A number of methods of prevention can be used that can either totally prevent, delay, or otherwise reduce food spoilage. Maintaining or creating nutritional value, texture and flavor is an important aspect of food preservation, although, historically, some methods drastically altered the character of the food being preserved.
In many cases these changes have now come to be seen as desirable qualities, as with cheese, yogurt, and pickled onions. Drying is one of the most ancient food preservation techniques, which reduces water activity sufficiently to prevent bacterial growth. Refrigeration preserves food by slowing down the growth and reproduction of microorganisms and the action of enzymes which cause food to rot. Freezing is also one of the most commonly used processes for preserving a very wide range of food including prepared foodstuffs which would not have required freezing in their unprepared state.
Vacuum-packing stores food in a vacuum environment, usually in an air-tight bag or bottle. The vacuum environment strips bacteria of oxygen needed for survival, thereby slowing spoiling. Vacuum-packing is commonly used for storing nuts to reduce the loss of flavor from oxidation.
Salting or curing draws moisture from the meat through a process of osmosis. Meat is cured with salt or sugar, or a combination of the two. Nitrates and nitrites are also often used to cure meat and contribute to the characteristic pink color, as well as inhibition of Clostridium botulinum. Sugar is used to preserve fruits, either in syrup with fruit such as apples, pears, peaches, apricots, plums, or in crystallized form where the preserved material is cooked in sugar to the point of crystallisation and the resultant product is then stored dry.
This method is used for the skins of citrus fruit candied peel , angelica, and ginger. The use of sugar is often combined with alcohol for preservation of luxury products such as fruit in brandy or other spirits. These should not be confused with fruit flavored spirits such as cherry brandy.
Smoking is used to lengthen the shelf life of perishable food items. This effect is achieved by exposing the food to smoke from burning plant materials such as wood.
Most commonly subjected to this method of food preservation are meats and fish that have undergone curing. Fruits and vegetables like paprika, cheeses, spices, and ingredients for making drinks such as malt and tea leaves are also smoked, but mainly for cooking or flavoring them.
It is one of the oldest food preservation methods, which probably arose after the development of cooking with fire. Preservative food additives can be antimicrobial. These inhibit the growth of bacteria or fungi, including mold, or antioxidant, such as oxygen absorbers, which inhibit the oxidation of food constituents. Common antimicrobial preservatives include calcium propionate, sodium nitrate, sodium nitrite, sulfites sulfur dioxide, sodium bisulfite, potassium hydrogen sulfite, etc.
Other preservatives include formaldehyde usually in solution , glutaraldehyde kills insects , ethanol, and methylchloroisothiazolinone. Pickling is a method of preserving food in an edible anti-microbial liquid. Pickling can be broadly categorized into two categories: chemical pickling and fermentation pickling.
An advantage of short term storage on agar medium is that it is easy to recover the microorganisms later. Just streak them onto fresh culture plates or inoculate them into liquid culture medium and incubate at the optimum growth temperature! However, over a prolonged period of time, the agar dries up and nutrients in the culture media are depleted by the microorganisms.
Metabolic waste products also accumulate to toxic levels. Nutrient starvation and toxic waste product build-up eventually cause the death of the stored microorganisms. Cryopreservation is suitable for a wide range of bacteria, algae, fungi, viruses, and protozoa.
Ultralow temperatures drastically reduce enzymatic activity and, thus, metabolic processes of microorganisms. Alternatively, cells can also be resuspended in skim milk before being frozen [2]. To maximise the recovery rate, it is recommended to freeze cultures at the stationary phase of growth when cell concentration is highest. Freeze drying lyophilisation removes moisture from frozen samples by converting water into ice in a vacuum, thus arresting microbial metabolic processes and transforming cells into dry pellets for convenient storage and transportation in ampoules Figure 3.
Freeze drying has been used to preserve bacteria, algae, yeasts, viruses, and sporulating fungi [4]. However, lyophilisation is not suitable for certain bacteria and non-sporulating fungi as they cannot survive the stresses of freeze drying. Lyophilized cultures can be recovered by rehydrating the cell pellets in growth media bacteria and algae , growth medium containing the bacterial hosts bacteriophages , or water fungi and yeasts before inoculating into fresh culture media [5].
Microencapsulation, where cells are entrapped in a matrix prior to storage, has been proposed as a long term microbial preservation method which does not expose the microorganisms to the harsh stresses of freezing and drying. The matrix shields the cells and increases stability during storage.
The temperature and the type of medium chosen should support a slow rather than a rapid rate of growth so that the time interval between transfers can be as long as possible. The transfer method has the disadvantage of failing to prevent changes in the characteristics of a strain due to the development of variants and mutants. It is a simple method, any special apparatus are not required.
Freeze-drying is a process where water and other solvents are removed from a frozen product via sublimation. Sublimation occurs when a frozen liquid goes directly to a gaseous state without entering a liquid phase.
It is recommended using slow rates of cooling, as this will result in the formation of vertical ice crystal structures, thus allowing for more efficient water sublimation from the frozen product.
Lyophilization Freeze-Drying Freeze-dried products are hygroscopic and must be protected from moisture during storage. Under these conditions, the microbial cells are dehydrated and their metabolic activities are stopped; as a result, the microbes go into dormant state and retain viability for years.
Freeze-drying method is the most frequently used technique by culture collection centers. Many species of bacteria preserved by this method have remained viable and unchanged in their characteristics for more than 30 years. Advantage of Lyophilization Only minimal storage space is required; hundreds of lyophilized cultures can be stored in a small area. Small vials can be sent conveniently through the mail to other microbiology laboratories when packaged in a special sealed mailing containers.
Lyophilized cultures can be revived by opening the vials, adding liquid medium, and transferring the rehydrated culture to a suitable growth medium. The slants are incubated for 24hr or more and are then stored in a refrigerator. These cultures are periodically transferred to fresh media. Time intervals at which the transfers are made which varies with the origin and condition of growth.
They are then covered with sterile mineral oil to a depth of 1 cm above the tip of slant surface. Many bacterial species can preserved by covering the growth on the agar slants with steril mineral oil. Oil must cover the slants completely. Oil reduces the loss of water and also slow down the exchange of gas within organism and sorrounding The layer of paraffin prevents dehydration of the medium and by ensuring an aerobic condition, the microorganism remain in dormant state. The tubes are stored at room temperature.
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