Fish Microbiology and Quality Assurance

Unit 1 - Role and significance of microorganisms in food

1.1 Introduction to fish microbiology

Fish /aquatic foods contain a variety of microorganisms from different sources. These contaminants in food cause problems of spoilage and health risk to consumers.
Fish are not sterile and the microflora includes natural flora of the waters from which fish are harvested and acquired transient flora from environment especially during handling, processing, storage etc.

Inner tissue of healthy fish is sterile and microorganisms are mainly associated with outer slime, gills and intestine. Microbial load is higher in intestine followed by gills and skin. Natural microflora of fish varies depending on the habitat of fish (freshwater / marine / river/ water/ lake/ etc.), its feeding habit and life history stages. Generally, fresh warm water fish have more mesophilic bacteria than cold water fish. Transient flora include microorganisms entering the food from fish contact surface (crafts/gears/ baskets/ fish holds etc), the air, soil, water/ice used for washing, food handlers, packaging material and storage environment.

Fish harvested from polluted water contain variety of microorganisms depending on nature of pollutant, and also human pathogens such as bacteria, fungi, viruses, protozoans, parasites etc. Once the fish dies the associated microorganisms affect quality due to spoilage. Thus, it is necessary to maintain the quality by destroying associated microorganisms and preventing growth of surviving microorganisms. The different preservation methods mainly aim at maintaining fish quality by reducing, killing or inactivating associated spoilage microorganisms.

History of microorganisms in food
The role of microorganisms in spoilage and food poisoning was realised only after the establishment of bacteriology or microbiology as a science during the early 18th century. However, adoption of several approaches to avoid/ reduce spoilage was known since early civilization. The use of salted meat, fish, fat, dried animal skin and cereals was practiced by the Sumarians at about 3000 B.C. Romans were known for adopting preservation techniques for meat and also use of snow for perishables including prawns at around 1000 B.C. Smoking of meats and making of cheese and wines emerged during the same time. But the people at that time were not fully aware of nature of the preservation techniques and also the role of foods in the transmission of diseases and the danger of eating infected animal meat.

A Kircher, a monk, was first person in 1658 to observe the decaying bodies, meat, milk and other substances and believed it to be due to invisible worms, thus suggesting the role of microorganisms in food spoilage. During that time the theory of spontaneous generation of life was gaining importance and L. Spallanzani in 1765 was first to oppose it with his experiment using beef broth which remained sterile after boiling and sealing. But he was not able to convince the proponents of spontaneous generation who felt that air was necessary for life to begin. Later in 1837, Schwann’s experiment of passing heated air in to the boiling broth caused it to remain sterile thus disproving spontaneous generation of life. But it was not until Pasteur’s well planned swan neck flask experiment that the theory of spontaneous generation of life was put to rest.

The development of canning of meat in glass bottles by Nicholas Appert in 1809 had practical application in food preservation. Pasteur was the first person to appreciate and understand the presence and role of microorganisms in food. He demonstrated the role of microorganisms in souring of milk in 1837, and applied heat to destroy undesirable microorganisms in wine and beer, in about 1860. Involvement of of microorganisms in food spoilage and food poisoning has led to the development of preservation methods to arrest and kill microorganisms. The science of food microbiology has application in all kinds of food meant for human consumption and aims at ensuring the wellbeing of consumers.