ASHRAE REFRIGERATION SI CH 22-2010 FOOD MICROBIOLOGY AND REFRIGERATION《食品微生物学和制冷》.pdf

1、22.1CHAPTER 22FOOD MICROBIOLOGY AND REFRIGERATIONBasic Microbiology . 22.1Critical Microbial Growth Requirements . 22.1Design for Control of Microorganisms. 22.3The Role of HACCP 22.4Sanitation 22.4Regulations and Standards . 22.5EFRIGERATIONS largest overall application is the preven-R tion or reta

2、rdation of microbial, physiological, and chemicalchanges in foods. Even at temperatures near the freezing point,foods may deteriorate through growth of microorganisms, changescaused by enzymes, or chemical reactions. Holding foods at lowtemperatures merely reduces the rate at which these changes tak

3、eplace. A few spoilage organisms can grow at or below temperaturesat which food begins to freeze.Refrigeration also plays a major role in maintaining a safe foodsupply. Overall, the leading factor causing foodborne illness isimproper food-holding temperatures. Another important factor isimproperly s

4、anitized equipment. Engineering directly affects thesafety and stability of the food supply in design of cleanable equip-ment and facilities, as well as maintenance of environmental condi-tions that inhibit microbial growth. This chapter briefly discussesthe microbiology of foods and the effect of d

5、esign decisions on theproduction of safe and wholesome foods. Methods of applyingrefrigeration to specific foods are discussed in Chapters 30 to 42.BASIC MICROBIOLOGYMicroorganisms play several roles in a food production facility.They can contribute to food spoilage, producing off-odors and fla-vors

6、, or altering product texture or appearance through slime pro-duction and pigment formation. Some organisms cause disease;others are beneficial and are required to produce foods such ascheese, wine, and sauerkraut through fermentation.Microorganisms fall into four categories: bacteria, yeasts, molds

7、,and viruses. Bacteria are the most common foodborne pathogens.Bacterial growth rates, under optimum conditions, are generallyfaster than those of yeasts and molds, making bacteria a prime causeof spoilage, especially in refrigerated, moist foods. Bacteria havemany shapes, including spheres (cocci),

8、 rods (bacilli), or spirals (spi-rochetes), and are usually between 0.3 and 5 to 10 m in size. Bacteriacan grow in a wide range of environments. Some, notably Clostrid-ium and Bacillus spp., form endospores (i.e., resting states withextensive temperature, desiccation, and chemical resistance).Yeasts

9、 and molds become important in situations that restrict thegrowth of bacteria, such as in acidic or dry products. Yeasts cancause gas formation in juices and slime formation on fermentedproducts. Mildew (black mold) on humid surfaces and mold forma-tion on spoiled foods are also common. Some molds p

10、roduce verypowerful toxins (mycotoxins) that, if consumed, may be fatal.Viruses are obligate intracellular parasites that are specific to anindividual host. All viruses, including human viruses (e.g., hepatitisA), cannot multiply outside living cells or tissue. Refrigerationdesign features must incl

11、ude facilities for good employee handwash-ing and sanitation practices to minimize potential for product con-tamination. Bacterial viruses (phages), however, may contribute tostarter culture failure in bacterial fermentations if proper isolation,ventilation, and sanitation procedures are not followe

12、d. The use ofcommercial concentrated cultures, selected for phage resistance, hasgreatly reduced this problem.Sources of MicroorganismsBacteria, yeasts, and molds are widely distributed in water, soil,air, plant materials, and the skin and intestinal tracts of humans andanimals. Practically all unpr

13、ocessed foods are contaminated with avariety of spoilage and, sometimes, pathogenic microorganismsbecause foods act as excellent media for bacterial multiplication.Food processing environments that contain residual food materialwill naturally select for the microorganisms that are most likely tospoi

14、l the particular product.Microbial GrowthChanges in microbial populations follow a generalized growthcurve (Figure 1). An initial lag phase occurs as organisms adapt tonew environmental conditions and start to grow. The lag phase isvery important because the maximum extension of shelf life andlength

15、 of production runs are directly related to the length of the lagphase. After adaptation, the culture enters into the maximum (loga-rithmic) growth rate, and control of microbial growth is not possiblewithout major sanitation or other drastic measures. Numbers candouble as fast as every 20 to 30 min

16、 under optimum conditions.Toxin production and spore maturation, if possible, usually occurat the end of the exponential phase as the culture enters a stationaryphase. At this time, essential nutrients are depleted and/or inhibitoryby-products are accumulated. Eventually, culture viability declines;

17、the rate depends on the organism, medium, and other environmentalcharacteristics. Although refrigeration prolongs generation timeand reduces enzyme activity and toxin production, in most cases, itwill not restore lost product quality or safety.CRITICAL MICROBIAL GROWTH REQUIREMENTSFactors that influ

18、ence microbial growth can be divided into twocategories: (1) intrinsic factors that are a function of the food itselfThe preparation of this chapter is assigned to TC 10.9, Refrigeration Appli-cation for Foods and Beverages.Fig. 1 Typical Microbial Growth CurveFig. 1 Typical Microbial Growth Curve22

19、.2 2010 ASHRAE HandbookRefrigeration (SI)and (2) extrinsic factors that are a function of the environment inwhich a food is held.Intrinsic FactorsIntrinsic factors affecting microbial growth include nutrients,inhibitors, biological characteristics, water activity, pH, and pres-ence of competing micr

20、oorganisms in a food. Although engineeringpractices have little effect on these parameters, an understanding ofhow intrinsic factors influence growth is useful in predicting thetypes of microorganisms that may be present.Nutrients. Like other living organisms, microorganisms requirefood to grow. Car

21、bon and energy sources are usually sugars andstarches. Nitrogen requirements are met by the presence of protein.Vitamins and minerals are also necessary. Lactic acid bacteria haverather exacting nutritional requirements, but many aerobic sporeformers have tremendous enzymatic capabilities that allow

22、 growthon a wide variety of substrates. Cleanable systems facilitate removalof residual food material and deprive microorganisms of the nutri-ents required for growth, thus preventing a buildup of organisms inthe environment.Inhibitors. Either naturally occurring or added as preservatives,inhibitors

23、 may be present in food. Preservatives are not substitutesfor hygienic practices and, with time, microorganisms may developresistance. A cleanable processing system is still essential in pre-venting development of a resistant population.Competing Microorganisms. The presence of one type ofmicroorgan

24、ism affects other organisms in foods. Some organismsproduce inhibiting compounds or grow faster; others are better ableto use the available nutrients in a food matrix.Water Activity. All life-forms require water for growth. Wateractivity awrefers to the availability of water in a food system and isd

25、efined at a given temperature asVapor pressure of solution (food)aw= Vapor pressure of solute (water)The minimum water activities for growth of a variety of micro-organisms, along with representative foods, are listed in Table 1.These awminima are also factors in environmental humidity controldiscus

26、sed in the section on Extrinsic Factors.When food is enclosed in airtight packaging or in a chamberwith limited air circulation, an equilibrium awis achieved that isequal to the awof the food. In these situations, the awof the fooddetermines which organisms can grow. If the same foods areexposed to

27、reduced environmental relative humidity, such as meatcarcasses hanging in a controlled aging room or vegetables dis-played in an open case, surface dehydration acts as an inhibitor tomicrobial growth. Likewise, if a dry product, such as bread, isexposed to a moist environment, mold may grow on the s

28、urface asmoisture is absorbed. Environmental relative humidity thus signif-icantly affects product shelf life.pH. For most microorganisms, optimal growth occurs at neutralpH (7.0). Few organisms grow under alkaline conditions, but some,such as yeasts, molds, and lactic acid bacteria, are acid tolera

29、nt. Fig-ure 2 depicts pH values of a variety of foods and limiting pH valuesfor microorganisms.Extrinsic FactorsExtrinsic factors that influence the growth of microorganismsinclude temperature, environmental relative humidity, and oxygenlevels. Refrigeration and ventilation systems play a major role

30、 incontrolling these factors.Temperature. Microorganisms can grow over a wide range oftemperatures. Minimum growth temperatures for a variety of spoilageand pathogenic bacteria of significance in foods are summarized inTable 2. Previously, 7C was thought to be sufficient to controlTable 1 Approximat

31、e Minimum Water Activity for Growth of MicroorganismsOrganism awFoodsPseudomonads 0.98 Fresh fruits, vegetables, meatsSalmonella spp., E. coli 0.95 Many processed foodsListeria monocytogenes 0.93Bacillus cereus 0.92 Salted butterStaphylococcus aureus 0.86 Fermented sausageMolds 0.84 Soft, moist pet

32、food0.80 Pancake syrup, jam0.70 Corn syrupXerotrophic molds 0.65 CaramelsOsmophilic yeasts 0.62Limit of microbial growth 0.60 Wheat flour0.40 Nonfat dry milkTable 2 Minimum Growth Temperatures for Some Bacteria in FoodsOrganismPossibleSignificanceApproximateMinimum GrowthTemperature, CStaphylococcus

33、 aureus Foodborne disease 10Salmonella spp. Foodborne disease 5.5Clostridium botulinum, proteolytic nonproteolyticFoodborne disease1038Lactobacillus and LeuconostocSpoilage of fresh and cured meats0Listeria monocytogenes Foodborne disease 1Acinetobacter spp. Spoilage of precooked foods 1Pseudomonads

34、 Spoilage of raw fish, meats, poultry, and dairy products1Fig. 2 pH Ranges for Microbial Growth and RepresentativeExamplesFig. 2 pH Ranges for Microbial Growth and Representative ExamplesFood Microbiology and Refrigeration 22.3growth of pathogenic organisms. However, the emergence of psy-chrophilic

35、pathogens, such as Listeria monocytogenes, has dem-onstrated the need for lower temperatures. In the United States,5C is now recognized as the upper limit for safe refrigeration tem-perature, although in some cases 1C or lower may be more appro-priate. Foods that can support growth of pathogenicmicr

36、oorganisms should not be held between 5 and 60C for morethan 2 h.Temperature is used to categorize microorganisms. Those capa-ble of growth above 45C, with optimum growth at 55 to 65C, arethermophiles. Thermophilic growth can be extremely rapid, withgeneration times of 10 to 20 min. Thermophiles can

37、 become a prob-lem in blanchers and other equipment that maintain food at elevatedtemperatures for extended periods. These organisms die or do notgrow at refrigeration temperatures.Mesophiles grow best between 20 and 45C. Most pathogens arein this group, with optimum growth temperatures around 37C (

38、i.e.,body temperature). Mesophiles also include a number of spoilageorganisms. Growth of mesophiles is quite rapid, with typical gener-ation times of 20 to 30 min. Because mesophiles grow so rapidly,perishable foods must be cooled as fast as possible to prevent spoil-age or potential unsafe conditio

39、ns. Also, slower cooling rates causemesophiles to adapt and grow at lower temperatures. With mild tem-perature abuse, prolific growth can occur, leading to spoilage or apotential health hazard.Psychrophiles can grow at 5C; some are able to grow at tem-peratures as low as 5C and are a primary cause o

40、f spoilage of per-ishable foods. Psychrophilic growth is slow compared to mesophilicand thermophilic growth, with maximum growth rates of 1 to 2 h orlonger. However, control of psychrophilic growth is a major require-ment in products with extended shelf life. Because many psychro-philes have optimum

41、 temperatures in the mesophilic range, whatmay seem to be an insignificant increase in temperature can have amajor effect on the growth rate of spoilage organisms. Growthroughly doubles with each 3 K increase in temperature. In practice,shelf life of fresh meat is maximized at 1.7C and is reduced 50

42、%by holding at 2.2C. Meat freezes at 2.2C.For all critical growth factors, the range over which growth canoccur is characteristic for a given organism. The range for growth isnarrower than that for survival. For example, the maximum temper-ature for growth is slightly above the optimum, and death us

43、uallyoccurs just slightly above the maximum. This is not the case at thelower end of the temperature range. Survival of psychrophilic andmost mesophilic microorganisms is enhanced by low storage tem-peratures. Freezing is not an effective lethal process; some organisms,notably gram-negative bacteria

44、, are damaged by freezing and may dieslowly, but others are extremely resistant. In fact, freezing is used asan effective means of preserving microbial cultures at extremely lowtemperatures (e.g., 80C).Environmental Relative Humidity. Water, previously dis-cussed as an essential intrinsic growth fac

45、tor, is also a major extrin-sic factor. Environmental water acts as a vector for transmission ofmicroorganisms from one location to another through foot traffic oraerosols. Refrigeration drain pans and drip coils have been identi-fied as significant contributors of L. monocytogenes contaminationin f

46、ood processing environments. Aerosols have also transmitted theagent that causes Legionnaires disease. High relative humidity incold rooms is a particular problem and leads to black mold build-upon walls and ceilings as well as growth of organisms in drains andother reservoirs of water. Condensation

47、 on ceilings supports micro-bial growth and can drip onto product contact surfaces. Inade-quately drained equipment collects stagnant water and supportsmicrobial growth that is easily transported throughout a productionfacility when people walk through puddles. It is extremely impor-tant to control

48、environmental relative humidity in food productionenvironments. Control measures are discussed further in the sectionon Regulations and Standards.Oxygen. Microorganisms are frequently classified by their oxy-gen requirement. Strictly aerobic microorganisms, such as moldsand pseudomonads, require oxy

49、gen for growth. Conversely, strictanaerobes, such as Clostridium spp., cannot grow in the presence ofoxygen. Facultatively anaerobic microorganisms (e.g., coliforms)grow with or without oxygen present, and microaerophiles, such aslactobacilli, grow best in conditions with reduced oxygen levels.Controlled-atmosphere (CA) chambers for fruit storage use loweroxygen levels to prolong storage life by retarding growth of spoilageorganisms as well as influencing ripening. Vacuum packing also usesthis extrinsic growth factor by inhibiting the growth of strict aero