MICROBIAL GROWTH & FOOD PRESERVATION

Water, air, and especially food are habitats for microorganisms. A variety of foods serve as an appropriate hub for colonization of these microorganisms, leading to contamination and food spoilage. There are several techniques used to manipulate microbial growth. For this discussion we will only be concerned with temperature/refrigeration. “When we have nutrients, moisture, time and favorable temperatures, they grow rapidly increasing in numbers to the point where some can cause illness. Therefore, understanding the important role temperature plays in keeping food safe is critical.”

F OOD POISONING & FOOD INFECTION

The cause of food borne illnesses:

v Failure to decontaminate

v Failure to preserve properly

Clostridium botulinum is an example of an exotoxin associated with food poisoning. Food poisoning is the ingestion of foods containing preformed microbial toxins. These microorganisms that produced the toxins do not grow in the host and are not alive during the consumption.

“Members of the Clostridium genus are anaerobic (grows in the absence of oxygen) spore formers. Cooking procedures do not kill spores and under appropriate anaerobic conditions, the spores germinate and toxins are produced.”

Salmonella is an example of a food infection. “Food infection is from the ingestion of pathogen-contaminated food. Food infection is a very common type of food borne illness. Symptoms begin only after the pathogen colonizes the intestinal epithelium.”

PROPER STORAGE TO PREVENT FOOD SPOILAGE

While the invasion of microorganisms in our food is inevitable, here is some useful information to extend the shelf as long as possible.

DAIRY PRODUCTS:

Dairy products such as cheese, milk, and yogurt should be refrigerated as quickly as possible. These commodities spoil quickly if not stored at proper temperatures. The suitable temperature range is 34°F - 38°F.

“Spoiled milk exhibits a fruity off odor, acid taste (sour is how we taste acid), and may curdle. As microorganisms grow, they utilize the food as a nutrient source and produce acids.”

MEAT PRODUCTS:

Beef, poultry, lamb, and fish are all examples of “highly perishable and are potentially hazardous due to their moisture and high protein content.” Bacteria produce a slimy film on the outer surface. To prevent this type of spoilage meats should be consumed within one day of purchase. However, poultry can be stored in the freezer for up to one year. Fish up to 3 to 6 months.

The proper storage temperature is 33°F - 36°F.

FRUITS AND VEGGIES:

The appearance of spoilage is more obvious to spot on fruits and vegetables. It is usually a type of fussy mold. To avoid this always refrigerate! Most fresh veggies can be stored up to 5 days. However, there are some exceptions:

“Potatoes and onions should be stored in a cool place between 50°F - 60°F.

Tomatoes should be stored at room temperature because they continue to ripen after harvesting.”

MICROBIAL SPOILAGE OF FRESH FOODS:

FOOD PRODUCT	TYPE OF ORGANISM: BACTERIA	COMMON SPOILAGE ORGANISM
FRUITS & VEGGIES		*Pseudomonas, & Corynebacterium*
MEAT, POULTRY, & FISH		*Salmonella, &* *Campylobacter*
MILK		*Streptococcus, Pseudomonas, & Proteus*
HIGH-SUGAR FOODS		*Clostridium & Bacillus*

	FUNGI
FRUITS & VEGGIES		Aspergillus, Rhizopus, & Penicillium
MEAT, POULTRY, & FISH		Micrococcus, & Flavobacterium
HIGH-SUGAR FOODS		Saccharomyces, & Penicillium

The organisms listed are some of the most commonly observed spoilage agents of fresh, perishable foods. Genera in bold face include possible human pathogens.

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Here is a very helpful table to provide you with recommended storage times from date of purchase.

Storage Times For Home-Refrigerated Foods

NOTE: These short but safe time limits will help keep home-refrigerated food from spoiling or becoming dangerous to eat.

Eggs
Fresh in shell	3-5 weeks
Raw yolks, whites	2-4 days
Hard cooked	1 week
Liquid pasteurized eggs	Unopened, 10 days Opened, 3 days
Cooked egg dishes	3-4 days
Mayonnaise, commercial	2 months
Deli & Vacuum-Packed Products
Store-prepared salads (or homemade) Egg, chicken, tuna, ham, macaroni, etc.	3-5 days
Pre-stuffed pork & lamb chops and chicken breasts	1 day
Store-cooked convenience meals	3-4 days
Commercial brand vacuum-packed dinners with/USDA seal, unopened	2 weeks
Raw Hamburger, Ground & Stew Meat
Ground beef, turkey, veal, pork, lamb	1-2 days
Stew meats	1-2 days
Ham, Corned Beef
Ham, canned, labeled "Keep Refrigerated"	Unopened, 6-9 months Opened, 3-5 days
Ham, fully cooked, whole	7 days
Ham, fully cooked, half	3-5 days
Ham, fully cooked, slices	3-4 days
Corned beef in pouch with pickling juices	5-7 days
Hot Dogs & Luncheon Meats
Hot dogs	Unopened package, 2 weeks Opened package, 1 week
Lunch meats	Unopened package, 2 weeks Opened package, 3-5 days
Soups & Stews	3-4 days
Bacon & Sausage
Bacon	7 days
Sausage, raw from meat or poultry	1-2 days
Smoked breakfast links, patties	7 days
Summer sausage labeled "Keep Refrigerated"	Unopened, 3 months Opened, 3 weeks
Pepperoni, sliced	2-3 weeks
Cooked Meat, Poultry & Fish Leftovers
Pieces and cooked casseroles	3-4 days
Gravy and broth, patties & nuggets	1-2 days
Fresh Meat (Beef, Veal, Lamb & Pork)
Steaks, chops, roasts	3-5 days
Variety meats (Tongue, kidneys, liver, heart, chitterlings)	1-2 days
Fresh Poultry
Chicken or turkey, whole	1-2 days
Chicken or turkey, parts	1-2 days
Giblets	1-2 days
Fresh Fish & Shellfish	1-2 days

INTRODUCTION

INFECTION VS POISONING

NOTE: This table was copied/paste directly from website http://www.fsis.usda.gov/

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EXPERIMENT #36

1st Period

Materials:

v (3) 10 g samples of ground salt pork, cottage cheese, or ground beef

v 8 Nutrient agar plates

v (1) 90 ml dilution bottle of sterile saline

v (4) 9 ml dilution tubes of sterile saline

v 1 and 10 ml pipettes

v Spatulas

v 95% ethyl alcohol

Procedure:

NOTE: To obtain a statistically valid count, each dilution will be plated in duplicate.

1. Place one sample of the food product assigned in the refrigerator and one in the desk drawer or on the shelf in the laboratory. Using aseptic technique (dipping the spatula in 95% ethyl alcohol and then flaming it), place the third sample into a 90 ml dilution bottle (10^-1 dilution).

2. Shake the dilution bottle vigorously for 20 sec., then let the sample settle to the bottom.

3. Pipette 1.0 ml of the solution into a 9 ml dilution blank (10^-3 dilution); roll vigorously between the palms for 5 sec.

4. Pipette 0.1 ml of the above dilution (10^-2 dilution) onto Nutrient agar plates and spread with a sterile glass spreader. Mark the plates 10^-3. Pipette 1.0 ml of the above dilution into a 9.0 ml dilution blank (10^-3dilution). Swirl the tube between the palms of the hand to mix thoroughly.

5. Pipette 0.1 ml of the above (10^-3 dilution) onto a Nutrient agar plate and spread as above. Mark the plate 10^-4, then pipette 1.0 ml of the dilution into a 9.0 ml dilution blank (10^-5 dilution). Mix thoroughly.

6. Pipette 0.1 ml of the above dilution onto a Nutrient agar plate and spread with the glass spreader. Mark the plate 10^-5 then pipette 1.0 ml of the dilution into a 9.0 ml dilution blank (10^-5 dilution). Mix thoroughly.

7. Pipette 0.1 ml of the above dilution onto a Nutrient agar plate and spread with the glass spreader. Mark the plate 10^-6.

8. Incubate the plates at 37°C for 48 hr.

2^nd Period

Materials:

v Glass spreader

v 16 Nutrient agar plates

v Spatulas

v (3) 99 ml dilution bottles of sterile saline

v 95% ethyl alcohol

v (2) 90 ml dilution bottles of sterile saline

v Glass spreaders

v (7) 9 ml dilution tubes of sterile saline

v Quebec® Colony Counters

v 1 and 10 ml pipettes

Procedure:

1. Utilizing a Quebec® Colony Counter, count the number of colonies on each plate from (1) and record the results.

2. Repeat the dilution procedure outline in steps 1 through 8 for the sample stored in the refrigerator.

3. For the sample stored in the desk drawer or shelf, dilutions of 10^-6 to 10^-9 should be prepared.

3rd Period

Materials:

v Quebec® Colony Counter

Procedure:

1. Utilizing a Quebec® Colony Counter, count the number of colonies on each plate from the previous exercise. Record results.

FIG. 36.1 Procedure for determining the microbial contamination of food samples.

"Well if you don't like the toxins, then just eat the noodles"

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SOURCES:

1.http://www.fsis.usda.gov

2.http://www.agen.ufl.edu

3.http://www.findarticles.com

4.http://www.ces.uga.edu

5.http://www.content.health.msn.com

6.Kerr, Thomas J. and Barbara B. McHale. Applications In General Microbiology: A Laboratory Manual. 6th ed. Winston-Salem: Hunter Textbooks, Inc., 2003.
7.Prescott, Lansing M., John P. Harley, and Donald A. Klein. Microbiology. 5th ed. New York: McGraw-Hill, 2002.

MICROBIAL GROWTH & FOOD PRESERVATION

FOOD POISONING & FOOD INFECTION

PROPER STORAGE TO PREVENT FOOD SPOILAGE

DAIRY PRODUCTS:

MEAT PRODUCTS:

FRUITS AND VEGGIES:

Storage Times For Home-Refrigerated Foods

Procedure:

NOTE: To obtain a statistically valid count, each dilution will be plated in duplicate.

F OOD POISONING & FOOD INFECTION