Food and Industrial Microbiology

Module 4. Microbiology of food preservation

Lesson 13
PHYSICAL METHODS: PART-II: NON-THERMAL PROCESSING

13.1 Introduction

The application of extreme heat treatments used for food preservation affect the nutritional and organoleptic properties of food. In recent years, the consumers demand for fresher, higher quality and safe food has increased. Therefore, nonthermal methods of food preservation for the inactivation of microorganisms and enzymes as an alternative to thermal processes are being used. However, the high resistance of certain enzymes and microorganisms to nonthermal processes, especially bacterial spores, limit their application. During nonthermal processing, the temperature of foods is held below the temperature normally used in thermal processing; therefore, a minimal degradation of food quality is expected. Nonthermal process of food preservation improves food quality and enhances safety levels. Overall, most nonthermal preservation techniques are highly effective in inactivating vegetative cells of bacteria, yeast, and molds. Bacterial spores and most enzymes are however, difficult to inactivate with these procedures. Thus their use is limited to foods where enzymatic reactions do not affect food quality or where spore germination is inhibited by other prevailing conditions, such as low pH.

13.2 Low Temperature Preservation

Storage at low temperatures prolongs the shelf life of many foods. In general, low temperatures reduce the growth rates of microorganisms and slow many of the physical and chemical reactions that occur in foods. Low temperatures are used to preserve food by lowering microbial activity through the reduction of microbial enzyme activity. However, psychrophic bacteria are known to grow even at commercial refrigeration temperatures (7°C). These bacteria include members of the genera Pseudomonas, Alcaligenes, Micrococcus and Flavobacterium. Some of the fungi also grow at refrigeration temperatures. Slow freezing and quick freezing are used for long-term preservation. Freezing reduces the number of microorganisms in foods but does not kill all of them . In microorganisms, cell proteins undergo denaturation due to increasing concentrations of solutes in the unfrozen water in foods, and damage is caused by ice crystals.

13.2.1 Refrigeration

Refrigeration slows down the biological, chemical, and physical reactions that shorten the shelf life of food. Exposure of microorganisms to low temperatures reduces their rates of growth and reproduction. This principle is used in refrigera­tion and freezing. Microbes are not killed at refrigeration temperature for a considerable period of time. In refrigerators at 5°C, foods remain unspoiled but in a freezer at -5°C the crystals formed tear and shred microorganisms. It may kill many of the microbes. However, some are able to survive. Salmonella spp. and Streptococcus spp. Survive freezing. For these types of microorganisms rapid thawing and cooking is necessary.

Refrigerators should be set to below 12°C to control the growth of micro-organisms in foods. This lowered temperature also reduces the respiration rate of fruits and vegetables, which retards reactions that promote spoilage. All perishable foods should be refrigerated as soon as possible, preferably during transport, to prevent bacteria from multiplying. Refrigeration is generally used to: i) reduce spoilage during distribution of perishable foods, ii) increase the holding period between harvesting and processing; and iii) extend the storage life of commercially processed foods. All foods are not benefited from cold temperatures. For example, bananas turn black and bread goes stale when refrigerated.

13.2.2 Freezing

Freezing is also one of the most commonly used processes commercially and domestically for preserving a very wide range of food stuffs including prepared food stuffs which would not have required freezing in their unprepared state. For example, potato waffles are stored in the freezer, but potatoes themselves require only a cool dark place to ensure many months storage. Freezing makes water unavailable to microorganisms. The chemical and physical reactions leading to deterioration are slowed by freezing. White or grayish patches on frozen food caused by water evaporating into the packages air spaces called freezer burn occurs which causes deterioration of taste and appearance. This occurs in fruits, vegetables, meat, poultry and fish. While many home freezers are held at -10°C, commercial freezers are under -18°C. At this temperature, the growth of micro-organisms is almost stopped. Deteriorative microbial reactions will still occur, but over a much longer time. In addition, deteriorative enzymatic reactions will still take place during frozen storage. Uncooked fruits and vegetables must be blanched before freezing to prevent these reactions. During freezing, the water in food forms ice crystals. The rate of this phenomenon has a big impact on the quality of frozen foods:

Slow freezing

Slow freezing (e.g. home freezer) forms large ice crystals which puncture cell walls and cellular fluid is released and also results in shrunken appearance of thawed food. In this process the freezing is done for 3-72 h. This method is used in home freezer and temperature is lowered to -15 to -29°C.

Rapid freezing

During rapid freezing small, numerous ice crystals are formed and cell structure is not changed. In this process the temperature of food is lowered to about -20°C within 30 min. This process blocks or suppresses the metabolism.

The shelf life of frozen foods is largely dependent on storage conditions. Under ideal conditions, frozen foods can have a shelf life of one year. However, if foods are continuously exposed to warmer temperatures, such as the opening and closing of freezer doors, then heat shock occurs. Heat shock is when ice melts and re-forms into larger ice crystals. The best example is ice cream, which has a gritty texture if large ice crystals have developed.

Advantages of freezing are generally good retention of nutrients and prevention of microbial growth by low temperature and unavailability of water. However, disadvantages of freezing are loss of some B-Group vitamins and vitamin C due to blanching of vegetables prior to freezing and unintended thawing can reduce product quality.

13.2.3 Preservation by freeze drying

The process of freeze drying or lyophilization is commonly used these days for preservation. The food is deep frozen, after which the water is drawn off by a vacuum pump in a machine. The dry product is then sealed in foil and is reconstituted with water. This method is very useful for storing, transporting and preserving bacterial cultures. Drying or dehydration involves the removal of water from the food by controlled processes. This may be done by evaporation due to heating of the product, e.g., drying of fruits, osmotic dehydration, e.g. brining of fish and sublimation, or freeze drying e.g. in the drying of coffee.

There are two distinct stages in this technology. In the first stage, the removal of surface water depends solely on the state of the air surrounding the food, such as its temperature, relative humidity and speed. In the second phase of drying, the moisture within the food moves to the surface. As the air is heated, its relative humidity decreases, resulting in more absorption of water. Here the rate of drying is dependent on the time the moisture takes to get to the surface. The heating of the air around a food product can, therefore, cause it to dry more quickly. The principle of sublimation is used in freeze drying and lyophilization. This is the process in which a solid changes directly to a vapor without passing through the liquid phase.

13.3 Irradiation

Generally alpha, beta and gamma radiation particles are used for the preservation of food. These radiations are of high frequency with a high energy content and they have the power to break molecules into oppositely charged units termed as ions. These radiations, are, therefore, called ionizing radiations. The treatment has a range of effects, including killing bacteria, molds and insect pests, reducing the ripening and spoiling of fruits and at higher doses inducing sterility. The technology may be compared to pasteurization. It is sometimes called 'cold pasteurization', as the product is not heated. Irradiation is useful only for foods of high initial quality. A spoiled food cannot be reverted to un-spoiled state. Irradiation is not effective against viruses and prions. It cannot eliminate toxins already formed by microorganisms.

The radiation process is unrelated to nuclear energy, but it may use the radiations emitted from radioactive nuclides produced in nuclear reactors. Irradiated food does not become radioactive. National and International expert bodies have declared food irradiation as 'wholesome'. The food is exposed to controlled levels of ionizing radiation in the form of gamma radiation, X-rays and electron beams to kill harmful bacteria, pests, or parasites, or to preserve its freshness. The particle sources are readily available in the form of radio-isotope Cobalt 60. This is the most suitable gamma-particle emitter. The penetration power of different radiation particles is different. Alpha particles are stopped by a sheet of paper, beta particles can penetrate through 1-2 cm thick sheet and gamma particles can penetrate through 30-120 cm thick sheet. UV-radiation are also used as an alternative means of treatment of foods but penetration power is very low. Ultraviolet radiation is valuable for reducing surface contamination on several foods. This short-wavelength light has been used in the cold storage units of meat processing plants.

A Roentgen (r) is the quantity of gamma or X-rays which produces one electrostatic unit of electric charge of either sign in a cubic centimeter of air under standard conditions. An electrovolt (eV) is the energy gained by an electron in moving through a potential difference of 1 volt. A meV is 1 million electrovolts.

13.3.2 Mechanism of microbial inactivation by radiation

Radiations causes disruption of internal metabolism of cells by destruction of chemical bonds, DNA cleavage results in loss of cells ability to reproduce . Free radicals are formed upon contact with water containing foods. These free radicals react with cellular DNA causing radiation damage. DNA is considered “radiation sensitive” portion of cells. Shorter wavelengths have enough energy to “knock off” an electron to form a “free radical” but not high enough to “split” an atom and cause target to become “radioactive”. Interaction between free radicals and DNA is responsible for “killing effect” of ionizing radiation (IR). The ionizing radiations can cause chemical reaction and alterations of chemicals in tissues and which can be toxic or fatal to humans in high dose. Much of the reactivity of ionizing radiations in organism is with water and produces superoxide radicals (O₂^-), hydroxradicals (HO^-), hydropyroxyl radicals (HOO^-) and hydrogen peroxide. These are produced during high energy collisions of gamma rays and heavy elements (i.e. Tungsten)

Alpha Particles are positively charged particle ejected spontaneously from the nuclei of some radioactive elements and have low penetrating power and short range. The most energetic alpha particles will generally fail to penetrate the dead layer of cell covering the skin. Alpha particles are hazardous when an alpha–emmitting isotope is inside the body.

Gamma Radiations are the most widely used type of ionizing radiation as they have good penetrating effect upto 20 cm in food depending on exposure time and emitted in all directions continuously. These are produced by exposure of natural Cobalt-59 to neutrons in a reactor where reaction between the two species produces Cobalt-60. Cobalt-60 is specifically manufactured, for radiotherapy, medical device sterilization and food irradiation and is not a waste product of nuclear reactors.

Beta Particle is a charged particle emitted from a nucleus during radioactive decay. It has mass equal to 1/1837 that of a proton. A negatively charged beta particle is identical to an electron and a positively charged beta particle is called a positron. Large amount of beta radiations may cause skin burns and beta emitters are harmful if they enter in the body. Beta particle may be stopped by thin sheets of metal or plastics. The lethal doze for vegetative bacteria is 0.50-10 Kgy, bactrial spores is 10-50 Kgy, human beings and animals is 0.005-0.01 Kgy and for insects is 0.1-1.0 Kgy. There are many benefits of irradiations. These are to reduce or eliminate harmful food borne pathogens e.g. E.coli O157:H7, Camplyobacter, Salmonella, Trichinella, Listeria and many others, delays ripening of fruits and vegetables, eliminates insects in fruits and vegetables, inhibits sprouting in onions, potatoes, etc (Figure 13.1). These irradiations also replace the need for chemical fumigation. This method is cheaper than freezing and refrigeration.

Fig. 13.1 Sprout inhibition in bulbs and tubers (0.03-0.15kGY)

Irradiation dose reduced microbial spoilage (1.5 - 3 kGy) and eliminated pathogenic microbes (3-7 kGy) to improve shelf-life of meat, poultry and sea foods under refrigeration. It also reduced number of microorganisms in spices to improve hygienic quality (10 kGy)

13.3.3 Food irradiation processes

Radurization: (0.75-2.5 Kgy)

This method mimics pasteurization. It inhibits sprouting and delays ripening, kills insects and it is used for shelf life extension,

Radicidation: (2.5-10KGy)

This method will eliminate spoilage microorganisms and non spore forming pathogens. Food will not get spoiled but still may contain some pathogens

Radapperization: (10-50 kGy)

This method is also called radiation sterilization. Here reduction of microorganisms occurs to the point of sterility.

Effect of Radiation on Microorganisms : Gram negative bacteria are generally more sensitive than Gram positive forms, bacterial spores are strongly resistant, yeasts tends to be rather more resistant than molds and smallest viruses required doses of >200 kGy to achieve a million-fold reduction in their numbers. The principal targets of irradiation are nucleic acids and membrane lipids. Approximate Killing Doses of Ionizing Radiations in Kilorays (kGy for various organisms are shown in Table 13.1

Table 13.1 Approximate killing doses of ionizing radiations in kilorays (kGY)

13.4 pH Control

Almost every food, with the exception of egg whites and soda crackers, has a pH value of less than 7. Foods can be broadly categorized on the basis of their pH as high acid, acid, medium acid or low acid. Examples of each category include:

· High acid (pH: 3.7) : apples, lemons, raspberries

· Acid (pH: 3.7 to 4.6) : oranges, olives, tomatoes (some)

· Medium acid (pH: 4.6 to 5.3) : bread, cheese, carrots

· Low acid (pH: over 5.3) : meat, fish and most vegetables

Most micro-organisms grow best in the pH range of 6.5 to 7.5. Yeasts and moulds are capable of growing over a much broader pH range than bacteria. Most spoilage yeast and molds grow at pH value greater than 2.0. Few pathogens will grow below pH 4.0. This information is important, because it will help in determining food stability with respect to microbial spoilage.

13.5 Osmotic Pressure

The principle of osmosis is applied. Foods are preserved by adding salts and sugars to them. These chemicals remove the water out of microbial cells causing them to shrink in hypertonic environment, thus stopping their metabolism. Jams, jellies, fruit syrups, honey etc. are preserved by high sugar concentration. Fish, meat beef and vegetable products are preserved with salt.