Bacteriology/Food Science 324:
KEY TO THE SAMPLE WATER ACTIVITY
AND THERMAL DEATH TIME PROBLEMS
to go along with Exercises 3 and 4
in the 1999 edition of the lab manual


  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.  Both graphs follow. Note how the D- and Z-values can be determined directly from the graphs.

    2. Summarize the D and Z values and thermal death times (for a 109 cells/ml culture) on the following table:  (D values and TDTs are in minutes.)
      54°C 57°C 60°C Z-value
      D-value TDT D-value TDT D-value TDT
      27 243 15 135 2.7 24.3 6°C


  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.)
      before
    drying
    after
    drying
    water content 80g 30g
    NaCl content 6g 6g
    other stuff 14g 14g
    total 100g 50g

    As drying causes the food to lose half its weight, 50g of water is lost, and the brine strength increases accordingly. On a weight/weight basis, the brine strength of the fresh product is 6/(80+6) = 6/86 = 7.0%; that of the dried product is 6/(30+6) = 6/36 = 16.7%.


  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?

      I:   between 0.77 and 0.82
      II:  between 0.82 and 0.85 
      III:  between 0.98 and 0.99  

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

Return to the 1999
Bact./Food Sci. 324
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Page last modified on 7/20/00 at 6:00 PM, CDT.
John Lindquist, Department of Bacteriology,
University of Wisconsin – Madison