High Heat Processing of Milk and Dairy Products
HTST is the acronym for High Temperature Short Time. The actual time-temperature combination varies depending upon the quality of the raw milk, the type of product treated, and the required keeping qualities.
Milk
The HTST process for milk involves heating the product to 72 – 75°C (generally 72°C) with a hold of 15 – 20 seconds (usually 15 seconds) before it is cooled (to 4°C or less). The phosphatase enzyme is destroyed by this time/temperature combination. The phosphatase test is therefore used to check whether the milk has been properly pasteurized. The test result must be negative. There must be no detectable or traces of phosphatase activity in the processed milk.
Cream and cultured products Use of phosphatase test as an indicator of efficiency of pasteurization doesn’t always hold good, especially with regard to fat rich dairy products (products with fat content above 8%) like cream as some reactivation of the enzyme takes place a fairly short time after pasteurization. The heat treatment must also be stronger, as fat is a poor heat conductor. Another enzyme, peroxidase is therefore used for checking the pasteurization results for cream. The product is heated to a temperature above 80°C with a holding time of about 5 seconds. This more intense heat treatment is sufficient to inactivate peroxidase. The test must be negative in samples of cream that is properly pasteurized and there should be no detectable peroxidase activity in the product. As the phosphatase test cannot be used for acidified products either, heating control is based on the peroxidase enzyme. Milk intended for cultured milk production is normally subjected to intense heating to |
Ultra pasteurization
Ultra pasteurization is another method that is frequently employed when a product of particular shelf life is required. By this method, it is possible to extend the shelf life of milk and dairy products by two extra days while it is indeed possible to increase the keeping quality by 30-40 days. The basic principle involved is to reduce the primary causes of reinfection of the product during processing and packing stages so as to extend its shelf life.
This warrants extremely high levels of hygiene during production and a distribution of temperature of no more than 7°C and a lower temperature is always preferred to extend the shelf life. In this process, milk is heated to 125–138°C for 2 – 4 seconds and immediately cooled to less than 7°C and it forms the basis of extended shelf life, popularly called by its acronym ESL. Extended Shelf Life is a general term for heat treated products which have been given improved keeping qualities by one means or another. Nevertheless, ESL products must still be kept refrigerated during distribution and in the retail stores.
Ultra High Temperature (UHT) processing
UHT is the abbreviation for Ultra High Temperature. UHT heat treatment is a technique for preserving liquid food products by exposing them to brief but intense heating. It is a continuous form of heat processing and it employs intense form of heat treatment where the product is usually packaged aseptically after heating and cooling without exposure to environment. Aseptic filling to avoid re-infection of the product is an integral part of the process. Naturally, this process increases the shelf life of milk and dairy products, sometimes up to 3 months of duration even when stored at room temperature.
The temperature – time combination used in this process is usually between 135-150°C for a fraction of second. The advantage of this method is less nutrients destruction because of brief exposure to heat and destruction of all pathogenic and spoilage causing microorganisms, thereby ensuring the safety of the products.
The following are the two methods of UHT treatments that are commonly used.
1. Indirect heating and cooling in heat exchangers
2. Direct heating by steam injection or infusion of milk into steam and cooling by expansion under vacuum.
Sterilization
The original form of sterilization still in vogue is in-container method of sterilization. The temperature employed is usually between 115–120°C for 15-30 minutes depending on the product. After standardizing the fat in the milk to the desired level, homogenization and heating it to about 80°C, the milk is packed in clean containers generally glass or plastic bottles (heat resistant) for milk, and cans for evaporated milk. The product, still hot, is transferred to autoclaves in batch production or to a hydrostatic tower in continuous production.