MIRROR MIRROR ON THE WALL, UHT MILK IS THE FAIREST OF THEM ALL

What is UHT milk?

Based on Food Regulation 1985, Regulation 87, ultra high temperature or U.H.T. milk shall be milk which has been subjected to heat treatment by being retained at a temperature of not less than 135 ◦C for at least 2 s to render it commercially sterile and immediately aseptically packed in the sterile container. There shall be written in the label on a package containing ultra high temperature or U.H.T. milk the words “ultra high temperature milk” or “U.H.T. milk”, as the case may be.

UHT processing of milk is used to produce shelf stable product with minimal chemical damage and it may have a shelf life of up to 12 months. UHT milk usually made from cow milk, however, recently there is UHT milk made from other sources of milk, such as UHT goat milk in the UK and Italy and UHT buffalo milk in India, UK and Egypt. There are two methods to produce UHT milk including direct heating and indirect heating of UHT milk.

Direct heating performed by steam injection (steam into milk) and steam infusion (milk into steam). Steam injection involves the injection of superheated steam into a stream of milk while for steam infusion, milk is sprayed into or allowed to fall as a thin film or fine streams, through a chamber of superheated steam. The feature of these two systems is the rapid rise in the temperature of the milk from preheat to sterilization temperature by transferring the latent heat of vaporization of the steam to the milk. In this method, the steam is condensed, and the milk is diluted with water. The water is removed in the vacuum chamber after the milk passes through the high heat holding tube which cools the milk rapidly to the temperature as that of the preheated milk to ensure there is no dilution of final milk. The steam used must be of high-quality, culinary grade because poor-quality steam can lead to the off-flavors of the milk.

Indirect heating is performed by a plate or tubular heat exchanger. Heat is transfer to the milk by contact with a metal surface which heated in the other side by steam or superheated water. Water flows in the reverse direction to that of the milk to minimizes the temperature difference between the two liquids and amount of burn-on. With respect to burn-on and flavor of the product, hot water is better heating medium than steam as it enables a smaller temperature differential. Heating from the preheat temperature to sterilization temperatures and initial cooling of the sterilized milk is much slower in indirect systems than the direct systems. Indirect heating processes are more severe than direct processes because the heating times from preheat to final UHT temperature can be up to 100 s but it has equivalent bactericidal effectiveness with direct heating.

Processing line of UHT milk.

Direct Heating vs Indirect Heating

Table of comparison of Direct Heating & Indirect Heating.

Composition of UHT milk

      Table of UHT milk composition.

      Table of milk properties.

Physical and biochemical changes 

Due to the UHT treatment, the milk may taste cooked or burnt resulting from the development of a sulfurous flavor. However, the flavor has dissipated by the time of consumption. The color of the milk became slightly brown due to the reaction between protein and lactose (Maillard reaction). Polymeric reaction induced the formation of brown pigment (melanoidins). The viscosity of the milk increases slightly because most proteins are denatured which are rendered insoluble and due to aggregation of casein micelle which leads to coagulation. Moreover, gases including CO2 and O2 are lost which prevent oxidation.

UHT treatment also altered the nutrients content of the milk.

Table of the effect of UHT treatment to the nutrients content of the milk.

Examples of commercial UHT milk.

                                           

                Nestle UHT full cream milk.                        Dutch Lady UHT full cream milk.

                                                

                                                   F&N fresh UHT milk.

Conclusion

Heat treatment is the major processing applied to milk. The types of heating vary from the mildest treatment to the most severe treatment. UHT milk can be produced by direct heating and indirect heating treatment combined with aseptic packaging to produce a shelf-stable product with minimal chemical damage by killing the microorganism present.