A Study of Psychorophilic Bacteria in Frozen Foods

Abstract

The bacteria examined in this study are facultative psychrohiles, which have their origin in the mesophilic group. Different members of the group have their optimum growth at temperatures slightly below the mesophilic range, but will grow at temperatures ranging from below zero to 25 degrees C. they have become increasingly important with the advent of the frozen food industry. Their presence in frozen foods is often the cause of objectionable flavor, putrid odor, or unnatural appearance. In this respect they are as important to the homemaker to the scientist and industrialist. It was with the hope of adding to the knowledge of this relatively little known group of microorganisms that this study was undertaken. The preservation of foods by freezing goes far back in history. Freezing foods was a common practice in northern Europe in ancient times. Meat and fish were allowed to freeze naturally in order to preserve them. Mechanical refrigeration did not appear until the nineteenth century. Meat packers were using natural ice refrigeration by 1860(12), but it was not until the ideal method. Birdseye and Fitzgerald (6) mention several man who pioneered in mechanical refrigeration devices. In 1861 Piper of Canden, Maine invented a process in which a mixture of salt and ice in metal pans was placed directly over fish which were held in the lower compartment of an insulated container. Davis, in 1869, received a patent for freezing fish in flat metal pans with telescoping metal covers, the fish being packed tightly in pans so there was contact on all sides. The pans were then surrounded with a freezing mixture of salt and ice. Hesketh and Marcet proposed in 1889 to freeze meat or other parishables by direct or indirect immersion in cold brine or other refrigerants. According to Jensen (12), G.H. Hammond in 1875 designed the first refrigerator car. This first car, crude but practical, subsequently enabled Swift to begin carrying dressed meats from Chicago to the east coast. The first frozen beef to be exported from the United States left New York City for England in 1875. (12) Preservation was accomplished by alternating layers of beef with layers of ice in the hold of the ship. The first method of cold storage used large blocks of ice as the cooling agent. These were placed in a room containing the food to be preserved. The ice cooled the air in the room sufficiently to prevent spoilage of the food. Gradually methods of mechanical refrigeration were developed. These were the compression system, the flooded system, and the absorption system. These systems use a variety of refrigerants, including amounts, carbon dioxide, sulfur dioxide, and methyl chloride. Each of the refrigerants is adaptable to some particular service. The compression system has the wildest use today. It enables the development of large warehouse near the production centers, where perishable foods can be stored before shipping to markets in other parts of the country. Cold storage, as we know it today, was developed about 1890, at the time when mechanical refrigeration first began to assume a practical importance in the food industry. A decade earlier, ammonia refrigeration machines were used to freeze fish. This same method was slowly expanded to include other foods as well. Eggs were first frozen in 1889. The freezing of fruits was begun in the eastern part of the United States about 1900. Freezing of berries in the Pacific Northwest was begun in 1910. The Birdseye organization started the commercial freezing of vegetables in Oregon in 1929 (30).The rapid freezing of foods has been known for a long time. Early scientists in the nineteenth century developed some theories as to the probable changes caused by freezing, but they never applied their work to practical processes of the industry. According to Tressler and Evers (30) in 1916, Plank Ehrenbaum and Reuter showed the practical advantages of the rapid freezing of meat. Since that time, rapid strides have been made both in the scientific and industrial fields. Today quick-freezing is considered one of the best methods of preserving many different types of food.There are three principle methods of quick-freezing: 1. Freezing by direct immersion in a refrigerating medium, such as low temperature brine; 2. Freezing by indirect contact with a refrigerant, by applying the product to a metal surface which is cooled by freezing brine; 3. Freezing in a blast of cold air. When water is frozen, it is changed to ice crystals. By the time the temperature has reached -3.9 degrees C most of the water is frozen and the ice crystals no longer increase in size. The temperature zone from -.6 C to -3.9 C was shown by Birdseye (4) to be the zone in which maximum growth of crystals occur. Crystals will be smaller if food is passed through this temperature zone quickly. The larger the crystals rupture the walls of the cells, changing the texture of the food and releasing nutrients which encourage bacterial growth