Pasteurization

Pasteurization is a relatively mild heat treatment, in which food is heated to below 100ºC. In low acid foods (pH > 4.5, for example, milk) it is used to minimise possible health hazards from pathogenic micro-organisms and to extend the shelf life of foods for several days. In acidic foods (pH 4.5, for example, bottled fruit) it is used to extend the shelf life for several months by the destruction of spoilage micro-organisms (yeasts or moulds) and/or enzyme inactivation. In both types of food, minimal changes are caused to sensory characteristics or nutritive value. Processing containers of food, either which have a naturally low pH (for example fruit pieces) or in which the pH is artificially lowered (for example pickles) is similar to canning. It is often termed pasteurization to indicate the mild heat treatment employed.

The process was named after the French microbiologist, Louis Pasteur, whose research in the 1880s demonstrated that thermal processing would inactivate unwanted microorganisms in wine. Spoilage enzymes are also inactivated during pasteurization. Today, pasteurization is used widely in the dairy industry and other food processing industries to achieve food preservation and food safety.

Purpose of pasteurisation for different foods

Pasteurization process

Pasteurization is a mild heat treatment of liquid foods (both packaged and unpackaged) where products are typically heated to below 100 °C. The heat treatment and cooling process are designed to inhibit a phase change of the product. The acidity of the food determines the parameters (time and temperature) of the heat treatment as well as the duration of shelf life. Parameters also take into account nutritional and sensory qualities that are sensitive to heat.

Equipments-

Food can be pasteurized in two ways: either before or after being packaged into containers.

Batch method

The batch method uses a vat pasteurizer which consists of a jacketed vat surrounded by either circulating water with added steam or heating coils of hot water or direct steam. In the vat the milk is heated and held throughout the holding period while being agitated. The milk may be cooled in the vat or removed hot after the holding time is completed for every particle. As a modification, the milk may be partially heated in tubular or plate heater before entering the vat. This method has very little use for milk but some use for milk by-products (e.g. creams, chocolate) and special batches. The vat pasteurizer is used extensively in the ice cream industry as it allows for dissolution and blending of ingredients during the heating stage.

Continuous Method

Continuous process method has several advantages over the vat method, the most important being time and energy saving. For most continuous processing, a high temperature short time (HTST) pasteurizer is used. The heat treatment is accomplished using a plate heat exchanger. This piece of equipment consists of a stack of corrugated stainless steel plates clamped together in a frame. There are several flow patterns that can be used. Gaskets are used to define the boundaries of the channels and to prevent leakage. The heating medium can be vacuum steam or hot water.

Temperature	Time	Pasteurization Type
63ºC (145ºF)*	30 minutes	Vat Pasteurization
72ºC (161ºF)*	15 seconds	High temperature short time Pasteurization (HTST)
89ºC (191ºF)	1.0 second	Higher-Heat Shorter Time (HHST)
90ºC (194ºF)	0.5 seconds	Higher-Heat Shorter Time (HHST)
94ºC (201ºF)	0.1 seconds	Higher-Heat Shorter Time (HHST)
96ºC (204ºF)	0.05 seconds	Higher-Heat Shorter Time (HHST)
100ºC (212ºF)	0.01seconds	Higher-Heat Shorter Time (HHST)
138ºC (280ºF)	2.0 seconds	Ultra Pasteurization (UP)

Effect on foods

Pasteurisation is a relatively mild heat treatment and even when combined with other unit operations there are only minor changes to the nutritional and sensory characteristics of most foods. However, the shelf life of pasteurised foods is usually only extended by a few days or weeks compared with many months with the more severe heat sterilisation. Minimising postprocessing contamination is essential to ensure adequate shelf life.

Colour, flavour and aroma

In fruit juices, the main cause of colour deterioration is enzymic browning by polyphenol oxidase. This is promoted by the presence of oxygen, and fruit juices are therefore routinely deaerated prior to pasteurisation. The difference between the whiteness of raw milk and that of pasteurised milk is due to homogenisation, and pasteurisation alone has no measurable effect. Other pigments in plant and animal products are also mostly unaffected by pasteurisation. A small loss of volatile aroma compounds during pasteurisation of juices causes a reduction in quality and may also unmask other ‘cooked’ flavours. Volatile recovery may be used to produce high-quality juices but this is not routinely used, due to the high cost. Loss of volatiles from raw milk removes a hay-like aroma and produces a blander product.

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