Bacteriology/Food Science 324:
to go along with Exercises 3 and 4
in the 1999 edition of the lab manual

Solutions to the problems below are posted here.

  1. In a thermal death time experiment, the class results obtained for an Escherichia coli culture in September, 1999 were as follows:

    minutes 54°C 57°C 60°C
    0 3.0 X 109 2.1 X 109 3.8 X 108
    5 3.1 X 109 1.6 X 109 4.5 X 106
    10 1.0 X 109 4.5 X 108 5.8 X 104
    15 9.2 X 108 2.7 X 108 (too few to count)
    20 6.5 X 108 1.1 X 108 (too few to count)

    1. On the appropriate graph paper, plot the class results and determine the D-values, thermal death times (for a 109 cells/ml culture) and Z-value.

    2. Summarize the D and Z values and thermal death times on the following table:
      54°C 57°C 60°C Z-value
      D-value TDT D-value TDT D-value TDT

  2. Included in a sample of a fresh food is 80% moisture and 6% NaCl by weight. Subsequent drying of the food causes it to lose half of its weight. Compare the brine strength (w/w) in the fresh and dried products. (Solution hint: Suppose the food sample weighed 100 grams originally. Then consider what happens during drying.)

  3. The following results were obtained in an aw experiment for three food products by the use of our filter-strip technique:
    Product aw value of salt on strip
    0.99 0.98 0.96 0.95 0.93 0.91 0.85 0.82 0.77 0.71
    I dry dry dry dry dry dry dry dry wet wet
    II dry dry dry dry dry dry dry wet wet wet
    III dry wet wet wet wet wet wet wet wet wet
    1. What was the approximate aw range for each food product?

    2. Which product is best protected against bacterial spoilage on the basis of this test?

Return to the 1999
Bact./Food Sci. 324
Home Page
Solutions are here.

Page last modified on 7/20/00 at 6:00 PM, CDT.
John Lindquist, Department of Bacteriology,
University of Wisconsin – Madison