Factors affecting survival and growth FS

Traditional food preservation - Stored in cool caves Cheese, fruits, vegetables - Frozen / freeze-dried at Meat (Switzerland), high latitudes and altitudes potato (Peru) - Dried in the sun - Smoked / dried over fires - Salted and dried - Fermented FS Tomato (Italy), fish, meat (South America), Biltong (South Africa) Kipper (UK, smoked herring) Bacalhau (Portugal - dried cod) Tempeh, cheese, wine, beer, yoghurt, kefir

Pasteur’s germ theory Traditional methods work by ‹ Preventing contamination ‹ Destroying microorganisms ‹ Inhibiting growth of undesirable microorganisms FS

Traditional food preservation Traditional preservation sometimes fails to preserve the food, or to kill the pathogens. In these cases, spoilage or food poisoning may occur. Examples: salmonellosis from dried beef (Salmonella St. Paul ) and poisoning from tempeh bonkrek in Indonesia. FS

Traditional food preservation Certain “traditional” foods can be manufactured using industrial processes Example: Soy sauce FS

Factors affecting growth of bacteria in food - Temperature - Time - pH - Water activity (a w ) - Oxygen tension - Preservatives - Microbial interactions FS

Temperature Boiling point Pasteurising temperature Body temperature Fridge Freezer FS ° 72° 60° 36.5° 10° 0° SAFETY DANGER SAFETY

How temperature affects growth rate of a bacterial population B (Optimum) C (Minimum)A (Maximum) ColdHot Temperature FS

Growth of S. typhimurium at different temperatures ° 6 20° 5 15° 4 10° Time (Days) FS

Temperature range for growth of pathogens Temperature°C Min.Opt.Max. Salmonella Campylobacter E. coli S. aureus C. botulinum (proteolytic)1050 C. botulinum (non-proteolytic) B. cereus FS

Temperature range for growth of toxigenic moulds Temperature °C Min. Opt. Max. Penicillium verrucosum Aspergillus ochraceus Aspergillus flavus Fusarium moniliforme FS

Growth of different bacteria at 25°C Log 4 CFU FS Time (Days) S. typhimurium L. monocytogenes Ps. fluorescens B. cereus C. bot-Proteolytic G+ve Spoilers 3 4

Effect of temperature on time to botulinum toxin production FS Vacuum-packed hot smoked trout Salt concentration = 0.5% Temperature (°C) 20 25

Temperature affects bacteria ‹ Lag phase ‹ Growth rate ‹ Final cell numbers through the change in ‹ Enzymatic and chemical composition of cells ‹ Nutritional requirements ‹ Limits for other factors influencing growth FS

Effect of salt concentration on time to botulinum toxin production FS °C 14°C 18°C 24°C Salt Concentration (%) 1.5 2

Limits of pH for growth of pathogens pH MinMax. Escherichia coli Salmonella typhi Bacillus cereus Clostridium botulinum Staphylococcus aureus49.8 Saccharomyces cerevisiae Aspergillus flavus Fusarium moniliforme Penicillium verrucosum FS

Definition of water activity (a w ) a W = p / p o a w is the ratio of the water vapour pressure of the food (p) to that of pure water (p o ) at the same temperature. FS

Effect of water activity on lag time of S. aureus in UHT milk at 12°C Water activity (a w)w) FS

NaCl and glucose concentrations and corresponding a w values at 25°C aWaW % w/w NaClGlucose FS

Minimum levels of a W permitting growth at near optimum temperatures aWaW MouldsAspergillus chevalieri0.71 Aspergillus ochraceus0.78 Aspergillus flavus0.80 Penicillium verrucosum0.79 Fusarium moniliforme0.87 YeastsSaccharomyces rouxii0.62 Saccharomyces cerevisiae0.90 Bacteria Bacillus cereus0.92 Clostridium botulinum (proteolytic)0.93 Clostridium botulinum (non-proteolytic)0.97 Escherichia coli0.93 Salmonella0.95 Staphylococcus aureus0.83 FS

Range of a W in foods and their microbial flora a w range > FS foods Fresh meats Fresh fish Fresh fruits Fresh vegetables Canned vegetables in brine Canned fruit in light syrup (<3.5% salt, 26% sugar) Fermented sausages Processed cheese Bread Evaporated milk Tomato paste (10% salt, 50% sugar) microbial flora (C. perfringens, Salmonella) (Pseudomonas) (B. cereus, C. botulinum, Salmonella) lactobacilli, bacilli and micrococci

Range of a W in foods and their microbial flora a w range < 0.6 FS foods Dry fermented sausages Raw ham (17% salt, saturated sucrose) Dried fruit Flour Cereals Salted fish Nuts Confectionery Honey Noodles Dried egg, milk microbial flora S. aureus Mycotoxin producing moulds Spoilage yeasts and moulds Xerophilic fungi Halophiles Osmophilic yeasts No growth but may remain viable

Factors affecting microbial growth Key messages Temperature, pH, water activity and oxygen tension are the principal factors affecting microbial growth There are optimum ranges for these parameters These optima are interdependent They can be selected to inhibit the growth of certain organisms within limits related to the palatability of food Certain foods are suited for the growth of certain flora FS