Biol 251 Lecture 24 Food Microbiology

problems detecting microbes in food

Low levels (enrichment methods) May be stressed or injured (enrichment methods) Not evenly dispersed (test multiple samples) Testing can take several days (development of rapid methods)

Calculate the number of microorganisms in a food sample based on the results of a serial dilution

CFU/ml = # colonies/(dilution factor x volume)

Describe how the following methods are used to reduce the microbial load of foods: canning,

Canning: use of high heat and pressure

Name foods that are produced using microorganisms

Cheese Yogurt Bread Beer and wine Chocolate Vinegar Soy sauce Fermented meats Saurkraut

The ability of microbes to ferment different compounds is used to produce a variety of foods. Which of the following are possible products of fermentation? Select all that apply.

Lactic acid, acetic acid, carbon dioxide, and ethanol are all products of fermentation. Other possible products of fermentation are shown below.

Describe how microorganisms are detected in food, including selective enrichment, nonselective enrichment, cultural isolation, and biochemical identification

Non-selective enrichment -allow all microorganisms to grow Selective enrichment -inhibits growth of some microorganisms and encourages growth of target organisms Cultural enrichment -non-selective pre-enrichment - selective secondary enrichments Cultural isolation -selective/differential agars -allows for detection of presumptive positive colonies Biochemical identification -individual biochemical tests

Describe how serial dilutions are used to enumerate microorganisms in food

Number of colonies on plate * reciprocal of dilution of sample = number of bacteria/ml

pasteurization, aseptic packaging, radiation, and high pressure

pasteurization: Mild heat for 30 minutes Aseptic packaging: Heat hydrogen peroxide UV radiation High energy electron beams Aseptic packaging consists of filling and sealing a sterilized packaging material with a sterilized product. Radiation: X-rays or gamma rays High pressure

Describe the processes used to produce cheese, yogurt, chocolate, beer, and wine and the roles that microorganisms play

Cheese: Milk + bacteria + rennin Curd (solid) Whey (liquid) Ripening Lactic acid bacteria (cheddar) Propionibacterium (swiss) Penicillium (blue cheese) Yogurt: Evaporated milk Lactic acid bacteria Streptococcus thermophilus - acid production Lactobacillus delbrueckii - flavor and aroma Probiotic yogurts Bifidobacterium spp. Lactobacillus spp. Lactic aid curdles the milk Chocolate: Yeasts, Lactic acid bacteria, acetic acid bacteria, and spore-forming bacteria are used to ferment the bulb surrounding the beans and for flavoring. Wine: 1. Grapes are harvested and crushed 2. Sulfites are added to kills unwanted microbes 3. yeast are added and fermentation occurs 4. Wine is pressed, clarified, and filtered 5. Wine is aged then bottled Beer: Naturally fermented and artificially fermented Artificial: Mashing (wort) Hops and other flavors are added Fermentation Bottling and aging Natural: Mashing Wort cooled overnight Microbes in the air settle in the wort Fermentation (in casks) 4 stages of fermentation: Enterobacteriaceae Main fermentation Acidification Maturation

Describe the procedures used to ensure food safety at home

Clean separate cook chill

Name and describe the types of spoilage that can occur in canned foods and identify at least one organism that causes each type of spoilage.

Thermophilic anaerobic spoilage -improper storage -thermophilic bacteria grow and produce gas Flat, sour spoilage: -inadequate processing -no gas production -thermphilic bacteria Food is underprocessed: -endospore-forming bacteria Leakage during cooling -non-endospore-forming bacteria

Differentiate between index organisms and indicator organisms

index organisms -suggests that pathogens may be present Indicator organisms -indicates a process failure