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Separation in Food Processing and Milk Processing

Nov 22, 2014






It is the set of methods and techniques used to transform raw ingredients into food or to transform food into other forms for consumption by humans or animals either in the home or by the food processing industry. Food processing typically takes clean, harvested crops or butchered animal products and uses these to produce attractive, marketable and often long shelf-life food products. Similar processes are used to produce animal feed. Food processing dates back to the prehistoric ages when crude processing incorporated slaughtering, fermenting, sun drying, preserving with salt, and various types of cooking (such as roasting, smoking, steaming, and oven baking). Salt-preservation was especially common for foods that constituted warrior and sailors' diets, up until the introduction of canning methods. This holds true except for lettuce. Evidence for the existence of these methods exists in the writings of the ancient Greek , Chaldean, Egyptian and Roman civilizations as well as archaeological evidence from Europe, North and South America and Asia. These tried and tested processing techniques remained essentially the same until the advent of the industrial revolution. Modern food processing technology in the 19th and 20th century was largely developed to serve military needs. In 1809 Nicolas Apart invented a vacuum bottling technique that would supply food for French troops, and this contributed to the development of tinning and then canning by Peter Durand in 1810. Although initially expensive and somewhat hazardous due to the lead used in cans, canned goods would later become a staple around the world. Pasteurization, discovered by Louis Pasteur in 1862, was a significant advance in ensuring the micro-biological safety of food. In the 20th century, World War II, the space race and the rising consumer society in developed countries (including the United States) contributed to the growth of food processing with such advances as spray drying, juice concentrates, freeze drying and the introduction of artificial sweeteners, colouring agents, and preservatives such as sodium benzoate. In the late 20th century products such as dried instant soups, reconstituted fruits and juices, and self-cooking meals such as MRE food ration were developed. In Western Europe and North America, the second half of the 20th century witnessed a rise in the pursuit of convenience; food processors especially marketed their products to middle-class working wives and mothers. Frozen foods (often credited to Clarence Birdseye) found their success in sales of juice concentrates and "TV dinners". Processors utilised the perceived value of time to appeal to the post-war population, and this same appeal contributes to the success of convenience foods today.

Separations are vital to all areas of the food processing industry. Separations usually aim to remove specific components in order to increase the added value of the products, which may be the extracted component, the residue or both. Purposes include cleaning, sorting and grading operations, extraction and purification of fractions such as sugar solutions or vegetable oils, recovery of valuable components such as enzymes or flavour compounds, or removal of undesirable components such as microorganisms, agricultural residues or radionuclides. Operations range from separation of large food units, such as fruits and vegetables measuring many centimetres, down to separation of molecules or ions measured in nanometres. Separation processes always make use of some physical or chemical difference between the separated fractions; examples are size, shape, colour, density, solubility, electrical charge and volatility. The separation rate is dependent on the magnitude of the driving force and may be governed by a number of physical principles involving concepts of mass transfer and heat transfer. Rates of chemical reaction and physical processes are virtually always temperaturedependent, such that separation rate will increase with temperature. However, high temperatures give rise to degradation reactions in foods, producing changes in colour, flavour and texture, loss of nutritional quality, protein degradation, etc. Thus a balance must be struck between rate of separation and quality of the product. Separations may be classified according to the nature of the materials being separated, and a brief overview is given below.

Solid foods include fruits, vegetables, cereals, legumes, animal products (carcasses, joints, minced meat, fish fillets and shellfish) and various powders and granules. Their separation has been reviewed by Lewis and can be subdivided as follows.

Particle size may be exploited to separate powders or larger units using sieves or other screen designs. Air classification can be achieved using differences in aerodynamic properties to clean or fractionate particulate materials in the dry state. Controlled air streams will cause some particles to be fluidised in an air stream depending on the terminal velocity, which in turn is related primarily to size, but also to shape and density. Also in the dry state, particles can be separated on the basis of photometric (colour), magnetic or electrostatic properties. By suspending particles in a liquid, particles may be separated by settlement on the basis of a combination of size and density differences. Buoyancy differences can be exploited to separate products from heavy materials such as stones or rotten fruit in flotation washing, while surface properties can be used to separate peas from weed seeds in froth flotation.

Plant materials often contain valuable components within their structure. In the case of oils or juices, these may be separated from the bulk structure by expression, which involves the application of pressure. Alternatively, components may be removed from solids by extraction, which utilises the differential solubilities of extracted components in a second medium. Water may be used to extract sugar, coffee, fruit and vegetable juices, etc. Organic solvents are necessary in some cases, e.g. hexane for oil extraction. Supercritical CO2 may be used to extract volatile materials such as in the decaffeination of coffee. A combination of expression and extraction is used to remove 99% of the oil from oilseeds.

Liquid foods include aqueous or oil based materials, and frequently contains solids either in true solution or dispersed as colloids or emulsions.

Discrete solids may be removed from liquids using a number of principles. Conventional filtration is the removal of suspended particles on the basis of particle size using a porous membrane or septum, composed of wire mesh, ceramics or textiles. A variety of pore sizes and geometric shapes are available and the driving force can be gravity, upstream pressure (pumping), downstream pressure (vacuum) or centrifugal force. Using smaller pore sizes, microfiltration, ultrafiltration and related membrane processes can be used to fractionate solids in true solution. Density and particle size determine the rate of settlement of dispersed solids in a liquid, according to Stokes Law. Settlement due to gravity is very slow, but is widely used in water and effluent treatment. Centrifugation subjects the dispersed particles to forces greatly exceeding gravity which dramatically increases the rate of separation and is widely used for clarifying liquid food products. A range of geometries for batch and continuous processing are available.

Centrifugation is again used to separate immiscible liquids of different densities. The major applications are cream separation and the dewatering of oils during refining.

Differences in solubility can be exploited by contacting a liquid with a solvent which preferentially extracts the component(s) of interest from a mixture. For example, organic solvents could be used to extract oil soluble components, such as flavour compounds, from an aqueous medium.

An alternative approach is to induce a phase change within the liquid, such that components are separated on the basis of their freezing or boiling points. Crystallisation is the conversion of a liquid into a solid plus liquid state by cooling or evaporation. The desired fraction, solid or liquid, can then be collected by filtration or centrifugation. Alternatively, evaporation is used to remove solvent or other volatile materials by vaporisation. In heat-sensitive foods, this is usually carried out at reduced operating pressures and hence reduced temperature, frequently in the range 4090oC. Reverse osmosis is an alternative to evaporation in which pressure rather than heat is the driving force. Ion exchange and electrodialysis are used to separate dissolved components in liquids, depending on their electrostatic charge.

These separations are not common in food processing. Removal of solids suspended in gases is required in spray drying and pneumatic conveying and is achieved by filter cloths, bag filters or cyclones. Another possibility is wet scrubbing to remove suspended solids on the basis of solubility in a solvent.

The separation by sedimentation of two immiscible liquids, or of a liquid and a solid, depends on the effects of gravity on the components. Sometimes this separation may be very slow because the specific gravities of the components may not be very different, or because of forces holding the components in association, for example as occur in emulsions. Also, under circumstances when sedimentation does occur there may not be a clear demarcation between the components but rather a merging of the layers. For example, if whole milk is allowed to stand, the cream will rise to the top and there is eventually a clean separation between the cre