WATER ACTIVITY AND MICROBIAL STABILITY

Transcription

1 WATER ACTIVITY AND MICROBIAL STABILITY Larry R. Beuchat Center for Food Safety and Department of Food Science and Technology University of Georgia 1109 Experiment Street, Griffin, GA

2 Ecology of Foodborne Microorganisms Large variety of initial microflora ; Selective pressure of ecological factors ; Characteristic microbial community for each type of food ; Specific associations are influenced by a w

3 Factors Influencing Growth Temperature Water activity (a w ) ( ) Oxygen Nutrients Acidity and ph Natural and added inhibitors

4 Water content of foods at a w 0.70 Food Nuts Whole milk powder Cocoa Soybeans Dried whole egg Skim milk powder Dried lean meat and fish Rolled oats Rice Pulses Dried vegetables Wheat flour, pasta Dried soup mixes Dried fruits Water Content (%, w/w) Adapted from Mossel, Crit. Rev. Environ. Control, 5:1-139 (1975)

5 Lower limits of moisture for growth of molds on cereal grains and oilseeds Moisture (%, w/w) Mold Corn, wheat, sorghum, rice Soybeans Peanuts, sunflower seeds, copra Aspergillus candidus A. flavus A. glaucus A. halophilicus A. ochraceus A. restrictus Penicillium spp Wallemia sebi Adapted from Christensen, Food and Beverage Mycology, p (1987)

6 Definition and Measurement Proportion of water available for biological (biochemical) and chemical reactions For an ideal solution: a n 1 n1 + n 2 p p0 w = = = ERH 100 Where n 1 = moles of solvents (water) n 2 = moles of solute p = vapor pressure of solution p 0 = vapor pressure of solvent

7 Water Activity Affects the Behavior of Microorganisms Growth Sporulation Toxin production, stability Survival during processing, storage Recovery on agar media

8 Minimum a w for Growth Depends on Other Factors Temperature ph Nutrient availability Antimicrobial substances Procedure for formulation

9 Xerophilic Fungi Growth characteristics y Xerotolerance depends on genus y Range for growth is a w y y Spoilage of foods with a w < 0.85 usually involves xerotolerant yeasts or molds Lower a w limit for growth depends on availability of nutrients, ph, type of solute, temperature

10 Minimum a w for growth of microorganisms Range of a w Microorganisms inhibited by lowest a w in this range Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Bacillus, Clostridium perfringens, some yeasts Foods generally within this range Highly perishable (fresh) foods and canned fruits, vegetables, meat, fish, and milk; cooked sausages and breads, foods containing up to approximately 40% (w/w) sucrose or 7%

11 Range of a w Microorganisms inhibited by lowest a w in this range Salmonella, V. parahaemolyticus, C. botulinum, Serratia, Lactobacillus, Pediococcus, some molds, yeasts (Rhodotorula, Pichia) Foods generally within this range Some cheese (Cheddar, Swiss, Muenster, Provolone), cured meat (ham), some fruit juice concentrates, foods containing 55% (w/w) sucrose or 12%

12 Range of a w Microorganisms inhibited by lowest a w in this range Many yeasts (Candida, Torulopsis, Hansenula), Micrococcus Foods generally within this range Fermented sausage (salami), sponge cake, dry cheeses, margarine, foods containing 65% (w/w) sucrose (saturated) or 15%

13 Range of a w Microorganisms inhibited by lowest a w in this range Most molds (mycotoxigenic penicillia), Staphylococcus aureus, most Saccharomyces, Debaryomyces Foods generally within this range Most fruit juice concentrates, sweetened condensed milk, chocolate syrup, maple and fruit syrups, flour, rice, pulses containing 15-17% moisture, fruit cake, country-style ham, fondants, high-ratio cakes

14 Range of a w Microorganisms inhibited by lowest a w in this range Most halophilic bacteria, mycotoxigenic aspergilli Xerophilic molds (Aspergillus chevalieri, A. candidus, Wallemia sebi), Saccharomyces bisporus Foods generally within this range Jam, marmalade, marzipan, glacé fruits, some marshmallows Rolled oats containing approximately 10% moisture, grained nougats, fudge, marshmallows, jelly, molasses, raw cane sugar, some dried fruits, nuts

15 Range of a w Microorganisms inhibited by lowest a w in this range Osmophilic yeasts (Zygosaccharomyces rouxii), few molds (Aspergillus echinulatus, Monascus bisporus) Foods generally within this range Dried fruits containing 15-20% moisture, some toffees and caramels, honey

16 Range of a w Microorganisms inhibited by lowest a w in this range No microbial proliferation No microbial proliferation Foods generally within this range Cookies, some crackers, bread crusts, etc. containing 3-5% moisture Whole milk powder containing 2-3% moisture, dried vegetables containing approximately 5% moisture, corn flakes containing approximately 5% moisture, some crackers

17 Range of a w Microorganisms inhibited by lowest a w in this range No microbial proliferation No microbial proliferation Foods generally within this range Pasta containing approximately 12% moisture, spices containing approximately 10% moisture Whole egg powder containing approximately 5% moisture

18 Adaptation to Reduced a w Plasmolysis occurs when cells are subjected to lower a w (higher osmotic pressure) Compatible solutes are synthesized or taken up from the environment to increase internal osmotic pressure y y y Bacteria: free amino acid pool increases, with glycine betaine (trimethyl glycine) predominating., trehalose, glycerol, and/or glutamate and K + may also accumulate Yeasts and molds: mannitol, glycerol, arabitol, sorbitol, erytritol, and K + are most often accumulated. Glucose, fructose, sucrose, and proline may also increase within cells Compatible solutes can also serve as energy reserves

19 Solutes accumulated by fungi to response to reduced a w A w -Controlling Fungus solute Saccharomyces cerevisiae Zygosaccharomyces rouxii Debaryomyces hansenii NaCI Solute(s) accumulated Glycerol Glycerol Glycerol, arabitol, K + Penicillium ochro-chloron Penicillium chrysogenum Chrysosporium fastidium KCl Glucose Glucose KCl Glycerol Glycerol and K + Glycerol, glucose, K + Glucose and glycerol Glucose and glycerol Xeromyces, bisporus, Wallemia sebi, Eurotium chevalieri, Penicillium digitatum Mucor hiemalis, Pythium debaryanum Sorbitol Glucose and fructose KCl Glycerol Sorbitol and glycerol Glucose, fructose, and glycerol K + and proline or glucose and fructose, proline

20 Spore Production and Germination When the lower a w limit for growth is approached, sporulation often occurs Minimum a w for germination is usually higher than the minimum a w for sporulation

21 Effects of ph and solute on minimum a w for germination of fungal spores at 25oC Aspergillus ochraceus Wallemia sebi Solute ph Minimum a w Germ. time (d) Minimum a w Germ. time (d) Glucose/fructose Adapted from Pitt and Hocking, J. Gen. Microbiol., 101:35 (1977)

22 Minimum a w for growth and toxin production by bacteria Minimal a w Bacteria Staphylococcus aureus Salmonella Vibrio parahaemolyticus Clostridium botulinum Clostridium perfringens Bacillus cereus Growth (A, B) 0.95 (E) Toxin Production 0.87 (enterotoxin A) 0.97 (enterotoxin B) 0.94 (A, B) 0.97 (E)

23 Minimum a w for growth and mycotoxin production Mycotoxin Aflatoxin Ochratoxin Penicillic acid Patulin Stachybotryn Mold Aspergillus flavus A. parasiticus A. ochraceous Penicillium cyclopium P. viridicatum A. ochraceous P. cyclopium P. marensii P. patulum P. expansum Stachybotrys atra Minimum a w Toxin Growth production

24 Estimates of half-lives of patulin and citrinin for various grain-a w combinations at 25oC Half-life (days) Grain a w Patulin Citrinin Barley Corn Wheat Adapted from Harwig et al., J. Food Sci., 42:1225 (1997)

25 Heat Resistance Heat resistance increases as the a w of the environment decreases At a given a w, the type of solute can influence the rate of thermal inactivation

26 Influence of a w and type of solute on D values of mold conidia Fungus a w Temp. (oc) Solute D value (min) Aspergillus flavus None Glucose Aspergillus parasiticus None Glucose Aspergillus niger None None None None Penicillium puberulum None

27 Influence of a w and type of solute on D values of fungal ascospores Temp. Fungus oc) Solute Byssochlamys nivea a w Control D value (min) Zygosaccharomyces bailii Kluyveromyces marxianus Control Control

28 Influence of a w and type of solute on D values of vegetative cells of yeasts Yeast a w Temp. (oc) Solute D value (min) Geotrichum candidum None Saccharomyces cerevisiae None Debaryomyces hansenii None

29 Heat resistance of yeasts as affected by solute at a w 0.95 Solute 48% 35% Sorbitol 35% Glucose 35% Fructose 25% Glycerol S. pombé Z. rouxii Adapted from Corry, J. Appl. Bacteriol., 40: (1976)

30 Heat resistance of Bacillus subtilis spores as affected by a w and solute Solute None a w 1.00 D 95oC value (min) Glucose Glycerol

31 Fungicidal activity of propylene oxide at various relative humidities Mold D value (h, room temp.) at various RH (%) Aspergillus niger Penicillium thomii Adapted from Tawaratani and Shibasaki, J. Ferment. Technol., 50:349 (1972)

32 Considerations When Designing Challenge Studies to Determine the Minimum a w for Growth of Microorganisms Use food system Select formula and processing conditions Inoculate with test microorganism(s) Select appropriate incubation conditions Select appropriate detection and enumeration procedures

33 Control of a w in Foods Equilibrate with atmosphere of lower or higher equilibrium relative humidity (ERH) Remove or add water Add solute(s) Change temperature

34 Solutes and Humectants Sodium chloride Sugars (sucrose, glucose, fructose) Sorbitol Glycerol Propylene glycol

35 Detection/Enumeration of Xerotolerant Fungi Diluent - Add 20-30% sucrose, glucose, or glycerol Agar Media - Various, supplemented with solute y Czapek or potato dextrose agar with up to 50% sucrose y DG18 agar y Malt extract yeast extract glucose (50-70%) agar y Malt salt agar