Lesson 24. Physical Methods Of Food Preservation -II

24. Separation / Filtration

Separation operations are based on the principles that depend on the physical properties of the components of the liquid product and their differences. A separation process such as centrifugation relies on differences in solubility and specific gravity to separate blends of immiscible materials. In case of membranes, separation takes place with the use of difference in size of osmotic pressure of the components of liquid food.

In last two decades, separation processes have emerged in which membranes have played a prominent role in the isolation and enrichment of functional ingredients by concentrating, fractionating and purification of fluids or gases.

Membrane processing can be applied to vegetable protein concentrates and isolates derived from soybean, peas and canola. It permits concentration and separation without the use of heat. They make it possible to standardize the concentrations of specific components, and to decrease the concentration of undesirable components.

Table: Applications of Membranes in Food Industry.


Application in Food Industry


Removal of particles from beverages and dairy products, clarification of alcoholic beverages, waste water management.


Concentration of egg white, blood serum proteins, whey protein concentrate, clarification of dextrose, waste water recovery and use, recovery of by-products from waste water, pre-concentration of dilute sugar solutions and syrup concentration.


Desalting of lactose, whey protein concentrates and antibiotics, recycling of caustic cleaning solutions, softening water to reduce scaling on equipment and heat exchange surfaces.

Reverse Osmosis

Production of pure water, concentration of maple sap, recovery of sugar from candy, recovery of oil-seed protein.

(GEA,Technology Watch,2005)

 24.1 Concentration

Foods are concentrated for many of the same reasons that they are dehydrated. Concentration can be a form of preservation but this is true only for some foods. Concentration reduces weight and volume and results in immediate economic advantages.

Nearly all liquid foods which are dehydrated are concentrated before drying. This is because in the early stages of water removal, moisture can be more economically removed in highly efficient evaporators than in dehydration equipment. Further, increased viscosity from concentration often is needed to prevent liquids from running off drying surfaces or to facilitate foaming or puffing.

The more common concentrated fruit and vegetable products include items as fruit and vegetable juices and nectars, jams and jellies, tomato paste, many types of fruit purées used by bakers, candy makers and other food manufacturers.

The methods used to concentrate liquid foods depend on the composition of the product, the sensitivity of the components to decompose with heat, and the economics of the process.

Methods of concentration

  1. Solar concentration - One of the simplest methods of evaporating water. A typical example of this method is production at farm level in developing countries of fruit pastes/leathers (such as apricot or plum pastes).

  2. Open Kettles - Some foods can be satisfactorily concentrated in open kettles that are heated by steam, as in case of jellies and jams, tomato juices and purées and for certain types of soups. High temperatures and long concentration time should be avoided in order to avoid thickening and burn-on of product to the kettle wall.

  3. This type of evaporation is highly recommended for small scale operations in developing countries, widely used for jellies, jams and marmalades.

  4. Flash evaporators- Flashing is a process which gives rise to vaporization flow rate more significant thanthat obtained during simple evaporation. The industrial applications of flashing are varied. One of the most important concern is the desalination of seawater in multi-stage flash evaporators.

  5. Vacuum evaporators- Several vacuumised vessels in series are constructed, so that the product moves from one vacuum chamber to the next and thereby becomes progressively more concentrated in stages. In this way maximum use of heat energy is made. Such system is called a multiple effect vacuum evaporator and is a widely used system for concentrated tomato paste.

  6. Freeze Concentration - This process has been known for many years and has been applied commercially to orange juice. However, high processing costs largely due to losses of juice occlude [unclear] to the ice crystals.

  7. Ultrafiltration and reverse Osmosis-

    UF is used in a variety of industries mainly due to its low energy requirements, non-thermal nature and simplicity. It is ideal for removal of anti-nutritional factors (i.e. oligosaccharides, phytic acid, and some trypsin inhibitors from vegetable protein processing) to produce purified protein isolates or concentrates with superior functional properties

    Food products concentrated by reverse osmosis are generally more heat labile; they include high priced fruit juices, dairy products and pharmaceuticals. In most reverse osmosis applications, the liquid products are pre-filtered using membranes with larger pore openings, such as ultrafiltration and microfiltration membranes


  • Concentration exposes food to 100° C or above for prolonged periods, which can cause major changes in organoleptic and nutritional properties.

  • Cooked foods and darkening of colour are two of the more common heat induced results.

  • Concentration at a temperature of 100 °C or slightly above will kill many micro-organisms but cannot be depended on to destroy bacterial spores.

  • When the food contains acid, such as fruit juices, the micro-organisms kill will be greater but sterility is doubtful.

  • When concentration is done under vacuum, many bacterial types not only survive the low temperatures but multiply in the concentrating equipment.

24.2 Irradiation

Food irradiation is one of the food processing technologies available to the food industry to control organisms that cause food-borne diseases and to reduce food losses due to spoilage and deterioration. Food irradiation technology offers some advantages over conventional processes. Each application should be evaluated on its own merit as to whether irradiation provides a technical and economical solution that is better than traditional processing methods.


For products where irradiation is permitted, commercial applications depend on a number of factors including the demand for the benefits provided, competitiveness with alternative processes and the willingness of consumers to buy irradiated food products. There are a number of applications of food irradiation. For each application it is important to determine the optimum dosage range required to achieve the desired effect. Too high a dosage can produce undesirable changes in texture, colour and taste of foods.

The way in which the radiation dose absorbed is measured differs according to the source of radiation, and various dosimetry techniques exist.


Dose-range needed for effective treatment


Inhibition of sprouting in potatoes and onions

0 03—0 1

Sterilization of insects and parasites

0 03—0.2

Killing of insects and parasites

0 05—5

Reducing by 10* the number of vegetative bacteria, moulds, and fungi


Reducing by 10' the number of dried or

frozen vegetative bacteria, fungi, and spores


Reduction by 10' the number of viruses


Sterilization of food


• 1 kGy (Gray) = 100 000 rad (= 1 Joule/kg).


Irradiation can extend the shelf-life of foods in a number of ways. By reducing the number of spoilage organisms (bacteria, mould, fungi), irradiation can lengthen the shelf life of fruits and vegetables.

Since ionizing radiation interferes with cell division, it can be used as an alternative to chemicals to inhibit sprouting and thereby extend the shelf life of potatoes, onions and garlic. Exposure of fruits and vegetables to ionizing radiation slows their rate of ripening. Strawberries, for example, have been found to be suitable for irradiation. Their shelf-life can be extended three-fold, from 5 to 15 days. Ionising radiation can also be used as an alternative to chemical fumigants for disinfestations of grains, spices, fruits and vegetables.

The Expert Committee concluded that irradiation to an overall dose of 10 kGy (kilograys) presents no toxicological hazard and introduces no special nutritional or microbiological problems, thus establishing the wholesomeness of irradiated foods up to an overall average absorbed dose of 10 kGy. Data were insufficient to formulate conclusions on applications of food irradiation above 10 kGy.

More than 30 countries have given clearances for the use of food irradiation to process some 40 food items and approximately 30 facilities world-wide treat food by irradiation processing. Approvals for additional items are being considered in many countries and many food irradiation facilities are being planned.

24.3 MAP / CAP

 MAP and CAP can supplement proper temperature and relative humidity management in maintaining quality and reducing losses of tropical fruits. Its beneficial effects include reduction of respiration rate, inhibition of ethylene production and action, retardation of ripening and maintenance of nutritional quality.

Short term exposure of tropical fruits to O2 levels below 1% and /or CO2 levels below 12% can reduce incidence and severity of physiological disorders (such as chilling injury), pathogens and insects.

Controlled Atmosphere Packaging

A defined mix of gases is maintained or controlled over time by some external apparatus or internal chemical reactions. An example of a controlled atmosphere is the Transfresh container (used for ocean transport of fruit) which have a mechanical means of measuring the container atmosphere and adjusting the gas levels to maintain a predetermined mixture of CO2, O2 and N2 during shipment. Placing an oxygen absorbing sachet inside a barrier package is an example of a controlled atmosphere package using a chemical reaction. The sachet absorbs any oxygen that transmits through the package barrier. Sulfur dioxide producing pads used in long term shipment or storage of table grapes to prevent growth of gray mold are another example of controlling package atmosphere.

Modified Atmosphere Packaging

Gas atmosphere is modified by (1) direct injection of gases (often CO2 or nitrogen) into a package, (2) evacuating air from the package or (3) interaction between package contents and the air in the package causing the package atmosphere to modify over time. With proper packaging, the natural respiration of produce causes O2 levels to drop and CO2 levels to rise. Modified atmosphere packages have an atmosphere different from ambient air but, that atmosphere can change over time.

In the case of produce, package atmosphere is affected by the transmission rates of the packaging material and changes in storage temperatures. Higher temperatures lead to higher respiration rates, creating lower O2 levels in the package atmosphere and higher concentrations of CO2. Hence, the atmosphere inside the package is modified but not controlled.

Carbon Dioxide (CO2)

Carbon Dioxide is a natural gas, which is found in small concentrations in the air. It inhibits the increase of most aerobic bacteria and mildew. CO2 is the most important gas in the packaging of food under modified atmospheres. Higher the CO2 concentration the longer the durability of the perishable food. However fat and water absorb CO2 gases very easily and excessive CO2 concentrations causes quality failures regarding taste, loss of humidity and the concentration of the packaging (so called vacuum effect). If CO2 is intended to regulate the growth of bacteria and mildew a concentration of at least 20% is recommended.

Nitrogen (N2)

N2 is an inert gas that is used to expel air especially Oxygen out of the packaging. It is also used as a filling gas that equalizes the effect of CO2 absorption by the perishable food. It reduces the vacuum effect and is also a natural component of the air.

Oxygen (O2)

O2 is an essential gas for the respiration of all living beings and supports the decay of perishable food. It is the condition for the growth of aerobic micro organisms. In general Oxygen should be excluded for the MAP but in some cases a determined amount of Oxygen brings quite positive results.

  • It keeps the natural colour of the perishable food (effect of freshness).

  • It makes possible respiration, especially for fruits or vegetables.

  • It inhibits the growth of anaerobic micro organisms in several kinds of fish and vegetable.

Last modified: Thursday, 22 August 2013, 9:12 AM