Fermentation in food industry
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Fermentation is the controlled action of selected micro-organisms to alter the texture of foods, to preserve foods by the production of acids or alcohol, or to produce or modify flavours and aromas. It also preserves products by lowering the pH tolerance limits of many microorganisms.
Field of application
Fermentation is an important processing step for a number of FDM products. Typical applications include beer, wine, various dairy products, vegetables, meat and fish. Alcoholic fermentation is used in beer and winemaking and for the production of spirits, mostly with cereals, grape musts, sugar juices and molasses as a raw material. Lactic acid fermentation is used for making yoghurt and other fermented dairy products, fermented meat products such as certain types of sausages and vegetables, i.e. sauerkraut. In the lactic acid fermentation of vegetables, the sliced raw material, e.g. green cabbage in the case of sauerkraut production, is salted and then fermented under anaerobic conditions.
Description of techniques, methods and equipment
There are two types of fermentation processes, i.e. alcoholic fermentation and lactic acid fermentation. The breakdown of simple sugars into alcohol is normally referred to as alcoholic fermentation. Yeasts, usually Saccharomyces sp, e.g. cerevisae or bayanus, are used to produce ethanol from carbohydrates and very small amounts of other organic compounds. This conversion can be represented by the following equation:
2C2H5OH + 2CO2 = C6H12O6
(Ethyl alcohol)+ (Carbon dioxide)=(Glucose)
This is an anaerobic process, i.e. it does not require the presence of oxygen. The temperature of fermentation is usually in the range of 8 to 30 ºC. The temperature affects the rate of fermentation, the efficiency of conversion and the flavour and the aroma of the finished product. The pH may also be adjusted. This ensures that the fermentation is efficient and produces the required flavour. The type of yeast species used affects the rate, efficiency, flavour and aroma and is, therefore carefully selected to give the desired results. Selected strains of yeasts are often used to optimise the alcohol yield and the production of aromatics as secondary components. Nitrogen, vitamins and trace elements are usually added as yeast nutrients. Traditionally, fermentation in beer and winemaking has been carried out in open fermentation vessels. Some have been replaced by cylindrical closed fermenters, making recovery of CO2 possible.
In lactic acid fermentation, lactose or other sugars are converted into lactic acid and small amounts of other components. Lactic acid formation is accompanied by a decrease in pH, which is important for the taste, the aroma and the preservation of the product. There are several species of bacteria which are able to produce lactic acid. Each species gives its own typical taste and aroma. Lactic acid fermentation is an anaerobic process. It is sometimes necessary to remove as much of the oxygen as possible to enhance the fermentation process. Lactic acid fermentation is carried out at 20 to 40 ºC. To start the process, bacteria cultures known as starters are added to the raw material to be fermented. The preparation of starter cultures is a sensitive process since the risk of airborne infection must be reduced to an absolute minimum. Starter cultures must, therefore, be prepared in a separate room supplied with filtered air with a slight positive pressure compared to normal atmospheric pressure. The equipment cleaning system must also be carefully designed to prevent detergent and sterilant residues from coming into contact with the cultures and spoiling them. These very high hygiene constraints, coupled with the requirements of the temperature regulation, i.e. heat treatment of the substrate first and then cooling, require a specific energy consumption and use of cooling water.
European Commission, 2006. BREF Reference document on Best Available Techniques in the Food, Drink and Milk industries
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