Everything you need to know about food and dairy microbiology. Some of the most frequently asked questions are as follows:-
Q.1. What are naturally occurring antimicrobial substances in some foods and how do they work?
Ans: Eggs contain lysozyme which causes lysis of cell walls of bacteria as lysozyme breaks down peptidoglycan. Eggs also contain conalbumin and riboflavin both of which chelates iron.The waxy skin of fruits retards the entry of bacteria and fungi. Garlic possesses allyl iothiocyanate that inhibits bacterial growth best at 37°C. Onion possesses lacrimatory factors that inhibit the growth of bacteria and fungi and cranberries have benzoic acid which inhibits the growth of fungi.
Q.2. Which plants were consumed by the Egyptians who built pyramids to prevent and combat infections?
Ans: Garlics and radishes.
Q.3. Who was first to report the antibacterial potential of garlic?
Ans: Louis Pasteur in 1835.
Q.4. What is TDT?
Thermal death time.
Q.5. Give thermal death times of some spore forming bacteria in a tabulated form.
Q.6. What are the methods of preservation of food?
Ans: Some of the important methods of preservation of food are listed below:
(1) High temperature without harming the quality of food.
(3) Low temperature treatment.
(5) By chemical additives.
Q.7. Which are the three methods of pasteurization in general use?
Ans: These are:
(1) High temperature-short-time method (HTST) needs temperatures of 72°C for 15 seconds.
(2) Low temperature long time (LTLT) need temperatures of 62°C for 30 minutes.
(3) Ultra high temperature method (UHT) in which temperature of 140°C is maintained for 1.5 seconds and 149°C for 0.5 seconds.
Q.8. Which are the antibiotics used to preserve dairy products?
Ans: The antibiotics nisin and antamycin are used.
Q.9. Define 12 D treatments.
Ans: It is a process of sterilization which results in a decrease in the number of Clostridium botulinum endospores by 12 logarithmic cycles.
Q.10. Nicholas Appert’s discovery may be regarded as a forerunner of hermetically sealed cans. How do hermetically sealed cans prevent the spoilage of food?
Ans: Being completely airtight they do not permit the entry of contaminating organisms.
Q.11. Name three food borne infections.
Ans: Gastroenteritis, typhoid fever and dysentery.
Q.12. Give three examples of microbial intoxication.
Ans: Botulism, staphyloccal food poisoning and mycotoxicoses.
Q.13. What are the public health service standards for Grade A pasteurized milk?
Ans: Standard plate count of fewer than 20,000 bacteria per millilitre of milk and not more than 10 coliforms per millilitre.
Q.14. What are the public health service standards for dry milk products?
Ans: Standard plate count less than 30,000 bacteria per gram or a coliform count of less than 10 per gram.
Q.15. Why does casein, the milk protein curdle?
Ans: Due to action of lactic acid bacteria or the enzyme rennin (or chymosin).
Q.16. What is cheese?
Ans: It is the curd separated from the liquid portion of milk called whey.
Q.17. How is hard cheese produced?
Ans: By growing lactic acid bacteria in the interiors of the cheese.
Q.18. What is ripening of cheese?
Ans: Growth of microorganisms in cheese.
Q.19. How are semisoft cheeses ripened?
Ans: By having bacteria grown on the surface.
Q.20. What ripens soft cheeses?
Ans: Penicillium growing on the surface.
Q.21. How is commercial butter milk made?
Ans: By permitting lactic acid bacteria to grow for 12 hours.
Q.22. How are sour cream, yoghurt, kefir and kumiss produced?
Ans: By growing lactobacilli, streptococci and yeast in low fat milk.
Q.23. How is commercial yoghurt produced?
Ans: Commercial yoghurt is produced by evaporating low fat milk in a vacuum pan and the resulting thickened milk is inoculated with a mixed culture of Streptococcus thermopiles (for acid production) and Lactobacillus bulgaricus (for flavour). The temperature of fermentation is maintained at 45°C for several hours so that S. thermophilic outgrows L. bulgaricus.
Q.24. What is water activity of a food?
Water activity (Aw) is the amount of moisture available in foods.
Q.25. What is DEFT?
Ans: DEFT is the direct epifluorescent filter technique, which is a microscopic technique. In it the ‘ bacteria are concentrated from a significantly large volume of sample by filtering it through a polycarbonate membrane filter. The retained bacteria are then stained on the membrane with acridine organ and counted directly under an epifluorescene microscope.
The bacteria are trapped in a single focal plane which is achieved by using a polycarbonate membrane where relatively uniform pores are produced following neutron bombardment of a plastic film rather than cellulose acetate filters which have tortuous pores resulting in bacteria to be held at different levels.
Q.26. List the important methods for microbiological examination of food.
Ans: These are:
(1) Direct examination,
(2) Cultural techniques,
(3) Enumeration method, e.g., plate counts and most probable number counts (MPN),
(4) Alternative methods, e.g., dye reduction test, electrical methods, ATP determination,
(5) Rapid methods for the detection of specific organisms and toxins.