Enumeration of Bacteria Present in Food Samples

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tube and was mixed three times. In a similar manner, this dilution step was repeated with the rest of the tubes. At the end, tube 1:16 had 2ml of the diluted organisms. The optical density of each of the four tubes was measured, staring from the 1:16 tube first. The O.D. values were recorded in the Results and a graph of O.D. against dilution was plotted using Microsoft Excel.

Part C: Bacterial Count in Various Food Samples

Aseptic technique was used to weigh out 20 grams of food on a weighing paper to be tested. The food was added together with 180ml of sterile water to a sterile mechanical blender jar. The food was then blended of 5 minutes. A suspension of a 1:10 dilution was provided. The blended food samples were dispensed and distributed to the class. Then, using a micropipette, 0.1ml of a chosen food sample was dispensed to plate I and 0.1ml to the water blank. The water blank was shaken 25 tines in an arc fir 7 seconds with the elbow on the table. With a clean pipette, 0.1 ml from the water blank was dispensed to plate III and 1.0ml to plate II. Liquefied agar is then poured into three plates and the plates are incubated at 35C for 24 hours. The colonies are counted on the best plate and the results are recorded.

Results

Part A: Quantitative plating method

Table 1: Number of colonies found based on dilution factor

Dilution bottle ml plated Dilution Dilution factor Number of colonies

b (1:10,000) 1.0 1:10,000 10^4 >300

b (1:10,000) 0.1 1:100,000 10^5 >300

c (1:1,000,000) 1.0 1,000,000 10^6 >300

c (1:1,000,000) 0.1 10,000,000 10^7 65

Based on Table 1, the number of bacteria colonies decrease as the dilution factor increase. This is because of the decrease in concentration of the suspension used.

Part B: Optical density determination

Table 2: Optical Density based on dilution

Dilution Optical Density

1:1 1.350

1:2 0.757

1:4 0.412

1:8 0.242

1:16 0.080

Figure 1: Effects of dilution on optical densities

Based on Figure 1, in optical density increases proportionally as the concentration increases.

Part C: Bacterial Count in Various Food Samples

Table 3: Bacteria count of various food types

Type of Food Plate Count Dilution Organisms per mL

Kueh I >300 1:10 >3000

Kueh II 103 1:100 103

Kueh III 32 1:1000 320

Sandwich I >300 1:10 >3000

Sandwich II >300 1:100 >300

Sandwich III 8 1:1000 80

Curry puff I 19 1:10 190

Curry puff II 17 1:100 17

Curry puff III 15 1:1000 150

Figure 2: Bacteria Growth of Kueh I

Figure 3: Bacteria Growth of Kueh II

Figure 4: Bacterial Growth of Kueh III

Discussion

Based on the results obtained, the type of food that was expected to have the highest bacteria count was Sandwich as it contains deli meat in which was ham. However, the reason to the lower expected bacteria growth may be due to lactic acid bacteria dominating the components of meat microbial growth. This causes fermentation of meat and prolongs the shelf life. This usually occurs in packaged meat, such as ham and sausages, that are vacuum-packed that has low gas permeability (Egan, 1983).

The type of food that had the highest bacterial count is the ‘kueh’. The kueh that was used in this experiment contains raw coconut meat. It is found that contamination of bacteria could be caused by the contact of soils that contain bacteria and also dispersion via infected shells and coconut milk (Schaffner et al, 1966). In the same study, by pasteurizing raw coconut meat in an 80C water bath, these bacteria can be destroyed. However, the kueh that was used was stored at room temperature for approximately 6-10 hours. Hence, the bacterial growth would have increased. Another factor that could have cause the high bacterial count is Salmonella bacteria contamination. Salmonella, a Coliform, is usually found in eggs, poultry, and meat and its growth can be inhibited at a very low temperature of 5C and below (Malorny et al, 2008). The kueh that was used is possibly contaminated with Salmonella due to inappropriate storage, in which is in close vicinity with raw animal products above the inhibition temperature.

The type of food with the lowest bacteria count is the curry puff. This may be due to it being deep-fried in hot oil. Deep-frying food is found to kill bacteria and viruses in which inhibits the growth (Morrisona & Petticrew, 2004). The minimum heat of the oil for deep-frying to occur is 180C. Hence, the high temperature would terminate microorganisms.

The type of food with the second most bacterial count is the sandwich. The sandwich that was used in this experiment contained ham, butter and cheese. Coliform bacteria are commonly found in meat products. It is found in the digestive tracts of animals and may have been opened during the butchering process. This may have caused the dispersion of the bacteria onto the meat (Newton et al, 1976). Salmonella and E. coli are intestinal pathogens found in meat due to the increased Coliform count.

Dried fruits are knows to be resistant to spoilage due to its low water activity. According to the a study by Christian & Scott (1953) the minimal aw for growth is 0.945 and for Staphylococcus aureus it is 0.86 (Scott, 1962). An aw lower than 0.94 will inhibit the bacterial growth in