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1.Write – in full detail – how you would carry out the following counts. Describe the technique (or multiple techniques), media along with incubation temperatures and times you would use.
a.An aerobic plate count of a dry soup mix.
b.Water that is being supplied to a food processing plant.
c.Thermophiles in a sample of buttermilk.
d.Psychrophiles in an ice cream sample.
2.You would like to enumerate the micro-organisms present in a 25 g sample of chicken mince. You place the 25 g sample into 225 mL of sterile peptone water and perform a serial dilution to 10-4. You then performed the pour plate method with 1 mL of each dilution for both Plate Count Agar and Violet Red Bile Agar. You plated each dilution onto triplicate plates and incubated all the plates at 35 ºC.
a.What precautions would you take to ensure that the chicken sample was suitable for testing?
b.What does Plate Count Agar test for?
c.What does Violet Red Bile Agar test for?
d.Using the results in the Table 1 below, calculate the average CFU/g of the original chicken mince sample for both plate count agar and violet red bile agar. Show all working.
e.Why do differences occur in colony counts across triplicate plates of the same dilution?
| Dilutions | Plate Count Agar
(Triplicate Plates) |
Violet Red Bile Agar (Triplicate Plates) | ||||
| 1 | 2 | 3 | 1 | 2 | 3 | |
| Sample (10-1) | TNTC | TNTC | TNTC | TNTC | TNTC | TNTC |
| (10-2) | TNTC | TNTC | TNTC | 250 | 234 | 215 |
| (10-3) | 105 | 97 | 95 | 32 | 27 | 20 |
| (10-4) | 9 | 8 | 17 | 0 | 0 | 0 |
Table 1: Results showing colony counts for each dilution and agar.
(TNTC = Too numerous to count)
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