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.
Field of application
Fermentation is an important processing step for a number of food products. Typical applications include beer, wine, various dairy products, vegetables, meat, fish, etc.
Techniques, methods and equipment
a) Alcoholic fermentation
The breakdown of simple sugars into alcohol is normally referred to as “Alcoholic Fermentation”. Alcoholic fermentation is used in beer and wine making and for the production of spirits (mostly with grape musts, sugar juices and molasses as a raw material). Yeast, usually Saccharomyces sp. (e.g. cerevisae, 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.
C6H12O6 = 2C2H5OH + 2CO2 (Glucose) = (Ethyl alcohol) + (Carbon dioxide)
This is an anaerobic process, i.e. it does not require the presence of oxygen. The temperature of fermentation is usually in the range 8 -30 °C. The temperature affects the rate of fermentation, the efficiency of conversion and the flavour and aroma of the finished product.
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.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. Traditionally, fermentation in beer and wine making has been carried out in open fermentation vessels. These are now being replaced by cylindrical closed fermenters, making recovery of the CO2 possible.
b) Lactic acid fermentation
Lactic acid fermentation is used for making yoghurt and other fermented dairy products, fermented meat products (certain types of sausages) and vegetables (i.e. sauerkraut). To start the fermentation, bacteria cultures known as starters are added to the raw material to be fermented. 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 – 40 °C. 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 at 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 (heat treatment of the substrate first and then cooling) require a specific energy consumption and use of cooling water.
The objective of germination is to activate and develop the enzyme system in the grain kernel. This activated enzyme system is necessary to activate the starch and protein break down at the mashing stage before the later brewing process.
Field of application
Germination is an important processing step within the malting process of cereals. Malted cereals (mainly barley) are used for beer production and the production of distilled drinks (e.g. whisky). Malt is also used in food flavouring.
Description of processing techniques, methods and equipment
To start germination, the grain is steeped to obtain the necessery moisture content (see
soaking, Section 4.1). The steeped grain is then transferred into germination vessels (for pneumatic malting), which have perforated steel plates floors, or onto germination floors (floor malting). The grain is layered about 70 to 150 cm thick for pneumatic malting and about 5 to 15 cm for floor malting. The germination step generally lasts between 96 and 200 hours. Depending on the equipment, the process parameters, the raw material and the finished malt to be obtained the germination process can be shorter or longer. Conditioned air (cooled or heated and humidified) is blown through (i.e. in pneumatic malting), or over (i.e. floor malting), the grain layer, to control the temperature and moisture content during germination. To prevent the grain kernels from matting (felting) the layer of grain is turned from time to time, during which water spraying can also be applied. The germination process is stopped by drying.