011.General description

1. General

Ultra-high-temperature treatment is a continuous inflow 

process. It is based on the rapid heating of the product to the sterilisation temperature and short holding at that temperature, followed by rapid cooling. The purpose of UHT treatment is to achieve commercial sterility of the product. Full sterilisation efficiency requires rapid heat transfer which is only possible in liquid systems. If powders are used in the formulation of a product to be UHT-treated, special attention has to be paid to proper soaking: all powder particles must be completely wetted through.

1.1 Low-Acid, Liquid-Food Products

Low-acid food products are characterised by a pH value of > 4.5 or > 4.6, depending on local food legislation. These products require careful treatment because:

a) microorganisms can grow and multiply. Bacterial spores are also able to germinate and cause product spoilage;

b) practically all pathogenic (disease-causing) microorganisms can develop.

Consumption of products thus spoiled may lead to food poisoning and/or food-borne infections.

Typical processing temperatures for low-acid foods are 130-150°C with holding times of a few seconds (60), usually 4 seconds.

1.2 High-Acid Liquid-Food Products

High-acid food products have a pH value equal to 4.5 or 4.6 or less. These are mainly fruit juices, fruit juice drinks and “belly washers”. High-acid food products are safer than low-acid foods because:

a) they are not prone to pathogenic bacteria and are therefore regarded as safe from the point of view of public health;

b) bacterial spores cannot germinate under high-acid conditions and, consequently, cannot cause food spoilage;

c) the sterilising efficiency of any heat treatment increases with decreasing pH. Thus, lower temperatures can be applied in order to achieve commercial sterility;

d) in addition, some organic acids common in fruits specifically decrease the temperature resistance of possible spoilage organisms;

e) the main spoilage microorganisms are yeast, mould, and a few bacteria (Lactobacillus, Streptococcus, and some others). Processing temperatures for high-acid foods are rather low. With few exceptions temperatures of 85 -95°C with holding times of 30 to 15 seconds, sometimes a few minutes, are sufficient (60). However, there are some exceptions, particularly tomato products which may require considerably higher temperatures, often above 100°C. The pH values of a number of fruits and fruit juices (131) are given in table 1.

1.3 Acidified Liquid-Food Products

Acidified foods are low-acid products, the pH of which has been lowered into the high-acid range by pre-processing. Such products have the same microbiological advantages as high-acid food products. The acidification process is crucial. It is necessary to achieve an even and low pH throughout the product. Acidification can be carried out biologically by the addition and growth of a starter culture, usually Lactobacillus and/or Streptococcus followed by a ripening period at a suitable temperature. Another possibility is an artificial or chemical procedure in which an acid (usually citric or lactic acid) is added to the product which has to be mixed thoroughly afterwards (Figure 1). To achieve commercial sterility, the acidified product has to be processed after the acidification process, usually by heat treatment, and then packaged.