37. What is the relationship between temperature and

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 (Gänzle) 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. milleri)
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     0.052
14    0.11     0.11     0.078
16    0.14     0.15     0.011
18    0.19     0.20     0.16
20    0.24     0.26     0.23
22    0.30     0.29     0.30
24    0.37     0.37     0.37
26    0.45     0.46     0.42
28    0.49     0.55     0.42
30    0.61     0.64     0.35
32    0.66     0.70     0.20
34    0.66     0.70     0.05
36    0.58     0.54     0.00
38    0.39     0.31
40    0.1      0.055
41    0.00     0.00

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

growth rate = a (x^b)(e^cx)
for lactobacilli, x is (41-temperature), all temperatures are in deg.
centigrade
for the yeast, x is (36-temperature), all 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