In this article we will discuss about:- 1. Definition of Sterilized Milk 2. Requirements of Sterilized Milk 3. Method of Manufacturing 4. Distribution, Tests and Faults 5. Equipments 6. Advantages and Disadvantages.
Definition of Sterilized Milk:
Sterilized milk may be defined as (homogenized) milk which has been heated to a temperature of 100°C or above for such lengths of time that it remains fit for human consumption for at least 7 days at room temperatures. (Commercially, sterilized milk is rarely sterile in the strict bacteriological sense. This is because the requirements for complete sterility conflict with the consumer’s preference for normal colour and flavour in the sterilized product. The spore-forming bacteria in raw milk, which are highly heat-resistant, survive the sterilization temperature-time employed in the dairy and ultimately lead to the deterioration of sterilized milk.)
Requirements of Sterilized Milk:
Sterilized milk must:
(i) Keep without deterioration, i.e., remain stable and be of good commercial value for a sufficient period to satisfy commercial requirements;
(ii) Be free of micro-organisms harmful to consumer health, i.e., pathogenic, toxinogenic germs and toxins;
(iii) Be free of any micro-organisms liable to proliferate, i.e. it should not show signs of bacterial growth (which leads, inter alia, to an absence of deterioration).
Method of Manufacturing Sterilized Milk:
A. In-Bottle Sterilization:
Flow Diagram of Manufacture:
Details of Manufacture:
The raw milk, on receipt, should be strictly examined by the prescribed physico-chemical and bacteriological tests and only high-quality milk should be used for production of sterilized milk. Care should be taken to accept milk supplies which have no developed acidity and which contain the least number of spore-forming bacteria.
The intake milk should be promptly cooled to 5°C for bulk storage in order to check any bacterial growth. Next, it should be pre-heated to 35-40°C for efficient filtration/clarification, so as to remove visible dirt, etc., and to increase its aesthetic quality.
The milk should again be cooled to 5°C so as to preserve its quality. It should then be standardized to the prescribed percentages of fat and solids-not- fat content in order to conform to legal standards (which vary from State to State for both cow and buffalo milk). It must be stored at 5°C until processing.
The milk should be promptly pre-heated to 60°C for efficient homogenization to prevent any subsequent formation of a cream layer; usually single-stage homogenization is carried out at 2500 psi pressure. The homogenized milk must be clarified so as to remove the sediment formed during the homogenization process.
The hot milk from the homogenizer should be filled into the (hot) cleaned and sanitized bottles coming from the bottle- washing machine and then sealed with special caps (of the crown seal type). The filled and capped bottles should then be placed in metal crates for sterilization by the Batch Process, or fed into conveyors for the Continuous Process.
Usually the milk is sterilized at 108-111°C (225-230°F) for 25-30 minutes. The sterilized milk bottles should be gradually cooled to room temperature. Any sudden cooling may lead to bottle breakage. Finally the milk-in-bottles should be stored in a cool place.
Sterilizers:
These may be:
1. Batch;
2. Continuous.
1. Batch:
These may either be rotary or non-rotary in type. The batch (tank) sterilizers are rectangular, horizontal, boiler- shaped retorts with a steam inlet and condensate outlet, fitted with clamp-down covers, into which steam is adjusted for the required temperature and time for sterilization.
Advantages:
(i) Simplicity and flexibility of operation;
(ii) Less initial capital and recurring expenditure.
Disadvantages:
(i) Usually produces a brownish appearance and cooked taste in the finished product;
(ii) Sterilization may be faulty;
(iii) Cooling has to be slow to avoid breakage;
(iv) Economic advantages of large-scale processing are not obtained.
In the batch-rotary type, the filled bottles are put into holders which are rotated at 6-7 rpm. The sterilized milk is of a slightly better quality in rotary-type sterilizers than in non-rotary ones.
2. Continuous:
In this type, the filled and sealed milk bottles are automatically placed by means of a slat conveyor into the pockets of carrier cages. They then pass into water at or near boiling temperature; from there, they enter the sterilizing zone, which consists of a steam chamber at 108-111°C (225-230°F). Here the bottles remain for a pre-determined time, viz., 25-30 minutes, for milk sterilization.
Cooling:
After heat-treatment in the batch/tank sterilizers, the milk bottles may be cooled in air or water. If cooling is too rapid, the bottles may crack; if too slow, there is a danger of browning due to caramelization. In the continuous system, after leaving the sterilizing zone, the bottles enter a column of hot water where the cooling process begins.
This is followed by their passage through another tank of water (at a lower temperature than the previous one) for further cooling, and lastly through a shallow tank of cold water for final cooling. The bottles are then automatically discharged and conveyed to a point where they are placed in crates in which they are transferred to the storage room.
B. Ultra High Temperature (UHT) Methods of Sterilization:
In these processes, the milk is heated to 135-150°C for a few seconds, generally in a plate or tubular heat-exchanger. The milk, which is then almost sterile, has to be filled into containers for distribution; the filling has to be done aseptically. In many cases pre-sterilization, as above, is followed by in-bottle sterilization.
Distribution, Tests and Faults of Sterilized Milk:
Once a week:
This is why sterilized milk has great scope in warm countries as long as household refrigerators are not in common use.
Tests:
(i) Turbidity test. (This is the official test.)
(ii) Bacterial count.
Note:
Phosphatase test is not applicable to sterilized milk.
Faults:
The most common is browning. Because of this fault, ‘Plain’ sterilized milk is not so popular. Flavoured (and simultaneously coloured) sterilized milk is more popular.
Equipments Used for Sterilization of Milk:
There are two types of sterilizers in use, viz., batch and continuous.
I. Batch:
This consists of a large boiler-like, horizontal steam drum, opening at the top or at one or both ends. Its hollow interior is equipped with a revolving frame into which the cans are loaded. A perforated steam-distributing pipe near the bottom, with a steam inlet at both ends, extends over the entire length of the sterilizer drum.
Near the top, there is a water-distributing pipe with connections to the water main. At the bottom of the shell there is a drain. On the sterilizer drum are mounted a pressure-safety valve, water, steam and vacuum gauges, a high-temperature thermometer, etc.
The sterilizer rotates at a speed of 6 to 12 rpm. Rapidity and uniformity of heat distribution during sterilization are important. It has been found practicable to use just enough water in the sterilizer so that all the cans in the same position in the frame are submerged. Use of water also eliminates interfering air-pockets between cans.
The temperature-time for heating during sterilization should be such as to ensure sterility consistent with sufficient body (to prevent fat separation) without causing the formation of an unshakeable curd, objectionable discolouration or an excessively cooked flavour.
A successful sterilizing routine consists of:
Merits:
(i) Useful for both small and large plants.
(ii) Usable for cans of all sizes.
Demerits:
High labour cost.
II. Continuous:
This is so designed as to pre-heat the cans gradually by slowly moving them through progressively hotter hot-water chambers to a few degrees below boiling point. Then the cans enter the sterilizer proper where they are heated to sterilizing temperature by steam under pressure.
Later, the cooling process starts with the cans gradually moving through progressively less hot hot-water chambers and finally through cold water (under air pressure). One such type of sterilizer consists of three principal parts, viz., pre-heater, sterilizer and cooler, with two leak-detectors at the can-exit of the pre-heater and cooler respectively.
Cooling:
Immediately after the holding time is over, the evaporated milk is cooled within 15 minutes to 27-32°C (70-80°F). Rapid and uniform cooling is important. This requires, for the batch system, not only a plentiful supply of cold water but also its uniform distribution all over the sterilizer.
Further, delayed cooling may cause the cans to bulge badly. Bulging can be avoided, however, by admitting a sufficient quantity of compressed air. In the continuous system, the cooler operates at 10 psi.
Shaking:
The purpose of shaking is to mechanically break down any curd which may have formed during sterilization to a smooth homogeneous consistency. A shaking period from about one-fourth to two minutes is usually sufficient. Excessive shaking should be avoided as it decreases the viscosity.
Storage:
In recent years, the trend has been to store evaporated milk at below room temperature. A storage temperature of 5 to 16°C (40 to 60°F) is generally used. This checks the deterioration in quality. Studies have shown that commercial evaporated milk remains acceptable for 2 years when stored below 16°C (60°F) but deteriorates rapidly at 21°C (70°F) or above.
The humidity of the surrounding air should be low (below 50%) to inhibit spoilage of cans and labels. Inversion of the cans during storage will help minimize fat separation. A guideline for turning cases of cans in storage is given in Table 8.12.
Merits:
Low labour cost.
Demerits:
(i) Not useful for small plants.
(ii) Made for particular can size only.
(iii) High initial cost.
Advantages and Disadvantages of Sterilized Milk:
(a) Advantages:
(i) Remarkable keeping quality; does not need refrigerated storage;
(ii) No cream layer/plug;
(iii) Forms a soft digestible curd, and hence useful for feeding of infants and invalids;
(iv) Distinctive ‘rich’ flavour (due to homogenization);
(v) Economical to use;
(vi) Less liable to develop oxidized taints.
(b) Disadvantages:
(i) Increased cost of production;
(ii) More loss in nutritive value than pasteurization (50 per cent of the vitamin C and 33 per cent of vitamin B originally present, are destroyed, and there is a slight reduction in the biological value of the milk proteins);
(iii) Gerber test by normal procedure not so accurate.