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37. What is the relationship between temperature and activity?

Subject: 37. What is the relationship between temperature and sourdough activity?

Recent information indicates that the time-temperature relationship is steeper than was proposed, and not quite so log-linear as had been assumed.

Through the range 40 to 75 degrees, rates may double approximately for each 7 degrees F., rather than for each 18 degrees F. as I had assumed. That is based on information sent to me by Michael Gaenzle, who, with colleagues, has studied growth rates of sourdough yeasts and bacteria, and on (my) assumption that leavening activity and yeast growth are mutually proportional.

Further, Michael Gaenzle (GŠnzle) has shown that sourdough yeast growth (for the SF sourdough yeast organism) is severely retarded by temperatures much above 85 degrees F. and that culturing above that temperature can deplete the yeast, leaving the lactobacillis predominant.

Michael's study was published last July in _Applied and Environmental Microbiology_, but I have not seen it. I have appended the information that I have (an email received last June 28) in case some one would like to help me speculate on how a time-temperature table might be presented.

There is probably a simple answer about how to adjust "proofing" times for various temperatures, but I have come to understand that I am not exactly sure what it is just now.

Thanks to Michael for his contributions to sourdough science, and for his interest in our discussion group.

- Dick Adams

Dear Dick Adams

Please find attached the growth rates of L. sanfranciscensis and C. milleri as function of temperature. Growth rate is ln2/generation time, i.e. a growth rate of 0.7 is a generation (doubling time) of about 1 h.

The generation times measured in laboratory media are different from that in rye / wheat / white wheat dough, however, if the generation time at 20 C is 1/2 of that at 30 C in my medium, the organism will also grow 1/2 as fast at 20 C compared to 30 C in dough (we checked). So, it's not the absolute numbers that matter, but the ratio of growth rate to growth rate at optimum temperature.

Temp  L. sf I  L.
sf II Yeast (C.
2     0.019   
0.016    0.004
4     0.026   
0.022    0.008
6     0.035   
0.031    0.013
8     0.047   
0.043    0.021
10    0.063   
0.060    0.033
12    0.084    0.08
14    0.11     0.11
16    0.14     0.15
18    0.19     0.20
20    0.24     0.26
22    0.30     0.29
24    0.37     0.37
26    0.45     0.46
28    0.49     0.55
30    0.61     0.64
32    0.66     0.70
34    0.66     0.70
36    0.58     0.54
38    0.39     0.31
40    0.1     
41    0.00     0.00

The curves were
generated by
fitting the
following curve to
experimental data:

growth rate = a
for lactobacilli, x
(41-temperature), all
temperatures are in
deg. centigrade
for the yeast, x is
temperatures are in
deg. centigrade

A, b and c were
calculated as
follows for the
three organisms: 

     L. sf I   L.
sf II  Yeast

a    0.1267   
0.0682    0.0124
b    1.5404   
1.9782    2.9810
c   -0.1931  
-0.2233   -0.3355

If I didn't make a typing error this equation should generate the curve described above. The curve does not give the best approximation at temperatures below 10C, though.

Transfer of the curves from our strains to your starter may change things a bit, but nevertheless I think that it may serve as guideline for many sourdough starters.

Let me know if you think it works (or not).

- Michael


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