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Food Spoilage and Preservation Dr James Dooley School of Biomedical Sciences, University of Ulster, Coleraine Food Spoilage and Preservation An essential element of modern society Not appreciated by most individuals A changing environment requiring constant innovation Will always be a problem for humans Hunter-Gatherer society supported low numbers/ self-sufficient limited specialisation unreliable Industrial and Agricultural society supports high numbers/ produce excess supports specialisation generally predictable Vitamins Proteins Food Energy Carbohydrate Building materials Lipids Microbial Growth Human Growth What are microbes? “Organisms that are too small to be seen with the naked eye” Bacteria Viruses Fungi Protozoa What are bacteria? Unicellular organisms Very small!!!!!!! 1-10 microns Enormous diversity Shape Habitat Nutrition Many bacteria require similar growth and nutrition conditions to humans (very many do not!) Light Microscope x 1,000 Where do we find bacteria? Everywhere! Soil Plant roots Water Bodies of animals, fish, birds etc, Hot springs Dead Sea Hydrothermal vents spores Endospore formation • some bacteria produce endospores •response to stress • very resistant to heat •121oC • very resistant to harsh chemicals, drying, radiation • can remain dormant for a long time (years) • endospore - forming bacteria are common in soil What are microbes? “Organisms that are too small to be seen with the naked eye” Bacteria Viruses Fungi Protozoa General features of Fungi unicellular (yeasts) and multicellular (moulds) Non-photosynthetic,plant-like organisms Multicellular, filamentous organisms Normally inhabitants of the soil, rhizosphere and water Can tolerate acidic and dry conditions Fungi in Nature Metabolic by-products form the raw material for many industries: ethanol antibiotics enzymes (washing powders etc.) solvents food flavours Cholesterol-lowering drugs – mevacor Fungi are the main organisms involved in the decay of organic material and the recycling of essential elements (C, N, etc.) Yeast are good model organisms for genetic manipulation. Micro-organisms and food Agents of food production Micro-organisms and food Agents of disease Micro-organisms and food Agents of food spoilage Food Spoilage and How to Prevent it We need to know about how spoilage organisms live We need to understand their biology We need Microbiologists! Laboratory study of bacteria Bacterial growth 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 4 8 16 32 64 128 256 512 1,024 2,048 4,096 8,192 16,384 32,768 65,536 1,310,752 2,621,504 5,243,008 10,486,016 20,972,032 41,944,064 83,888,128 Doubling is a Big Deal Some bacteria can double every 30 min Bacterial growth Binary fission Divide in two Growth rate varies Escherichia coli 20 minutes Mycobacterium tuberculosis 15 hours What do bacteria need to grow? Source of nutrients amino acids, sugars, lipids, vitamins released by action of enzymes operating outside the cell Correct temperature Bacteria grow within temperature ranges mesophiles (10-45oC) psycrophiles (0-20oC) What do bacteria need to grow? pH 6-7.5 Absence of toxic chemicals Correct atmosphere (O2) Aerobic Anaerobic Bacillus Clostridium facultative anaerobes Salmonella Consequences of bacterial growth Nutrients Suitable Environment Time More Bacteria! Consequences of bacterial growth in food Nutrients: Starch, protein etc. Waste products Altered Food CO2 Alcohol Lactic acid etc. Altered Environment Microbial Food Spoilage Microbial growth introduces alterations in food appearance smell Taste Nutritional content Changes not necessarily harmful! Each food unique microbial environment unique spoilage agents Three groups of foods: based upon rate of spoilage highly perishable meat fruit milk vegetables eggs WET semi perishable potatoes nuts stable rice flour dry beans Dry What defines each group? Amount of water Food Spoilage Each food has it’s own unique microbial population Uncontrolled growth of the microbes results in food spoilage We can predict (and therefore control) food spoilage Milk spoilage (unpasteurised) Bacterial growth on milk sugars (Lactobacillus spp., Lactococcus spp.) pH reduction lactic acid build up (bitter taste!) Change in bacterial population further pH reductions and much more lactic acid, continues until all sugars depleted Yeasts and moulds dominate use lactic acid for growth. pH rise allowing further bacterial growth Bacteria use proteins as major nutrient (Primary amines produced- Smelly!!!!!) Food spoilage has major economic impacts Microbial food spoilage Foods are characteristically spoiled by known organisms Food Organism Type of Spoilage Chicken Pseudomonas spp. Sliminess Green colour Milk (pasteurised) Lactobacillus thermophilus Sour Bread Rhizopus nigrans Bread mould Food Spoilage Shapes History Nicholas Appert a Frenchman who invented a method to preserve perishable organic materials. In 1809, Appert received 12,000 francs for his method of enclosing food in airtight jars which were then heated. In 1810, Appert published the first known book on canning boiling products in jars for four to six hours and then pouring molten wax over the jars. By this method, food could be preserved indefinitely. Unfortunately, the glass jars often broke on their trip to the army!!!! Preservation of food by killing all microbes Temperature canning sterilization by heat 121oC for 15 minutes all bacteria and endospores killed Preservation of food by killing all microbes Removal or killing of all microbes from a food will prevent spoilage! Removal or killing of all microbes from a food will drastically alter the food –taste –texture –nutritional content Preservation of food by preventing microbial growth A number of parameters can be manipulated to slow down microbial growth Moisture content {water activity (Aw)} Perishable foods have a high Aw preserve by lowering Aw How to reduce water? drying sun heat freeze - dried (expensive!) How to reduce water? addition of salt or sugar water needed to keep salt and sugar in solution Preservation of food by preventing microbial growth pH very few bacteria grow below pH 5.0 How to make food acidic? Add acid e.g. acetic acid Allow bacteria to make acid from natural food components lactic acid bacteria Preservation of food by preventing microbial growth Temperature storage at 4oC degrees storage at -20oC degrees rate of spoilage decreased rate of spoilage extremely slow need -70oC to eliminate spoilage Preservation of food by preventing microbial growth Temperature Pasteurization mild heat treatment overall microbial population is reduced pathogens are eliminated since these tend to be more heat sensitive than other organisms. 63C for 30 min. (batch pasteurization) 72C for 15 sec. (flash pasteurization) Food Preservation by control of bacterial growth Radiation use of gamma rays from Co60 microbes killed by free radicals Food can be packaged! No recontamination possible Pasteurization of meat, poultry, cheese No alteration of food controversial claim Irradiation is controversial Irradiation of various foods accepted in US and many other countries UK only allows for irradiation of herbs, spices or vegetable seasonings Preservation of food by preventing microbial growth Modified Atmosphere Packaging Oxygen Nitrogen Carbon Dioxide Argon Mix depends on food in question A little extra material... BBC Radio 4 Science “On the shelf” http://www.bbc.co.uk/radio4/science/connect_2002103 0.shtml Food Safety Through the Ages Dr. Bill Grierson http://www.acsh.org/healthissues/newsID.767/healthiss ue_detail.asp Food Preservation site Good links to related material http://www.bookrags.com/sciences/biology/foodpreservation-wmi.html Food Standards Agency www.food.gov.uk/ Good site for general information Food Spoilage Tests- do not take these literally THE GAG TEST - Anything that makes you gag is spoiled (except for leftovers from what you cooked for yourself last night). Ditto for things that make you violently ill. EGGS - When something starts pecking its way out of the shell, the egg is probably past its prime. Especially if the something is NOT a chicken. DAIRY PRODUCTS - Milk is spoiled when it starts to look like yogurt. Yogurt is spoiled when it starts to look like cottage cheese. Cottage cheese is spoiled when it starts to look like regular cheese. Regular cheese is nothing but spoiled milk anyway and can't get any more spoiled than it is already. Cheddar cheese is spoiled when you think it is blue cheese but you realize you've never purchased that kind. Blue cheese, by definition, is never spoiled. MAYONNAISE - If it makes you violently ill after you eat it, the mayonnaise is spoiled. FROZEN FOODS - Frozen foods that have become an integral part of the defrosting problem in your freezer compartment will probably be spoiled - (or wrecked anyway) by the time you pry them out with a kitchen knife. EXPIRATION DATES - This is NOT a marketing ploy to encourage you to throw away perfectly good food so that you'll spend more on groceries. Perhaps you'd benefit by having a calender in your kitchen. MEAT - If opening the refrigerator door causes stray animals from a three- block radius to congregate outside your house, the meat is spoiled. (or the smell alone can make you violently ill and/or unconscious) BREAD - Sesame seeds and Poppy seeds are the only officially acceptable "spots" that should be seen on the surface of any loaf of bread. Fuzzy and hairy looking white or green growth areas are a good indication that your bread has turned into a pharmaceutical laboratory experiment. FLOUR - Flour is spoiled when it wiggles. SALT - It never spoils. CEREAL - It is generally a good rule of thumb that cereal should be discarded when it is two years or longer beyond the expiration date. LETTUCE - Bibb lettuce is spoiled when you can't get it off the bottom of the vegetable crisper without Comet. Romaine lettuce is spoiled when it turns liquid. CANNED GOODS - Any canned goods that have become the size or shape of a softball should be disposed of. Carefully. CARROTS - A carrot that you can tie in a clove hitch in is not fresh. RAISINS - Raisins should not be harder than your teeth. POTATOES - Fresh potatoes do not have roots, branches, or dense, leafy undergrowth. CHIP DIP - If you can take it out of its container and bounce it on the floor, it has gone bad. EMPTY CONTAINERS - Putting empty containers back into the refrigerator is an old trick, but it only works if you live with someone or have a maid. UNMARKED ITEMS - You know it is well beyond prime when you're tempted to discard the Tupperware along with the food. Generally speaking, Tupperware containers should not burp when you open them. GENERAL RULE OF THUMB - Most food cannot be kept longer than the average life span of a hamster. Keep a hamster in or nearby your refrigerator to gauge this.