Lecture 2


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Lecture 2

  1. 1. FOOD SPOILAGE Food spoilage Spoilage is the deterioration of food which makes it go sour, rotten or moldy. • Most fresh foods spoil within days when they are stored without care in warm surroundings. When is food spoiled? • Offensive smell • Mouldy surface • Strongly sour or sharp flavor • Different color • Gas development (e.g. in bulging tins) or shiny surface. 1
  2. 2. Spoilage When food is spoiled, the characteristics of the food are changed in such a way that it is no longer acceptable for consumption (not necessarily dangerous) 2
  3. 3. Mouldy Bread Phytophtora on potato 3
  4. 4. Monilia on peach 4
  5. 5. Gas production resulting in a blown can 5
  6. 6. Spoilage of food may be a result of one or more of the following:(1) Growth and activity of microorganisms in the food, often a succession of organisms is involved. (2) Contamination of food by bacteria (3) Action of enzymes of the plant or animal food product (4) Worms, bugs, weevils fruit flies and moths may damage food and renders it unfit for human consumption 6
  7. 7. The main microorganisms responsible for the contamination of food are:» Bacteria » Moulds » Yeast Each group has many species which are responsible for different forms of contamination. 7
  8. 8. 8 • All three groups require a medium in which to grow and reproduce. food is an ideal medium as it provides nutrient and adequate moisture. • Microorganism’s contaminate food by producing waste products or toxins (poisons) or simply makes the food inedible by their presence. • In some cases, if contaminated food is eaten it may cause illness due to food poisoning. Microorganism which causes food poisoning and /or infections are called pathogenic (harmful) bacteria. Note: Not all microorganisms are harmful; some are used in the food industry to produce foods such as cheese yogurt and soy sauce. •
  9. 9. Examples of Micro-organisms in Food Useful: Fermentation: Lactic acid bacteria, yeast, moulds Spoilage (quality/shelf life): bacteria, yeast and moulds Pathogenic (hazards): bacteria (Listeria monocytogenes, Salmonella, Campylobacter, E.coli, Clostridium botulinum Staphylococcus aureus, Bacillus cereus); parasites; 9 viruses ;moulds
  10. 10. Bacteria • Are single celled organisms found in many places including air, water (including man). • There are thousands species some of which are harmful to man and some are beneficial. • 1. 2. 3. They can be classified according to their shape Spherical bacteria Rod shaped bacteria Spiral shaped bacteria 10
  11. 11. These are called cocci and they form chains called streptococci which are cause of disease such as tonsillitis Others forms pairs called diplococci which are the cause of pneumonia Clusters: called staphylococci which are the cause of boils, septic wounds and food poisoning 1 1
  12. 12. » Many bacteria of this shape are called bacilli and are the cause of disease such as diphtheria, tuberculosis typhoid and food poisoning Some are called clostridia and cause food poisoning some have tiny hair projection called cilia to enable them to swim through fluids 1 2
  13. 13. » These can cause diseases such as cholera, syphilis and infection jaundice. 13
  14. 14. Under suitable conditions of temperature, moisture and food supply bacteria can multiply or reproduce very rapidly. They reproduce by binary fission and in a space of twelve hours under right conditions a single bacteria can give rise to 16,000,000. 1 1 1 1 1 1 1 1 4 1 1 1
  15. 15. » When a large number of bacteria are present are in one place, they form a colony which is usually visible to the naked eye. » If conditions of division are unfavourable e.g. moisture is lacking, bacteria are able to form spores which remain dormant until the right conditions return, when the spore will germinate. Such spores are often resistant to heat. » Many bacteria do not require a source of oxygen to grow and multiply. Such bacteria are called anaerobic bacteria and this account for their ability to grow in food and intestines where oxygen supplies are limited. Some bacteria do require oxygen and are 1 called aerobic bacteria 5
  16. 16. Effect of Temperature and time on growth of Bacteria 16
  17. 17. Some bacteria can withstand extreme temperatures, but most are destroyed at temperature of around 60oC. Bacteria are able to multiply most rapidly at around 37oC and this has important implications for the preservation of food by heat. At very cold temperatures most bacteria stop multiplying and become dormant until the temperature conditions become favourable again, this has important implications for the freezing and cold storage of foods and their use. 1 7
  18. 18. » Many bacteria are affected by high concentration of salt acid or sugar. These interfere with the normal metabolic processes of the bacterial cells and either destroy them or prevent reproduction such substances are used for food preservation. » There are some however some bacteria which thrive in such conditions and may therefore require different treatment to prevent their growth. 1 8
  19. 19. » Bacteria can cause food poisoning symptoms in different ways: » Physical presence of bacteria in the food if bacteria have had the opportunity to multiply in large numbers in a food, then physical present in the intestine may cause irritation and food poisoning symptoms. This is also referred to as infection » Production of waster products (Toxins) bacteria like all other living things has to dispose off waste products which are the result of metabolic processes. These can cause irritation to the intestine and food poisoning symptoms even when only a small when only a small number of bacteria are present. The toxins are not destroyed by normal cooking temperatures » Germination of spores: The germination of bacterial spores is usually accompanied by the productions of highly poisons substances (exotoxins) only a small amount of which can lead to severe illness or possibly death. In some cases, just one spore can produce sufficient toxin to cause food poisoning. 1 9
  20. 20. ˃ Using the same utensils to serve contaminated food and other foods ˃ Careless attention to personal hygiene while handling food e.g. not washing hands after visiting the toilet, touching nose while preparing food. ˃ Leaving skins infection and cuts covered while preparing food ˃ Coughing, sneezing or spitting while preparing food ˃ Incomplete cleansing of food utensils and serving dishes ˃ Pests e.g. houseflies, cockroaches, beetles and certain moths, rodent e.g. rats, mice etc 2 ˃ Contaminated water supply 0 ˃ Household pets e.g. dogs, cats, hamsters ˃ Soil and dust.
  21. 21. Incubation/ Duration Bacteria types Name Cocci Stamphylococcu 1-6hrs s 1-6days Bacilli Salmonella typhimurium 12-48hr 7days Clostridia Clostridium botulinum Improperly Rare but in most canned foods cases, exotoxin especially meat produced vegetables Clostridia Clostridium welchii 12-36hrs fatal Double vision difficulty with within 7days breathing taking and swallowing 8-24hrs Diarrhea Abdominal pain 1-2 days headache Bacilli Salmonella typhi 1-3 weeks 1-2 Headache Tiredness fever, months rash haemorrage Sewage, water Can fatal slow flies cream, cake recovery 21 water cress, canned meat Bacilli Bacillus cereus 2-18 hrs Symptoms Cause notes Severe vomiting Nose, skin cuts Poisoning due to diarrhea toxins heat does cooked meat not destroy exhaustion toxins Headache fever Faeces, sliced Cause food vomiting and cooked meat poisoning. Can abdominal pain poultry, be fatal sausages egg, pies Vomiting Meat, gravy, meat Faeces pies Poisoning due to canned toxin and bacteria in gut cold Poisoning
  22. 22. » Moulds are tiny microorganisms which are just visible to the naked eye. They grow on many types of food, especially cheese, bread and fruit. They require warm most conditions to grow, but are able to grow at a slower rate in cool places. Moulds reproduce by means of sporulation. Spores are released into the atmosphere and carried in the air. If they land on a suitable food, the spores germinate and a new mould appears. There are many types of mould, but among the more common types are Mouldy cheese Fusarium on maize 2 2
  23. 23. Aspergillus 23
  24. 24. » Food that is contaminated with mould often appears to be safe to eat as only the outer part is affected by mould growth. However, recent research has shown that metabolites (toxins) produced by mould which migrate into the food could be harmful to many organs of the body. » These toxins are called mycotoxins it is therefore advisable to discard mould food completely rather just remove the mouldy part. » Mould growth is prevented by cool dry storage, heating to destroy moulds and acidic conditions. However, not all moulds are harmful. Specially produced moulds are added to certain cheeses e.g. stilton, Danish blue, to develop characteristic flavours. 2 4
  25. 25. » Yeasts are microscopic single celled microorganisms which are found in the air and soil and on the surface of fruits. Some are able to tolerate fairly high acidic, salt and sugar concentrations and can grow without the presence oxygen. » Yeasts reproduce by budding in presence of water, warmth and food. Yeast cells 2 5
  26. 26. Budding yeast 26
  27. 27. » The cells remain dormant in very cold conditions and are killed at temperature approaching 100oC. » Growth is inhibited in the presence of high salt concentrations. Yeast can spoil foods such as jam and fruits by fermenting the sugar, to produce alcohol and carbon dioxide gas. » The time that this takes will depend on the concentration of sugar in the food and length of time it is stored. Fruit flavoured yogurts may also be affected in this way. 27
  28. 28. » Enzymes are found in all fresh food and although their actions is important to the ripening of certain foods (such as fruits and vegetables) the continuance of this action after the peak of maturity brings about undesirable changes in the food tissue these changes include darkness of cut tissue or surfaces the formation of soft sports and the development of off flavours. The bruises and cuts caused by these insects serve as pathways by which microorganisms reach the inner 2 tissue. 8
  29. 29. There are various sources of food spoilage microorganisms such as i. Animals ii. Sewage iii. Soil iv. Water v. Air vi. During handling and processing 2 9
  30. 30. Sources of microorganisms from animals include the surface flora, the flora of the respiratory tract and the flora of the gastrointestinal tract. The natural surface flora of meat usually is not as important as the contaminating microorganisms from their intestinal or respiratory tract. Howerer, hides, hoofs and hair contain not only large numbers of microorganisms from soil, manure, feed, and water but also important kind of spoilage organisms. Feathers and feet of poultry carry heavy contamination from similar sources, the skin of animals may contain micrococci and staphylococci may find their way into the carcass and then to the final raw products. The faeces and 3 fecal contaminated products of animals can contain many enteric organisms including salmonella 0
  31. 31. » The natural surface flora of plants varies within plants but usually include, Pseudomonas Micrococcus and Coliforms and lactic acid bacteria. » Exposed surfaces of plants become contaminated from soil, water sewage, air and animals some fruits have been found to contain viable microorganism in their interior. » Normal healthy tomatoes have been found to contain Pseudomas and Coliforms. Yeasts have been found inside undamaged fruits. 31
  32. 32. When untreated domestic sewage is used to fertilize crops, there is likelihood that the raw plant foods will be contaminated with human pathogens especially those causing gastrointestinal diseases. The use of night soil as a fertilizer still persists in some countries such as China. 32
  33. 33. SOIL » The soil contains the greatest variety of microorganisms than any source of contamination not only numerous kinds of microorganism but also large total numbers are present in fertile soil ready to contaminate surfaces of plants growing on or in them and the surfaces of animals coming over the land. » Modern methods of foods handling usually involve washing the surfaces, of foods and hence the removal of much of the soil from those surfaces. 33
  34. 34. » Natural water contains not only their natural flora but also microorganisms from soil and possibly from animals or sewage. » Surface water from animals or pools and stored water in lakes and dams vary considerably in their microbial content. Ground water from springs and or wells have passed through layers of rocks and soil to a definite level and hence most of the bacteria as well as the greater part of suspended material, have been removed by subsequent filtration. » The kinds of bacteria in natural water are chiefly species of Pseudomonas, Proteus Micrococcus Bacillus and Streptococcus. 34
  35. 35. » Contamination of foods from the air may be important for sanitary as well as economic reasons. » Disease organisms especially those causing respiratory infection may be spread among employees by air or to the food. » Microorganisms get into the air by means of dust, solid suspended materials and moisture droplets from coughing sneezing or talking. 35
  36. 36. Contamination of foods from the natural sources as discussed above take place before the food is harvested or gathered or during handling and processing of the food. Additional contamination may come from equipment coming in contact with foods, from packaging materials and from the personnel. The processor attempts to clean and sanitize equipment to reduce such contamination and to employ packaging materials that will minimize contamination. 3 6