New Trends in the Technology of Food Preservation—An Introduction

Recently, concern has grown about the safety and preservation of packaged foods. In the food industry around the world, a counterplan for food preservation has been devised, laying stress on Clostridium botulinum. Food preservation is composed of food packaging technology, and sterilization and control technology of food microorganisms. When these two technologies are done perfectly, packaged foods can be preserved for a long period. This article describes food packaging technology and the behavior of microorganisms, and the sterilization and control of microorganisms in cookie packaged foods in box.

Food Preservation Packaging

A. Vacuum Packaging

In general, molds growing on the surface of foods and aerobic bacteria cannot develop under anaerobic conditions. Vacuum packaging relies on this characteristic.

In food packaging, vacuum packaging does not provide a complete vacuum in the interior of packaged containers. The pressure there is usually 5-10 torr (760 torr = 1 atm). Therefore the definition of vacuum packaging is as follows: Vacuum packaging of food means to pack food in the container—mainly made of plastics—under conditions of the same or nearly the same internal pressure as the moisture pressure of the contents.

There are five technical methods for vacuum packaging:

1. Mechanical press fixing method.
2. Steam flash method.
3. Nozzle-style evacuation method.
4. Chamber-style evacuation method.
5. Skin pack.

However food is packaged with a packaging machine that draws a vacuum, if the proper methods are not used and if the appropriate transport and storage are not applied, putrefaction will occur. The microorganisms in packaged foods grow rapidly when the oxygen level reaches 0.5%. Especially when cooked processed food is packaged in vacuum, complete evacuation is required. The lower the original amount of bacteria in food, the less there will be growth of them and the better the preservation will be.

Modified Atmosphere Packaging

Foods that undergo moisture separation or adhesion under vacuum conditions are often packaged with gas. Thin-sliced ham, cheese, fresh meat, coffee, dry milk, and oily confectionary goods are typical examples of modified atmosphere packaged foods. Sliced cheese, sliced ham, cooked processed food, and so on, which are proteinaceous processed foods, are packaged with mixed nitrogen and carbon dioxide. This modified atmosphere packaging
prevents the oxidation of fat and pigments in meat and represses the growth of bacteria.

There are three modified atmosphere packaging technical methods:

1. Nozzle-style modified atmosphere packaging.
2. Chamber-style modified atmosphere packaging.
3. Gas-flash modified atmosphere packaging.

Tomioka et al. reported on the number of bacteria in modified atmosphere packaging beef as fresh meat. They reported that the increase of bacterial number was not observed when the meat was packaged with 100% carbon dioxide, but when it was packaged with air, the original number of 1.3 x 10 cells/g reached 1.0 x 105 cells/g after storage for 16 days at 1.1°C.

Packaging with Enclosed Free-Oxygen Scavenging Agent

In this method, oxygen in the packaged food is removed, for example by the reduction of iron oxide. The atmosphere of foods becomes closer to vacuum and the growth of bacteria and the oxidation of fat are prevented. Especially in processed meat products, dairy products, and processed fish products, a free-oxygen scavenging agent is used to repress mold growth and prevent oxidation of pigments in meat. Packaging with enclosed free-oxygen scavenging agent is very easy to practice. The procedure is simply to put food into the packaging material and enclose the free-oxygen scavenging agent, and then seal the package. However, it is necessary to go through following method.

1. Choose the proper free-oxygen scavenging agent to remove the oxygen in the container completely.
2. Check the water content of food to be packaged. Choose the proper free-oxygen scavenging agent matching the water content of the food.
3. Us e high-gas-barrier packaging materials.
4. After taking off the outer package of the free-oxygen scavenging agent, use it soon.
5. After putting food into the packaging container or material, then introduce the free-oxygen scavenging agent, and seal the lid of the container or the lip of the material perfectly.

Aseptic Packaging

The history of aseptic packaging foods began with aseptic canning. Using sterilized cans for aseptic filling of foods was tried in 1917, but it could not be accomplished then. After much trial and error for aseptic packaging,
Dole Co. Ltd. established the aseptic system. From this opportunity, the aseptic canning of pudding, meat spread, and other products came about. On the other hand, the application of the aseptic filling packaging system
to paper cartons is comparatively recent. It originated in the research of long shelf life milk in Switzerland in 1951, and it was industrialized by Tetra-Pak Co. Ltd. in 1961. Then it attracted worldwide attention and
made rapid expansion, with Europe as the center.

The Prime-Pak system for aseptic filling of pasteurized milk into completely sterilized plastic containers was established. Most packaged milk for coffee is filled with this system. Furthermore, the trial of aseptic packaging for solid foods was started in 1965, using sliced ham, and was
followed by the introduction of bio clean rooms, which had been used for the assembling of rockets, as packaging room of foods. Why was aseptic food born in Europe and America like this? There are various explanations. In Europe, as so many people were killed by illness in the Middle Ages, microorganisms growing in foods attracted increasing attention. The effort to exclude microorganisms, particularly pathogenic ones, growing on foods seems to have given birth to aseptic packaging of foods.

STERILIZATION AND CONTROL OF MICROORGANISMS IN PACKAGED FOODS

A. Sterilization of Microorganisms by Heat

With the growth of microorganisms, most watery processed foods are likely to putrefy. Fish paste products, processed meat products, daily meals, and so on are transported and marketed after killing the vegetative cells of microorganisms by the boiling of packaged foods. Canned foods, bottled foods, and thermoplastic foods are sold at normal temperature after sterilizing microorganisms with high temperatures (110-120°C). The best way to improve the preservability of packaged foods is to eliminate microorganisms by heating after packaging. The spores of Clostridium botulinum type A and Β are killed at 100°C for 360 min and at 120°C for 4 min.

In the case of spores of Bacillus subtilis, treatment is 120°C for 7.5-8 min; Lactobacillus, 71°C for 30 min; spores of molds, 65-70°C for 5-10 min; and spores of yeasts, 60°C for 15 min. For this reason, the vegetative cells of bacteria, spores of molds, and yeasts in common packaged foods are killed by boiling at 85-95°C for 15-60 min after packaging. Fish sausages weighing about 100 g and meat sausages are packaged with polyvinylidene chloride film and sterilized by retort at 120°C for 25 min. Foods packaged in retortable pouches like stewed beef and hamburger, are subjected to retort sterilization at 120°C for 30 min and at 118°C for 20 min, respectively. Short-time sterilization with ultra-high temperature (UHT) is used for aseptic packaging foods to kill the microorganisms in the contents.

Sterilization with Microwaves and Ultraviolet Rays

Microwaves, far-infrared rays, gamma rays, and ultraviolet rays are being used for the food sterilization. The electronic range, which is an application of microwave dielectric heating, is working as a home use cooking
tool. Recently in the food industry, microwave (2450 MHz) dielectric heating has been applied to thermal processing of foods, defrosting, and sterilization of packaged foods and joined with new system of food packaging.

The world-famous Swedish maker of food sterilizing apparatus, AlfaLaval Co., Ltd., has developed Multitherm, which is a continuous sterilizing system of solid foods with microwaves. This system is a series of packaging-thermal processing machines composed of the following processes: formation of containers from rolled film, filling of solid foods, evacuation and sealing, preheating to a temperature of 80°C, cooling down to 40°C, sterilization with microwave dielectric heating at 127°C in the water, and cooling down again.

An irradiation method with ultraviolet radiation sterilizing apparatus has been achieved as a way to sterilize foods. This apparatus is used to kill the microorganisms attached to food packaging materials, air-borne microorganisms in factories producing medicine and food, and microorganisms in water. Particularly for aseptic packaging foods, aseptic packaging systems combined with highly efficient ultraviolet sterilizing apparatuses are producing many kinds of aseptic products around the world. Brown Boveri Co. Ltd. (BBC) in Switzerland has developed highly efficient ultraviolet apparatus possessing an output of 1 kW for sterilizing packaging materials. The apparatuses are introduced in many aseptic packaging machines. It has been reported that 99.99-99.999% of Escherichia coli is killed with the irradiation of ultraviolet rays at 50 mW/cm2 for 1 s.

C. Control of Microorganisms at Low Temperature

After packaging, most fresh foods and processed foods are transported and marketed at low temperature. As proteinaceous foods is particularly apt to be polluted by microorganisms, it is sold under chilled or frozen
conditions. Microorganisms causing putrefaction or spoilage are hard to develop at low temperatures below 10°C. However, psychrophilic bacteria such as Pseudomonas fluorescens will double even at 5°C after 10.65 h.
Dairy products, dessert foods, and frozen foods are transported at low temperature, either chilled or frozen.
Dairy products, fish, and meat processed products and daily meals are transported at low temperature below 10°C.

The foods transported at low temperature are classified as follows:

1. Cool foods: transport and market at 5-10°C.
2. Chilled foods: transport and market at – 5 to 5°C.
3. Frozen chilled foods: produce under frozen condition, transport and market under chilled conditions.

Control of Microorganisms with Preservatives and pH

Preservatives and pH depress the growth of microorganisms in processed foods. Bacteria scarcely grow at pH below 4.0, above 10% salt concentration, above 30% sugar, and above 8% alcohol concentration. On the other hand, molds and yeasts can develop at pH 3.0 and at 40% of Brix degree but cannot grow at 8% alcohol. Microorganisms in processed foods are often controlled by the addition of organic acids. Some reports say that preservability of foods reaches a maximum at 50% alcohol, 48% water, and 2% citric acid or lactic acid. It is said that the quality of salad made in Europe does not change after the storage at 10°C for 21 days. Most of these salads involve organic acids and a pH below 5.0.

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