By: Mohmmed Sabah Book: Food processing tecnology Page

Pasteurisation Pasteurization: is a relatively mild heat treatment, in which food is heated to below 100 ºC. In low acid foods (pH>4.5, for example milk) it is used to minimize 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 destruction of spoilage micro-organisms (yeasts or moulds) and/or enzyme inactivation

1- Three main types of pasteurization used today: 1- Lower Temperature/Longer Time (e.g. milk at 63 °C for 30 min, less often used) 2- High Temperature/Short Time (e.g. milk at 71.7 °C for 15 s) 3- Ultra High Temperature (or flash pasteurization) (e.g. Milk at 100 °C for 0.01 s

The time-temperature treatment used in pasteurization depends on : (1)the heat resistance of the particular vegetative or pathogenic microorganisms the process is designed to destroy. (2) the sensitivity of product quality to heat. The high- temperature and short-time (HTST) method involves a comparatively high temperature for a short time (e.g., 72 °C for 1 s for milk), whereas the low temperature and long- time procedure involves relatively low temperatures for longer times (e.g., 63 °C for 30 min for milk).

Optimization of the pasteurization process depends on the relative destruction rates of organisms as compared to quality factors, but generally the HTST process results in maximum product quality.

Batch Pasteurization System Methods of Pasteurization There are two basic methods, batch or continuous. continuous

3. Equipment 3.1 Pasteurization of packaged foods Some liquid foods (for example beers and fruit juices) are pasteurized after filling into containers. Hot water is normally used if the food is packaged in glass, to reduce the risk of thermal shock to the container (fracture caused by rapid changes in temperature). Maximum temperature differences between the container and water are 20 ºC for heating and 10 ºC for cooling.

Metal or plastic containers are processed using steam–air mixtures or hot water as there is little risk of thermal shock. In all cases the food is cooled to approximately 40 ºC to evaporate surface water and therefore to minimize external corrosion to the container or cap, and to accelerate setting of label adhesives.

Time–temperature relationships for pasteurization. The hatched area shows the range of times and temperatures used in commercial milk pasteurization.

3.2 Pasteurization of unpackaged liquids Swept surface heat exchangers or open boiling pans are used for small-scale batch pasteurization of some liquid foods. However, the large scale pasteurization of low viscosity liquids (for example milk, milk products, fruit juices, liquid egg, beers and wines) usually employs plate heat exchangers. Some products (for example fruit juices, wines) also require de-aeration to prevent oxidative changes during storage. They are sprayed into a vacuum chamber and dissolved air is removed by a vacuum pump, prior to pasteurization.

Pasteurized Foods The most common pasteurized food is milk Originally designed to eliminate Mycobacterium tuberculosis and Coxiella burnetti Fruit juice Spoilage yeast and bacteria, E. coli O157:H7 Beer Spoilage bacteria and yeast

Pasteurized Foods Liquid egg Salmonella and spoilage bacteria Honey Spoilage yeast Meat surfaces (steam, hot water) E. coli O157: H7, Salmonella, Campylobacter

Milk Pasteurization Time/Temperature Combinations High Temperature Short Time (HTST) o C Low Temperature Long Time (LTLT) 30 min at 63 o C These pasteurization time/temperatures are considered to be equivalent. Heat treatments are established on the basis of safety first (elimination of pathogens) and spoilage (extension of shelf life) second.

Plate heat exchanger

4. Effect on foods Pasteurization is a relatively mild heat treatment and even when combined with other unit operations (for example irradiation and chilling there are only minor changes to the nutritional and sensory characteristics of most foods. However, the shelf life of pasteurized foods is usually only extended by a few days or weeks compared with many months with the more severe heat sterilization. Minimizing post processing contamination is essential to ensure an adequate shelf life.

4.1 Vitamin loss In fruit juices, losses of vitamin C and carotene are minimised by deaeration. Changes to milk are confined to a 5% loss of serum proteins and small changes to the vitamin content Vitamin losses during pasteurization of milk Holder