Question

A buffet restaurant uses heat lamps at a temperature of 50^oC to keep the food warm. If the restaurant opens for service at 4 pm and closes at 10 pm, some of the food items that are not refilled frequently may sit for 6 hours at this temperature.

The following experiment was conducted to determine whether this temperature is suitable to maintain the food and prevent bacterial growth:

Beef cubes were inoculated with 500,000 bacterial cells and incubated at 43-53^oC to establish temperature limits for bacterial growth. Table 1 contains the numbers of bacteria after incubation for 6 hours, and plating on nutrient agar given in the numbers of colonies for a particular dilution.

Calculate the cfu/ml for each organism at each temperature using the formula

cfu/ml = # colonies x dilution factor x plating factor (10)

(this is what i mostly need help with, how does one calculate the plating factor? or am i just suppose to think 10 is the plating factor and multiple the previous numbers by 10?)

and draw a line or bar graph to indicate the amount of growth after 6 hours for each temperature.

What holding temperature would you recommend to maintain the food with minimal contamination? Is 50^oC sufficient? Assuming that cooking kills bacteria in food, how could these bacteria contaminate the cooked foods? What disease does each organism

Organism	Temp. (oC)	# Colonies	Dilution
Staphylococcus aureus	43	15	1:1000000
	51	8	1:10000
	53	6	1:100
Salmonella typhimurium	43	32	1:100000
	51	9	1:10000
	53	2	1:1000
Clostridium perfringens	43	17	1:100000
	51	12	1:1000
	53	4	1:100

Answer 1

Organism	Temp (0C)	# of colonies	Dilution	Dilution factor	Cfu/ml
Staphylococcus aureus	41	15	1:1000000	10^6	15*106*10= 1.5 X 108
	51	8	1:10000	10^4	8*104*10= 8 X 105
	53	6	1:100	10^2	6*102*10= 6 X 103
Salmonella typhimurium	41	32	1:10000	10^5	32*105*10= 3.2 X 107
	51	9	1:10000	10^4	9*104*10= 9 X 105
	53	12	1:1000	10^3	12*103*10= 1.2 X 105
Clostridium perfringens	41	17	1:100000	10^5	17*105*10= 1.7 X 107
	51	12	1:1000	10^3	12*103*10= 1.2 X 105
	53	4	1:100	10^2	4*102*10= 4 X 103

10 is the plating factor. Plating factor is the amount of culture that is plated from the dilution on to the plate. It is given that this factor is 10. Hence, 1 ml must have been plated from total 10 ml of final dilution.

Cfu/ml= # of colonies * dilution factor*10

At 53⁰C, 6000 S. aureus, 120000 S. typhimurium and 4000 C. perfringens organism per ml were alive. Hence, a temperature of 50⁰C did not kill the organisms completely in food. Most microorganism are killed at a temperature of 121⁰C for 20 min. This temperature will kill the spores of Clostridium perfringens.

These microorganism can cause food poisoning, if there is contamination from outside source. Spore forming bacteria are not killed by cooking. Inadequate storage temperatures will also cause contamination from refrigerators. Also, improper handling of cooked foods may introduce contamination.

S .aureus   is a gram positive bacterium that causes food poisoning if ingested via food. Symptoms include Nausea, vomiting, diarrhea, loss of appetite, severe abdominal cramps, mild fever. Clostridium perfringes causes food poisoning that includes abdominal cramps and watery diarrhea. Salmonella typhimurium causes food poisoning that includes abdominal cramps, vomiting and watery diarrhea that last for 12-72 hrs.

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