Mixing, conching, homogenisation
The aim of this group of operations is to obtain a uniform mixture from two or more components or to obtain an even particle size distribution in a food material. This can also result in improved characteristics and eating quality.
Field of application
These operations are widely applied in almost all sectors of the food industry.
Description of techniques, methods and equipment
a) Mixing, blending
Mixing (blending) is the combination of different materials. In mixing, the spatial distribution of the separate components are reduced to obtain a certain degree of homogeneity. Various mixing operations can be distinguished in the food industry: • solid/solid mixing as, for example, carried out during the production of animal feed or food products, such as blends of tea and coffee, dried soup, cake mixes, custard, ice-cream mixes, malt blends etc. • solid/liquid mixing as, for example, carried out during the production of canned foods, dairy products, etc. This type of mixing is also used in the production of chocolates and sweets, where the ingredients are mixed in a more or less liquid state then and solidify on cooling • liquid/liquid mixing, used during the production of emulsions like mayonnaise, margarine and mixtures of solutions. This type of mixing is also used to blend different wines or mineral water with each other or with additives or extra ingredients. • liquid/gas mixing, used, for example, in making ice-cream, whipped-cream and some sweets. During spray drying the liquid-phase is mixed in a stream of gas.
Mixing equipment commonly used for solid/solid mixing includes: rotating drums, rotary mixers, paddle and ribbon mixers and mixing screws in cylindrical or cone-shaped vessels and variable speed drive screws. Generally in solid/solid mixing, cyclones are used as an integral part of the process to recover the particulate matter (dust) in extracted air. The recovered material is then reprocessed. For viscous solid/liquid mixing, kneading machines are used. For low viscous solid/liquid mixtures and liquid/liquid mixtures various types of stirrers, impellers and agitators are applied. For liquid/gas mixing, atomisers are used to produce small liquid droplets, which are brought into contact with a stream of gas. In making ice-cream, whipped-cream or a foam, small gas bubbles are fed into a liquid.
b) Homogenisation
The aim of homogenisation is to attain a more even particle size or a more homogeneous blend of materials. It is, for example, applied to whole milk to reduce the size of fat globules so that they stay evenly divided in the milk, thereby preventing skimming of the fat. The liquid (whole milk) is pressed under high pressure (200 -300 bar) through a small orifice.
c) Conching
Conching is a special method of kneading used in the chocolate industry. The molten chocolate mass is placed in a special trough-shaped vessel and is kneaded by a granite roller moving slowly back and forth. The aim of conching is to reduce the viscosity of the mass and to improve flavour and texture.
