1. ERT 426 Food Engineering Semester 1 Academic Session 2014/15 1

2. Subtopics 1. Food spoilage. 2. Physical changes to foods 3. Biochemical changes 4. Microbiological changes 5. Food safety 6. Types of food processing 2

3. 1. 1. Food Spoilage Microbial metabolism of organic matter is a naturally occurring process in the environment that is essential for the recycling of nutrients.  These activities are generally referred to as biodegradation;  however, when these organic materials are important for the well-being of humans, such as foods, then microbial metabolism is considered to be spoilage. 3

4. Food Spoilage Food spoilage is therefore a social construct and can be defined as the process or change leading to a product becoming undesirable or unacceptable for human consumption. The manifestations of food spoilage are many and varied, and may be visual  e.g. discoloration, slime production, colony formation, breakdown of structure, blowing of container) or apparent by smell (e.g. off- odour) or taste (e.g. off-flavour, increase in acidity). 4

5. Food Spoilage Alternatively foods may be said to have spoiled when the numbers and/or activity of pathogenic micro-organisms make them unsafe to eat and may cause illness or even death, or when deterioration of one or more nutrients means that a food no longer has its declared nutritional value. The time taken to reach one of these conditions is the `shelf-life' of the product and in most countries it is a legal requirement to identify a `best before', `sell-by‘ or `use-by' date on the package label. 5

6. Food Spoilage `best before' date:  when a food will retain its optimum condition is used for most foods. `use by' or ‘sell-by’ date:  foods that are microbiologically highly perishable and are therefore likely to cause a danger to health if consumed beyond this date (assuming they have been stored correctly). 6

7. Food Spoilage  The foods (dairy products, cooked ham, uncooked or partly cooked pastry dough ) have to be stored at low temperatures to maintain their safety rather than their quality.  Foods, such as bread and cakes, that deteriorate in quality rather than safety and chilled foods that do not support the growth of pathogens (e.g. butter and margarines) do not need a use-by date. 7

8. Food Spoilage Foods deteriorate over time and although this cannot be completely prevented, one aim of food processing is to slow the rate of deterioration by selecting appropriate methods of processing, ingredient formulations, packaging and storage conditions. Food spoilage may be caused by microbial, chemical or physical mechanisms. 8

9. Food Spoilage Table 1: Spoilage mechanisms for selected foods 9

10. 2. 2. Physical changes to Foods Physical damage caused by poor handling is a significant cause of spoilage in crisp products such as extruded, baked, fried or frozen foods. Physical damage to fresh fruits and vegetables causes bruising, which can in turn lead to accelerated microbial growth, enzymatic browning reactions and wilting due to moisture loss. Physical spoilage is the destabilisation or breakdown of emulsions, such as mayonnaise due to freezing, high temperatures or extreme vibration. 10

11. 2.1 2.1 Moisture migration Physical changes caused by moisture migration are temperature dependent and involve the water activity (a w ) of the food and glass transitions (brittle, glassy state to a softer, rubbery state).  An example of glass transition causing spoilage is staling of bakery products, which is due in part to moisture migration from the crumb (high a w ) to the crust (low a w ). 11

12. 2.2 2.2 Temperature Temperature influences the rate of spoilage: for example, rates of microbial growth, oxidation of lipids or pigments, browning reactions and vitamin losses. The rate of deterioration of a food and the prediction of its shelf-life can be made by studying one or more quality index that is characteristic of the food.  e.g. loss of a nutrient or characteristic flavour, growth of a target micro-organism, production of an off-flavour or discoloration. 12

13. Temperature The effect of temperature on these indices is measured using kinetic studies based on the Arrhenius equation: where K = reaction rate, K A = Arrhenius equation constant, E A (J mole -1 ) = Activation energy, R (8.3144 J mol -1 K -1 ) = universal gas constant and θ (K) = temperature. 13

14. Temperature The equation constant (K A ) is the value of the reaction at zero K, which is not useful for practical studies and the equation is therefore modified to include a reference temperature: where K ref (K) = rate constant at reference temperatures of 255 K for frozen foods, 273 K for chilled foods and 295 K for ambient temperature storage. 14

15. 3. 3. Biochemical changes Chemical reactions involving fats, carbohydrates, proteins and micronutrients can each produce changes to the colour, texture or flavour of foods that consumers find unacceptable.  The main factors that affect these reactions are the temperature of storage, exposure to light and oxygen, and the a w and pH of the food. 15

16. Biochemical changes Oxidation reactions include the development of off-flavours and colour changes due to oxidative rancidity in fats (autoxidation), and lipid oxidation in meat, fish and dairy products. The Maillard reaction (or `non-enzymic browning') is a complex series of reactions between reducing sugars and amino acids or amino groups on proteins. 16

17. Biochemical changes  Maillard reactions are important to develop the required sensory characteristics in bakery products, fried foods, roasted coffee or cocoa and in soy sauce.  The reactions also cause flavour deterioration in products such as fruit juices, dairy products and beer. 17

18. 3.1 3.1 Enzymatic reactions Naturally occurring enzymes in foods catalyse a wide variety of reactions that can adversely affect the flavour, colour and texture of foods during storage. A very large number of extracellular enzymes is also produced by micro-organisms and these are an important cause of food spoilage. The factors that affect the rate of enzyme reactions are similar to those that control microbial activity, although enzyme production can take place under conditions that do not support cell growth. 18

19. 4. 4. Microbiological changes In order to predict the changes to foods and their expected shelf-life, it is necessary to understand the types of microbial contamination and the factors that affect microbial growth, activity and destruction. The main factors that control the types of microorganisms that contaminate foods and the extent of their growth or activity: i. stage of growth of micro-organisms (lag, logarithmic (or exponential), stationary & death); ii. presence of other competing microorganisms; 19

20. Microbiological changes iii. availability of nutrients in the food (e.g. carbon and nitrogen sources, and any specific nutrients required by individual micro-organisms); iv. environmental conditions in the food (pH, moisture content or a w, redox potential (E h ), presence of preservative chemicals); v. storage conditions (temperature, exposure to light or oxygen); vi. the interaction of these factors. 20

21. 4.1 4.1 Bacteria Bacteria are single-celled micro-organisms, 1-5 µm in size that reproduce by binary fission. Most spoilage bacteria cannot grow below a w = 0.91, although halophilic bacteria can grow at a w = 0.75. The optimum pH for growth of most species is 6.0-7.0, but LAB have optimum growth at pH 5.5-5.8 and can grow in foods at pH 4. The redox potential (E h ) at which bacteria grow determines whether they are aerobic (E h is positive) or anaerobic (E h is negative). 21

22. Bacteria  Facultative aerobes can grow under both E h conditions. o Bacteria are also classified according to the optimum temperature for growth: 22 minimumoptimumrange psychrophiles -10 to 5 0 C12-18 0 C-10 to 20 0 C psychrotrophs <0-5 0 C20- 30 0 C<0-30 0 C mesophiles 5-10 0 C30-40 0 C10-45 0 C thermophiles 20-40 0 C55-65 0 C45-75 0 C hyperthermophile ≈80 0 C90-100 0 C65-120 0 C

23. 4.2 4.2 Fungi Yeasts are a small group (1%) of fungal species that are single-celled micro-organisms, 3-5 µm in size that reproduce by budding or sporulation. They are widely distributed in soils, on plants (especially fruits), and commonly associated with insects and the hides and feathers of animals. Most yeasts cannot grow below a w = 0.88-0.9, but some osmophilic yeasts can grow at a w = 0.6-0.7. 23

24. Fungi The optimum pH for growth of most yeasts is 4.5-5.5. Yeasts are aerobes but about half of the species can ferment sugars and these are facultative anaerobes. Most yeast species are mesophiles although some psychrotrophic yeasts can grow at several degrees below 0 0 C and have optimum growth below 20 0 C. 24

25. Fungi Foodborne spoilage yeasts: 25 TypesExamplesRemarks Strongly fermentative yeasts: Saccharomyces, Zygosaccharomyces, Torulaspora and Kluyveromyces genera. ferment lactose and are common causes of spoilage in dairy products Weakly fermenting yeasts Debaryomyces hanseniisalt-tolerant and occurs widely in food products Hyphal yeastsYarrowia lipolyticaspoilage of meat and dairy products Imperfect yeastsCandida and C. tropicalis, C stellata and C. zeylanoides spoilage yeasts in meat, fish and dried foods Red yeastsRhodotorula and Sporobolomyces spp. They have strong hydrolytic activity and can grow at low temperatures, but do not ferment foods.

26. 4.3 4.3 Moulds Moulds are a type of fungi that form a mycelium of branched cells, each 30-100 µm in size that reproduce by sexual or asexual sporulation. Most moulds cannot grow below a w = 0.8, but some xerophilic moulds can grow at a w = 0.65. The minimum pH for growth of some species (e.g. Fusarium spp.) is about 2.0 and others (e.g. Penicillium spp.) can grow at -6 0 C. 26

27. Moulds Moulds affect fresh fruits and vegetables and a wide range of foods, including bakery products, nuts, cheese and other dairy products. 27

28. 5. 5. Food Safety Injury to consumers by contaminated foods can arise from harmful chemicals or foreign bodies or from micro-organisms. Pathogenic bacteria may also produce toxins in the food in relatively low numbers or grow to sufficient numbers to cause poisoning. The main types of foodborne pathogens are bacteria, viruses and parasites and a few mould species.  No foodborne yeast species are known to cause food poisoning. 28

29. Food Safety: A brief description of pathogen: 29 TypeDescription Aeromonas spp.responsible for gastroenteritis Bacillus cereus(i) nausea and vomiting (ii) diarrhoea and abdominal pain without vomiting Brucella spp.it can be contracted by consumption of raw milk and unpasteurised dairy products. Enteropathogenic Escherichia coli invade mucosal cells causing severe diarrhoea, fever, vomiting and abdominal cramps. Salmonella spp.From mild/severe food poisoning (gastroenteritis) to severe typhoid, paratyphoid and septicaemia

30. 6. 6. Types of Food Processing Adequate food safety and shelf-life or the alteration of sensory characteristics cannot be achieved using a single type of processing (such as heating or pH control) and multiple methods (operations) are used. In traditionally preserved foods, such as smoked fish or meat, jams and other preserves, there are a combination of factors that ensure microbiological safety and stability of the food, and thus enable it to be preserved for the required shelf-life. 30

31. Types of Food Processing In smoked products: this combination includes heat, reduced moisture content (aw) and antimicrobial chemicals deposited from the smoke on to the surface of the food.  Some smoked foods may also be dipped or soaked in brine or rubbed with salt before smoking to add a further preservative mechanism.  Smoked products may also be chilled or packed in modified atmospheres to further extend the shelf-life. 31

32. Types of Food Processing In jams and other fruit preserves, the combined factors are heat, a high solids content (reduced aw) and high acidity.  These preservative factors also strongly influence the sensory characteristics of the product and contribute to important differences in flavour, texture or colour between different products. 32

33. 6.1 6.1 Hurdle concepts The demand by consumers for high-quality foods having `fresh' or `natural‘ characteristics but with an extended shelf-life has led to the development of foods that are preserved using mild technologies. Hurdle technology is also known as `combined processes', `combination preservation' or `combination techniques‘.  an understanding of the complex interactions of temperature, a w, pH, chemical preservatives, etc is used to design a series of hurdles to control the growth of spoilage or pathogenic micro- organisms and ensure microbiological safety of processed foods. 33

34. Hurdle concepts Examples of hurdles used to preserve foods 34

35. Hurdle concepts Examples of hurdles used to preserve foods 35

36. Hurdle concepts To be successful, the hurdles must take into account the initial numbers and types of micro- organisms that are likely to be present in the food.  The hurdles that are selected should be `high enough' so that the anticipated numbers of these micro-organisms cannot overcome them. As an example of hurdle technology, fermented sausages (e.g. salami) are produced using a sequence of hurdles: i. salt and sodium nitrite preservatives inhibit many contaminating bacteria, 36

37. Hurdle concepts ii. allowing other bacteria to multiply and use up oxygen, thereby causing the redox potential to fall; this inhibits aerobic micro-organisms and iii. selects lactic acid bacteria, which acidify the meat and increase the pH hurdle; iv. during ripening, the moisture content falls and causes the water activity to decrease. v. The final product therefore has low water activity as the main hurdle, with lower contributions from nitrite, redox potential and pH, making it stable at ambient temperature for an extended shelf-life. 37