- Compiled by Lalita Oraon
- MT (PPD)
Sterilization of milk is aimed at killing all microorganisms present, including bacterial spores, so that the packaged product can be stored for a long period at ambient temperature, without spoilage by microorganisms. Since molds and yeasts are readily killed, we are only concerned about bacteria. To that end, 30 min at 110°C (in-bottle sterilization), 30 sec at 130°C, or 1 s at 145°C usually suffices. The latter two are examples of so-called UHT (ultra-high-temperature, short time) treatment. Heating for 30 min at 110°C inactivates all milk enzymes, but not all bacterial lipases and proteinases are fully inactivated; it causes extensive Maillard reactions, leading to browning, formation of a sterilized milk flavor, and some loss of available lysine; it reduces the content of some vitamins; causes considerable changes in the proteins including casein; and decreases the pH of the milk by about 0.2 unit. Upon heating for 1 s at 145°C chemical reactions hardly occur, most serum proteins remain unchanged, and only a weak cooked flavour develops. It does not inactivate all enzymes, e.g., plasmin is hardly affected and some bacterial lipases and proteinases not at all, and therefore such a short heat treatment is rarely applied (Brennan and Grandison 2008).
The undesirable secondary effects of in-bottle sterilization like browning, sterilization flavor, and losses of vitamins can be diminished by UHT sterilization. During packaging of UHT-sterilized milk, contamination by bacteria has to be rigorously prevented. After UHT sterilization, certain enzymatic reactions and physicochemical changes still may occur.
Sterilized milk may be defined as (homogenized) milk which has been heated to a temperature of 100 degree Celsius or above for such lengths of time that it remain 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 requirement for complete sterility conflict with the consumer preference for normal color and flavor 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 (De, 2001).
The term “sterilization” when used in association with milk, means heating milk in sealed container continuously to a temperature of either 115 degree Celsius for 15 minutes or at least 130 degree Celsius for a period of 1 second or more in a continuous flow and then packed under aseptic condition in hermetically sealed containers to ensure preservation at room temperature for a period not less than 15 days from the date of manufacture (FSSA, 2013).
Sterilization of foods by the application of heat can either be in sealed containers or by continuous flow techniques.
Sterilized milk is kept for a long time so that it will show extensive gravity creaming if unhomogenized. Creaming as such is undesirable. Besides, partial coalescence of the closely packed fat globules will lead to formation of a cream plug, which is hard to mix throughout the remaining milk; oiling off may occur at somewhat elevated temperatures. Therefore, sterilized liquid milk is always homogenized.
(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) Distinction rich flavor (due to homogenization);
(v) Economical to use;
(vi) Less liable to develop oxidized taints.
(i) increased cost of production;
(ii) More loss in nutritive value than pasteurization
(iii) Gerber test by normal procedure not so accurate.
Sterilized milk must
(i) Keep without deterioration, i.e., remain stable and be of good commercial value for a sufficient period to satisfy commercial requirement
(ii) Be free of any 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).
The raw milk, on receipt, should be strictly examined by the physic-chemical and bacteriological test 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 percentage of fat and solids-not-fat content in order to conform to legal standards. 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 cleaned and sanitized bottle coming from the bottle washing machine and then sealed with special caps. 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 for 25-35 minutes. The sterilized milk bottles should be gradually cooled to room temperature. Any sudden cooling may led to bottle breakage. Finally the milk-in-bottles should be stored in a cool place (De, 2001).
Sterilizers may be: (i) Batch; (ii) Continuous.
(i) Batch Sterilizers
These may either be rotary or non-rotary in type. The batch 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.
Simplicity and flexibility of operation
Less initial capital and recurring expenditure
1. Usually produces a brownish appearance and cooked taste in the finished product.
2. Sterilization may be faulty
3. Cooling has to be slow to avoid breakage
4. Economic advantages of large-scale processing are not obtained.
In the batch-rotary type, the filled bottles are put in to 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.
(ii) Continuous sterilizers
In this type, the filled and sealed milk bottles are automatically placed by means of a slat conveyor in to the pockets of carrier cages. They then passed into water at or near boiling temperature; from there, they enter the sterilizing zone, which consists of a steam chamber at 108-111°C. Here the bottles remain for a pre-determined time, viz., 25-30 minutes, for milk sterilization
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 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.
Turbidity Test for Sterilized Milk
The turbidity test depends upon the denaturation of proteins of milk especially albumin after sterilization. When solution of inorganic salts or acids are added albumin separates with the casein. The sample after treatment with ammonia sulphate is filtered and heating of filtrate shows turbidity due to presence of albumin on account of sufficient heat treatment. If milk has been sterilized properly all albumin will have been precipitated and no turbidity will be produced. The test is not suitable for UHT milk (FSSA, 2012).
Pipette 20 ml of milk in a 50 ml conical flask, add 4.0±0.1 g of ammonium sulphate. Shake the flask till the ammonium sulphate is completely dissolved. Allow the mixture to settle for 5 minute, filter through a folded filter paper in a test tube. Keep about 5 ml of the above filtrate in a boiling water bath for 5 min. Cool the tube in a beaker of cold water and examine the contents for turbidity by moving the tube in front of an electric light shaded from the eyes of the observer.
The milk is considered sterilized when the filtrate shows no turbidity.
Ultra high temperature (UHT) processing
More recently, continuous sterilization processes have been introduced. UHT or aseptic processing involves the production of a commercially sterile product by pumping the product through a heat exchanger. To ensure a long shelf life the sterile product is packed into pre-sterilized containers in a sterile environment. An airtight seal is formed, which prevents re-infection, in order to provide a shelf life of at least three months at ambient temperature. It has also been known for a long time that the use of higher temperatures for shorter times will result in less chemical damage to important nutrients and functional ingredients within foods, thereby leading to an improvement in product quality (Brennan and Grandison, 2008).
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 distributions; the filling has to be done aseptically. Ideally, heating and cooling should be as quick as possible.
This applies only as long as the product remains under aseptic conditions, so it is necessary to prevent re-infection by packaging the product in previously sterilised packaging materials under aseptic conditions after heat treatment. Any intermediate storage between treatment and packaging must take place under aseptic conditions. This is why UHT processing is also called aseptic processing.
UHT plants are often flexibly designed to enable processing of a wide range of products in the same plant. Both low-acid products (pH > 4.5) and high-acid products (pH < 4.5) can be treated in a UHT plant. However, only low-acid products require UHT treatment to make them commercially sterile. Spores cannot develop in high-acid products such as juice, and heat treatment is therefore intended only to kill yeast and moulds. Normal high temperature pasteurisation (90-95°C for 15-30 seconds) is sufficient to make high-acid products commercially sterile (Bylund, 2003).
Various UHT systems
There are two main types of UHT systems on the market.
A. Direct UHT plants
In the direct systems the product comes in direct contact with the heating medium, followed by flash cooling in a vacuum vessel and eventually further indirect cooling to packaging temperature.
The direct systems are divided into:
• Steam injection systems (steam injected into product)
• Steam infusion systems (product introduced into a steam-filled vessel)
UHT processing means commercial sterility to ensure food safety and long shelf life at ambient temperature. It entails heating the product to a specific temperature for a specific length of time. The higher the temperature, the shorter the time required to destroy micro-organisms. The more rapidly the product can be heated and then subsequently cooled down again, the less impact the process has on the chemical changes in the product, such as changes in taste, colour and even to some extent, nutritional value. The most effective way of achieving rapid heating is to mix high temperature steam directly with the product, followed by flash cooling in a vacuum vessel. This is called a direct system.
Flash cooling is an operation, which as well as cooling, also involves deaeration and deodorisation of the treated product. In addition, deaeration secures higher homogenisation efficiency and the deaeration will also positively influence the storage stability of the processed product in terms of preventing oxidation during storage.
The rapid heating and cooling explains why direct systems deliver superior product quality and are often chosen to manufacture heat - sensitive products, such as premium quality market milk, enriched milk, cream, formulated dairy products, soy milk and soft ice mix, as well as dairy desserts and baby food.
B. Indirect UHT plants
In many cases, products must not only be attractive and healthy to eat and drink, but also economical to manufacture, store and distribute. The most cost-effective method of UHT processing is indirect heating – a heating method in which the processed product never comes into direct contact with the heating medium. There is always a wall in between. This technique applies to all types of heat exchangers.
In the indirect systems the heat is transferred from the heating media to the product through a partition (plate or tubular wall). The indirect systems can be based on:
• Plate heat exchangers
• Tubular heat exchangers
• Scraped surface heat exchangers
Indirect UHT plants are a suitable choice for processing of milk, flavoured milk products, cream, dairy desserts, yogurt drinks and other non-dairy applications, such as juices, nectars and tea.
Indirect UHT plant based on plate heat exchangers
This process solution is appropriate for products such as coffee cream and evaporated concentrated milk.
Indirect UHT plant based on tubular heat exchangers
A tubular system is chosen for UHT treatment of products with low or medium viscosity that may or may not contain particles or fibres. Soups, tomato products, fruit and vegetable products, certain puddings and desserts are examples of medium-viscosity products well suited to treatment in a tubular concept. Tubular systems are also frequently utilised when longer processing times are required for ordinary market milk products.
Indirect UHT plant based on scraped surface heat exchangers
Scraped surface heat exchangers are the most suitable type for treatment of high-viscosity food products with or without particles.
Aseptic packaging has been defined as a procedure consisting of sterilisation of the packaging material or container, filling with a commercially sterile product in an aseptic environment, and producing containers that are tight enough to prevent recontamination, i.e. that are hermetically sealed. The term “aseptic” implies the absence or exclusion of any unwanted organisms from the product, package or other specific areas (Bylund, 2003).
For products with a long non-refrigerated shelf life, the package must also give almost complete protection against light and atmospheric oxygen.
Loss of nutrients during sterilization
The nutritive value of pasteurized and UHT-sterilized milk changes little by the heat treatment and during storage. In-bottle sterilized milk shows a somewhat greater loss of nutritive value. Of special importance are the decrease of available lysine and the total or partial loss of some vitamins. Maillard reactions are responsible for the partial loss of lysine. They occur to some extent in UHT-sterilized milk during storage and in in-bottle sterilized milk during heating. The loss of lysine is not serious in itself because in milk protein, lysine is in excess (Walstra et al., 2006).
The losses of vitamins mainly concern vitamin C and some five vitamins of the B group. Vitamins A and E are sensitive to light and/or oxidation, but mostly their concentrations do not decrease in sterilized milk. Losses of vitamins in milk should be evaluated relative to the contribution of beverage milk to the supply of these vitamins in the total diet. Especially the loss of vitamins B1, B2, and B6 are considered undesirable. The loss of vitamin C is generally of minor importance as such, but it may affect the nutritive value in other ways. The breakdown of vitamin C is connected with that of vitamin B12; moreover, vitamin C protects folic acid from oxidation.
Loss of vitamins during storage can largely be avoided if O2 is excluded. Vitamins C and B9 may completely disappear within a few days if much O2 is present. The loss is accelerated by exposure to light, with riboflavin (vitamin B2) being a catalyst. Most of the riboflavin disappears on long-term exposure to light.
Sterilization of the product is achieved by rapid heating to required high temperature, holding it for few seconds followed by rapid cooling. Ideally, heating and cooling should be as quick as possible. UHT products are in a good position to be able to improve the quality image of heat-processed, ambient stable foods.
De S (2001) Special Milks. In: Outlines of Technology. Chapter 2. 1st Ed., Oxford University Press- New Delhi. pp 90-93.
Brennan J G and Grandison A S (2008) Thermal processing. In: Food Processing Handbook. 2nd chapter, Second Edn., Vol 1. Wiley-VCH Verlag GmbH & Co. KGaA. pp. 60-61.
Bylund G (2003) Long-life milk. In: Dairy Processing Handbook. Chapter 9. Teknotext AB (Ed.) Tetra Pak Processing Systems AB S-221 86 Lund, Sweden. pp. 227-245.
FSSA (2013). The Food Safety and Standards (Food Products Standards and Food Additives) Regulations, 2011. 3rd Ed., Commercial Law Publishers (India) Pvt. Ltd., p 169.
FSSA (2012) Manual of Methods of Analysis of Foods. Milk and Milk Products. Lab Manual 1. Ministry of health and Family Welfare, Government of India, New Delhi.pp 39-41
Walstra P, Wouters J T M and Geurts T J 2006. Milk and lipid consumption. In: Dairy Science and Technology. Part III, Products. Second Edn. Taylor & Francis Group, LLC. Boca Raton London New York. pp. 421-444.