Sterilised milk shall be milk which has been filtered, homogenized and thereafter heated to and maintained at a temperature of not less than 100 °C (e.g. 108-110 °C) for a length of time sufficient to render it commercially sterile and shall be packed in hermetically sealed containers that stated in Food Regulations 1985. Milk is a white liquid produced by the mammary glands of mammals. Based on the sterilised milk bear brand, the composition of milk consists of fat, protein, carbohydrates, calcium and vitamins. The purpose of sterilised milk is to kill microorganism capable of causing spoilage and also to extend the shelf life of the milk product. Milk can be sterilised either in bottles or others sealed containers, which involves continuous sterilization followed by aseptic packaging. Clostridium botulinum is the most heat resistant pathogenic spores and shall be inactivated them. This bacteria is rarely found in a raw milk, there are the most heat resistant spores which causes spoilage but not pathogenic.

Sterilised Milk 

Source: Google image 

Retrieved on 16 April 2018

Nutritional Labelling of sterilised milk

 Source: Google image

 Retrieved on 29 April 2018

Milk also known as emulsion or butterfat globules that contains dissolved carbohydrates and protein with minerals. Butterfat is a type of triglycerides or fat formed from fatty acid and glycerol such as oleic acid and palmate. The pH of the milk is range between 6.4 and 6.8 and may be change according to time. Milk also contains fat that secreted in the form of fat globules. Each of the fat globules acts as emulsifier which consist of hydrophilic part (water loving) and hydrophobic part (oil-loving). Each part of the phospholipid play an important role in oil- in- water emulsion. Therefore, milk shall be in emulsion formed to prevent immiscible solution and the milk shall be liked opaque. The opacity of milk is due to fat, proteins and minerals. The function of lactose that contain in milk act as sweetener and flavour enhancer. Milk contains all the essential nutrients for all physiological function of the body system.

Raw milk is comes from mammals such as cow, goats and others and nothing is added into it. Based on regulation, (1985), milk shall contain not less than 3.25% of milk fat and 8.5% of non-fat milk solids and shall not contain any added water, permitted food additive and other added substance. Milk is a low acid food, pH > 4.5. Decreasing in pH is due to production of lactic acid from lactose. The common sporeformers found in milk are Bacillus stearothermophilus and Bacillus sporothermodurans. They may cause spoilage, but they are not pathogenic. 

Flow chart of processing line of sterilized milk 

Sterilised milk should be filtered, homogenised and heated to at least 100 °C. Homogenisation is an important part to reduce milk from forming a layer of cream that rising to the top of the package. It is a process of giving milk its rich, white colour and smooth texture. A homogeniser forces the milk under high pressure through a valve that breaks up fat globules into small sizes. So, the higher the pressure, the smaller the fat globules will be produced. Homogenisation process will cause small fat globules leading to less cream formation at the top of the packaging. The milk is clarified and homogenised at about 50 °C and filled into bottles which are hermetically sealed. Physical changes might be occurred due to formation of air bubbles, besides creaming and fat crystallization at low temperature. 

Next, during heat treatment the milk will heat under the high temperature at 118 °C for 20 min. Biochemical changes might be occurred due to presence of enzymes, lipase, proteinases and phosphatases. So, sterilisation able to kill all microorganisms including spores, all enzymes inactivated and numerous chemical changes such as browning reaction occur and formic acid is formed. Milk can be either sterilised in bottle or sealed containers, such as cans or by continuous UHT processing followed by aseptic packaging. Sterilised milk consists of batch sterilization and continuous sterilization. Glass bottles are sealed with a brown cap, while plastic bottles are heat-sealed with an aluminium foil cap. The milk is sterilised at 108-111 °C for 25- 30 min with pasteurised steam. Cooling may be either by immersion in water or more gradually by heat loss to the atmosphere, following removal from autoclave. When milk is processed in plastic bottles, the steam chamber is over-pressurized to maintain the integrity of the seal and bottle. The advantage of using sterilization method enable it to be kept at room temperature. Therefore, the milk can be stored at room temperature for several months. The chemical changes could be occurred if the milk expose to light, thus it will induce off- flavour.

Apart from that, packaging material must be sterilised before being filled with the sterile milk. Combination of hydrogen peroxide and heat is very effective sterilant for the surface of packaging material. Gamma radiation also can be used to sterilise heat-sensitive packaging material. It is important that sterile environment is maintained during aseptic packaging. Food Regulations, (1985), states that there shall be written in the label on a package containing sterilized milk the words ‘sterilised milk’. 

In summary, sterilised milk has a creamy appearance due to Maillard browning compounds, distinct cooked flavour, browner colour than raw milk and more loss of nutrients. It is still produced in retortable disposable plastic bottles with a metal foil cap, rather than in returnable glass bottles.