FOOD PRESERVATION

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Food preservation is the technique used to prevent food spoilage. It encompass all the methods employed to inhibit or delay the growth of microorganisms especially food spoilage microbes in food and/or food products. Various methods exist for the preservation of food after production and usage. Food preservation amongst other things ensures that the shelf life of food products is extended and that the likelihood of food spoilage and food poisoning or intoxication through the ingestion of food is largely contained. Food whether for human or animal consumption contains nutritive factors that also support the growth of microorganisms (which are ubiquitous in nature); and when food products are not properly preserved after processing or production, they are likely to serve as a medium via which some infectious diseases especially those that are food-related (i.e. food borne diseases) can be spread or transmitted in a defined human preservation. It is because of this that food preservation is critical since it is mainly aimed at preventing food spoilage as well as the transmission of food borne diseases via the ingestion of contaminated food products. In food preservation processes or techniques, the intrinsic or extrinsic factors of food that encourage microbial activity leading to food spoilage are contained.

Food preservation techniques helps to maintain the nutritive value of foods while extending the shell life of the food and keeping the food safe for human consumption by limiting microbial growth in it as much as possible. Smoking, salting and drying are typical examples of some local or traditional methods of preserving foods and food products in most rural communities; and these practices however obsolete is still being used to preserve some food products in rural areas where the conventional methods of food preservation are not readily available. Radiation, heat treatment, fermentation, chemical preservation, filtration and the use of low or high temperatures are some conventional preservation techniques employed in food preservation. Pasteurization, canning and appertization are examples of heat treatment techniques employed in the preservation of food products especially milk and other canned foods. In the use of chemical methods, foods can be preserved by the incorporation of chemicals such as organic acids, sodium nitrite, sulphite, and ethyl formate which disrupt important metabolic processes in foods. Despite their efficacy and significance in food preservation, the use of chemical methods for food preservation is often restricted by laws and under strict control due to the carcinogenic effects of some chemicals used for such purposes.

The inhibition of vital cellular and metabolic processes in microbes present in foods will adversely affect the growth of the organism and in such circumstances the food product can be preserved and made safe for consumption. Ionizing and non-ionizing radiation including the use of ultraviolet (UV) radiation are commonly employed in food industries as a sterilization technique to sterilize equipment and instruments used for food production. Pasteurization is the process of heating milk to destroy food spoilage organisms and microbes that cause food borne diseases or infection present in them. Appertization is another food preservation technique that is similar to pasteurization; and it is also employed in food industries to extend the shelf life of food products and thus limit microbial activities in them. Some food products such as wine and beer may lose their taste, flavour or quality when they undergo an unusual heat treatment (e.g. pasteurization). For such food products or beverages that are heat-sensitive in nature as aforementioned, the best food preservation technique that can be employed is filtration – in which microorganisms are removed with the help of membrane filters (Figure 1). Membrane filters are thin sheets or layers which hold back microorganisms (e.g. bacteria) when fluids are passed or drawn though it. They are usually fitted into a membrane filtration system which holds back or remove certain microorganisms present in fluids passed through it.

Figure 1. Illustration of membrane filter. Typical applications for membrane filters are cell retention, particle collection, clarification and sterile filtration of aqueous solutions, particulate analysis, and other microbiological analysis. A wide selection of membrane filters exist for the microfiltration of liquid substances; and these membrane materials have varying pore sizes ranging from 0.1 µm to 8 µm and for the ultrafiltration with molecular weight cut offs from 300,000 to 1,000 Dalton. Photo courtesy: https://www.microbiologyclass.com

Membrane filters provide an efficient way of removing microorganisms from fluids via filtration technique. Membrane filters can also be used to enumerate the number of bacteria present in a given sample (e.g. water and beverages). To enumerate microbial numbers in a sample for example, the fluid or liquid is drawn through the membrane filter and the filter is aseptically placed on culture media and incubated. The membrane filter is macroscopically checked for microbial growth in the form of colonies. The number of bacteria present in the filtered sample can then be determined via colony counting. Membrane filters are also employed in water treatment plants to evaluate the microbiological quality of water; and they also play huge roles in beverage producing companies where they are used to filter heat-sensitive liquids.  

  • PASTEURIZATION: Pasteurization is simply defined as the process of heating food during its production in order to destroy pathogenic microorganisms or food spoilage organisms in them. Food products or foods such as milk, yoghurt, dairy products and other liquid products are heated in this manner to destroy microorganisms that cause disease and spoilage in them. Pasteurization is the reduction of microbial growth in heat-sensitive products such as milk in order to inhibit or kill pathogenic microorganisms present in them. This technique of reducing or killing food spoilage organisms in food was discovered and developed by Louis Pasteur in 18-19th century as a method of preserving wine during storage; and Pasteur showed that pasteurization (which is the mild heating of milk) could kill pathogenic microorganisms in broth. Pasteurization is an important technique in most industrial applications especially in food industries where it is applied to contain food spoilage in food or food products. It is usually carried out at a temperature range of between 60-80oC; and pasteurization does not last too long when applied in food production. Generally, temperatures below 100oC are used during pasteurization of milk and other liquid food products; and the heating is usually done at various time intervals and at different temperature range. Pasteurization of food products or food is usually carried out at varying temperatures such as high-temperature short time. High-temperature short time (HTST) is applied in both pasteurization and appertization techniques to destroy pathogenic and spoilage microorganisms present in food. HTST is defined as the heating of milk and other liquid food products at high temperatures and at varying time intervals that can range from seconds to minutes and hours in order to inhibit or kill pathogenic microorganisms and other spoilage microbes present in them. Apart from inhibiting or killing pathogenic microorganisms present in food, pasteurization technique also helps to extend the shelf-life of the food being processed. Pasteurization technique is effective for killing milk flora as well as other disease-causing microorganisms present in the food. In general, pasteurization is a mild heating of milk and other heat-sensitive products and it is carried out at temperatures below 100oC. Pasteurization technique is effective in the killing of some food-borne pathogens including those that cause salmonellosis (Salmonella species) and even tuberculosis (Mycobacterium species). Metabolic products such as enzymes produced by food flora in food products and which stimulate food spoilage can also be inactivated and rendered ineffective during the process of pasteurization. 
  • APPERTIZATION: Appertization is simply defined as the heat-treatment of food at certain temperature levels that inhibit or kill pathogenic microorganisms present in the food. Unlike pasteurization which uses temperatures below 100oC to kill microbes in food, appertization (which was discovered by Nicolas Appert in the 18th century) is generally a food preservation technique that is used to heat food at temperatures of 40oC which is lower than that used in pasteurization. In appertization, foods or food products are exposed to varying low temperatures and time which makes the food microbiologically fit for consumption. Food products are heated at this temperature in containers or cans in order to bring the microbial load in the food to levels that are generally safe for human consumption. Appertized food or food products are not necessarily sterile but they have been so treated at temperature levels that significantly reduce or limit the possibility of microbial growth in the food product during storage. This process of appertization is used in food industries such as in canning to produce canned food products. The rationale behind appertization is based on the fact that viable organisms that survive during processing will not be able to grow under normal storage conditions; and these organisms will be below certain levels after a particular temperature (e.g. 40oC) is applied to the food and at a specific time period during its processing. However, appertization can still be carried out at high temperatures (e.g. above 100oC) for some food products. High-temperature short time (HTST) is applied in both pasteurization and appertization techniques to destroy pathogenic and spoilage microorganisms present in foods including the spores of some food-borne pathogens such as Clostridium species. HTST is defined as the heating of milk and other liquid food products at high temperatures and at varying time intervals that can range from seconds to minutes and hours in order to inhibit or kill pathogenic microorganisms and other spoilage microbes present in them.       

Further reading

Bushell M.E (1998). Application   of   the   principles   of   industrial   microbiology   to   biotechnology (ed. Wiseman, A.) Chapman and Hall, New York.

Byong H. Lee (2015). Fundamentals of Food Biotechnology. Second edition. Wiley-Blackwell, New Jersey, United States.

Frazier W.C, Westhoff D.C and Vanitha N.M (2014). Food Microbiology. Fifth edition. McGraw-Hill Education (India) Private Limited, New Delhi, India.

Jay J.M (2005). Modern Food Microbiology. Fourth edition. Chapman and Hall Inc, New York, USA.

Bushell M.E (1998). Application   of   the   principles   of   industrial   microbiology   to   biotechnology (ed. Wiseman, A.) Chapman and Hall, New York.

Farida A.A (2012). Dairy Microbiology. First edition. Random Publications. New Delhi, India.

Nduka Okafor (2007). Modern industrial microbiology and biotechnology. First edition. Science Publishers, New Hampshire, USA.

Roberts D and Greenwood M (2003). Practical Food Microbiology. Third edition. Blackwell publishing Inc, USA.

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