By Patel Hardik and Jana Atanu
Milk is a highly perishable food. Heat treatment of milk was realised to be essential to render milk safe (microbiological safety) for human consumption. The main aims of heat treatment are to reduce the microbial population, both pathogenic and spoilage, to inactivate enzymes and to minimise chemical reactions and physical changes. The thermal processing of milk can cause undesirable reactions such as vitamin and essential nutrition loss and change in colour, aroma and taste.
The HTST (High Temperature Short Time) pasteurisation process is widely used by the food, beverage and dairy industries. Currently, bacterial spoilage is the most limiting factor in extending the shelf life of conventionally pasteurised HTST processed fluid milk products beyond 2-3 days in India; that too when kept under refrigerated condition.
Pasteurised and UHT milk are sometimes mislabelled as ‘fresh milk’ in some countries. The commercial heat treatment processes applied to milk are pasteurisation, Ultra High Temperature (UHT), ultra pasteurisation or super pasteurisation, sterilisation, and Innovative Steam Injection (ISI) (Table 1). All these heat treatment processes, to which milk is subjected to, render it safe and prolong its shelf-life.
HTST Pasteurisation of Milk
The HTST method of pasteurisation involves heating milk at 72-75?C for 15-20 seconds before it is cooled. Pasteurisation kills most of the microorganisms in milk, but does not render the milk sterile. Hence, pasteurised milk must be kept refrigerated, ?4?C throughout distribution and storage.
Ultra Pasteurisation or Super Pasteurisation
Ultra Pasteurisation (UP) or Super Pasteurisation involves heating at temperatures above those used for pasteurisation yet below those used for UHT processing (i.e. sterilisation). Such treatment prevents thermal alteration of the sensory properties, and extends the shelf life of milk without the need for refrigerated storage. Milk treated in such manner may be called ultra pasteurised, super pasteurised or extended long life milk. Super pasteurised milk is designed to have a shelf life of up to 90 days at refrigerated temperatures and have sensory properties superior to those of UHT milk. Super pasteurised milk is processed at 125-138?C for 2-4 sec.
The UHT treatment is suitable for tropical countries with high ambient temperatures as they are unable to afford refrigeration during marketing and distribution. The nutritional quality of protein is of significance since protein intake is generally low in such countries.
Extended Shelf Life (ESL) milk has a shelf life of about 3 weeks under cold chain conditions and fills the gap between HTST-processed milk (a shelf life ranging from 2-10 days under refrigeration) and Ultra-high Temperature (UHT) processed milk (a shelf life of few months at ambient temperature storage). A thermal treatment or combination of heat treatment and membrane filtration (Microfiltration)/Bactofugation of milk are involved in the production of ESL milk. The thermal process requires direct or indirect heating at 123–127ºC with a holding time of 1–5 s.
UHT puts the locally produced fresh, pasteurised milk at a disadvantage since the former has prolonged shelf life up to 6 months, that too at ambient temperature.
Definition of Pasteurisation and Means to check Pasteurisation Efficiency
Pasteurisation is the widely adopted milk process to ensure complete destruction of all pathogenic microorganisms, commonly found in milk and inactivation or reduction of other non-pathogenic spoilage bacteria and certain undesirable enzymes to safeguard the food value of milk.
FAO/WHO (2004) defines pasteurisation as amicrobiocidal heat treatment aimed at reducing the number of any pathogenic microorganisms in milk, if present, to a level at which they do not constitute a significant health hazard. Pasteurisation conditions are designed to effectively destroy the pathogenic organisms such as Mycobacterium tuberculosis and Coxiellaburnetii. Pasteurisation achieves at least a 5-log reduction of C. burnetiiin whole milk. The HTST pasteurisation kills 99.999 percent of pathogens.
Pasteurisation efficiency can be determined by alkaline phosphatase test. Alkaline phosphatase, an enzyme native to milk have a thermal resistance greater than that of the most heat resistant non-spore-forming pathogens commonly found in milk and hence, its destruction confirms proper pasteurisation.
The HTST process for milk involves heating up to 72-75°C with a holding time of 15-20 seconds before it is cooled. Depending upon the quality of the raw milk and the degree of refrigeration, the shelf life may be from 2-16 days. The actual time/temperature combination varies according to the quality of the raw milk, the type of product treated and the required keeping properties.
UHT is the sterilisation of food before packaging, then filling into pre-sterilised containers in a sterile atmosphere. Milk is processed in this way using temperatures in the range 135-150°C with a holding time of 2-5 s enabling a continuous flow operation that takes place in a closed system. The product passes through heating and cooling stages in quick succession. Aseptic filling is a must to avoid re-infection of the product. Despite the advantage of UHT processing, extended thermal treatment and subsequent ambient storage can result in the development of off-flavours, sometimes described as cooked flavour, as well as the loss of vitamins and amino acids.
To circumvent such lacunae and attain ESL, some processors have employed UHT processing temperatures at the minimal safe times, in conjunction with aseptic packaging. The main advantage of UHT milk is easy and cheaper storage and delivery without the need of refrigerated trucks or refrigerated display shelf. UHT treatment of milk saves time, labour, energy and space. It is high-speed process and has much less effect on the flavour of milk. The high sterilisation temperature of milk causes de-naturation of proteins, which are deposited onto the heat transfer surfaces to form fouling layer that reduces heat transfer. Such heat treatment does appreciably affect the nutritional quality of milk.
Changes in Milk upon Heat Treatment
Pasteurised and ultra-high temperature milks resulted in protein and lactose content similar to raw milk. Pasteurisation and sterilisation altered the composition of the milk slightly, decreasing total fat and total solids and increasing urea. Pasteurisation and then the commercial sterilisation increased the pH of milk, with consequent decrease in acidity. The Poly Unsaturated Fatty Acid (PUFA)/Saturated Fatty Acid (SFA) and n-6/n-3 ratio remained unaltered.
Formation of cooked flavour and nutritional value loss due to new substances formed by the Maillard reaction continues during storage of heated milks. The calcium (0.11±0.02%) and phosphorus (0.10±0.02%) concentrations in the milk samples did not show significant differences between the milks subjected to HTST pasteurisation and ultra pasteurisation.
Commercial heat treatment processes alter the heat sensitive nutrients, physico-chemical and functional properties of milk. Higher amounts of lysinoalanine were found in UHT milk than in pasteurised milk. Further reduction in nutritional value of UHT milk occurs in supermarket shelves until sold and consumed.
Colour of Processed Milk
Colour is an important parameter that highly affects the consumer acceptability. After UHT treatment in milk, changes in case in size and de-naturation of whey protein increase the amount of light scatter and milk appears whiter. UHT milk is invariably homogenised and hence, whiter. However, this improvement is balanced by browning, which lowers the degree of reflectance and gives a mild white colour.
Flavour of Processed Milk
Intensity of flavour notes in UHT correlates well with the accumulation of lactulose. Flavour acceptability of sterilised milk is critically dependent on the level of free-SH groups (originating from whey proteins), which are responsible for the strong hydrogen sulphide odour of fresh sterilised milk. The level of these compounds declines rapidly, due to oxidation, with storage time. The rate and extent of Maillard reaction that occurs during storage contributes to the decline in the flavour quality of UHT milk.
Loss in Vitamins
There were greater losses in Vitamin C, folic acid, vitamin B12, riboflavin and thiamine in UHT milk than in pasteurised milk. The loss of vitamin C was attributed to the heat instability of the oxidised form, dehydroascorbic acid and can be minimised by limiting the dissolved oxygen content of milk during handling. Table 2 depicts the nutrient losses in HTST and UHT milks.
Ascorbic acid loss was high during storage, for both pasteurised and UHT milks. Most of the Vitamin C loss during storage occurred in milk packaged in materials other than paper (i.e. polyethylene, glass), when such packages were exposed to light. Exposure to light was responsible for riboflavin loss for both pasteurised and UHT milk and so was the case with vitamin A.
Oil soluble vitamins (A, D, E) as well as some water soluble vitamins (riboflavin, nicotinic acid, biotin) are heat stable, and are not adversely affected by UHT process. Folic acid, vitamin B12 and ascorbic acid are lost to different extent. The oxidised form of vitamin C is likely to be completely destroyed during UHT process while loss of only 10-20 percent of the reduced form is expected. Loss of vitamin B12 is linked to oxidative destruction of vitamin C. The loss of folic acid is limited by the protection provided by the reduced form of vitamin C.
Effect on Milk Protein
UHT processing results in moderate de-naturation (25-80%) of whey proteins. Heat treatment of milk above 75?C, affects sulphur containing amino acids resulting into volatile compounds such as hydrogen sulphide and mercaptans that give the typical cooked flavour to sterilised milk. Lysine losses in UHT processed milk is about 4 percent as compared to 1-2 percent losses during HTST pasteurisation. Proteolysis of UHT milk during storage at room temperature is a major factor limiting the shelf life through changes in its flavour and texture.
In HTST-heated milk, the degree of de-naturation of ?-lactalbumin and ?-lactoglobulin (whey proteins) is typically 10–20 percent, whereas the de-naturation of immunoglobulins is approximately 50 percent.
Somatic Cells and Microbial Profile of Heated Milk
Somatic cell and total bacterial counts are strongly influenced by the health of animals and the hygiene conditions for milking. The types of spores, which have been investigated as of particular relevance in the UHT, are those of Bacillus stearothermophilus, Bacillus subtilisand Clostridium botulinum.
According to Food Safety and Standards Act (FSSA), ‘Pasteurisation’, ‘Pasteurised’ and similar terms shall be taken to refer to the process of heating every particle of milk of different classes at least 71.50C and holding at such temperature continuously for at least 15 sec or an approved temperature time combination that will serve to give a negative phosphatase test. All pasteurised milk of different classes shall be cooled immediately to a temperature of 100C, or less.
The Philippine food regulatory agency allows the ‘fresh milk’ label on UHT milks marketed in their country.
The choice of consumers for heat processed milk (i.e. HTST or UHT processed) would depend on the convenience, cost and prevailing climatic conditions in a country. Both HTST and UHT milk are safe for drinking and nutritious as well. However, UHT sterilisation leads to relatively more loss of nutrients than HTST processing. The processing equipment and package cost are greater for UHT milk as compared to HTST treated milk.
Patel Hardik is M.Tech. Scholar and Jana Atanu is Professor and Head, Department of Dairy Technology, SMC College of Dairy Science, Anand Agricultural University, Anand – 388 110, Gujarat