensed milk is rare.

Defects in Evaporated Milk: Evaporated milk is sterilized product. If sterilization is properly carried out the product will remain in good condition without any bacterial spoilage during storage. However, if sterilization process is improperly carried out, some microorganisms and spore may, survive and cause spoilage, during storage, Gassy fermentation, coagulation, bitterness and fishy flavour are some of the defects noticed in improperly sterilized evaporated milk. If the bacterial defect in evaporated milk is to be eliminated, it is essential that the sterilization process should be carried properly and adequately with strict cleanliness in the factory.

Chemical/Physical defects in Evaporated Milk

Age thickening

Age thinning

Fat separation

Sediments/mineral deposit

Brown colouration

(a) Age thickening: As in the case of sweetened condensed milk thickening or high viscosity of the product is due to improper preheating of milk, higher total solids concentration, inadequate homogenization and storage at higher temperature.

To eliminate this defect the milk should be heated at optimum preheating temperatures. The concentrated milk should be homogenized at proper pressure and the product should be stored at low temperature.

(b) Age thinning/low viscosity: Milk like consistency of evaporated milk indicates its low viscosity. This defect may be due to insufficient concentration of milk and inadequate or no homogenization of milk or improper sterilization. This defect is eliminated by following correct/adequate processing conditions and storing the product at low temperature.

(c) Fat Separation: A thick, heavy cream layer is noticed at the top of the can when it is opened.

The cream layer is very dense and will not easily mix with remainder of milk. Even after mixing creamy chunks of butter particles are noticed floating in the milk of relatively low viscosity. The reasons for this defect to appear are in adequate homogenization high storage temperature, long storage period and improper handling during storage. Proper homogenization, low temperature storage and correct handling of the product will eliminate this defect. A more viscous is desirable from fat separation point  of view.

(d) Sediments/Mineral deposit: Sediments noticed in evaporated milk cans may be due to crystallization of some of the calcium and magnesium salts, and denatured proteins. This gritty sediment formation takes place during prolonged storage of evaporated milk. The rapidity with which the sediments are formed is influenced by the nature of the milk, conditions of manufacture and temperature of storage. Selection of good quality milk, correct processing conditions and low temperature storage will help to eliminate this defect. Higher viscosity product is less prone for this defect.

(e) Brown Colour: The brown colour in evaporated milk is associated with high sterilization temperature, high storage temperature and longer storage period.The sterilization temperature should be as low as possible with germicidal efficiency. The brown colour defect in evaporated milk is eliminated by employing proper preheating temperature, correct sterilization process and low temperature for storage of the product.