FLORIDA AGRICULTURAL EXPERIMENT STATION
Department of Dairy Science
Dairy Science Mimeo 59-2
January 5, 1959
Freezing Lactic Starter Cultures
B. J. Liska
Having a sufficient supply ef active lactic starter culture is a problem
in many dairy plants producing fermented milk products. Lactic cultures can be
obtained from commercial supply houses in liquid or freeze dried forms. Freeze
dried cultures will remain active at refrigerator temperatures for a year or more
bt* liquid cultures must be kept frozen if held for longer than one week.
A ripened culture loses activity rapidly in frozen storage. Research
has shown that by reducing the acidity of a ripened culture with alkali before
freezing, the culture remained active for longer periods of time in frozen storage.
Instead of using alkali to reduce the acid, attempts were made in this study
to reduce the acidity by dilution with sterile skimmilk. Cultures were ripened to
an acidity of 0.80 0.95% and diluted 1-10 with sterile skimmilk. Ten ml.
portions of these diluted cultures were placed in sterile test tubes and stored at
-10 to -150F. The viability of the cultures was checked by making plate counts
and actual transfers to determine speed of acid production a2ter 1, 15, 30, 60,
90, 120 and 150 days storage. The results for one culture appear in Table 1.
Other cultures reacted in a similar manner. After 150 days storage the
cultures used in this study had lost only 10% of their original activity. After
the second transfer, activity and flavor production of the thawed cultures were
equal to the original unfrozen cultures in most cases, LM-.=
Table 1. Viability of H 4 Frozen Lactic Culture Diluted 1-10 with 12.5% Sterile
Reconstituted NDM Before Freezing. (a, b)
% Acidity of 1st Transfer
ime in Bacterial Count (c) After 16 Hours at
storage (days) Millions/ml. 700 F.
5 -- 0.84
0 23.5 0.84
0 -- 0.82
0 18.1 0.81
20 -- 0.80
50 8.5 0.76
a) Samples were stored at --100 to --150 F.
b) Samples were thawed in water bath at 400 F.
c) Bacterial counts on samples immediately after thawing.
Two additional factors studied were thawing temperature and percent solids
in the sterile skimmilk used for dilution of cultures prior to freezing. The
percentages of solids of the skimmilk (from 10% to 15%) had no noticeable effect
on survival of cultures. Therefore heated skimmilk containing 10% solids will
serve the purpose well. Tharing frozen cultures at 400 F. gave better results
than temperatures of 600 F. to 700 F.
Keeping good cultures in a frozen state can be helpful in several ways:
(1) additional cultures can be kept on hand for emergency use, (2) a plant can
dispense with caring for mother cultures over weekends and during periods of
slack operations, (3) a particularly good culture can be preserved for later
use, and (4) sufficient st-rter can be frozen at one time for use in making
small quantities of fermented products over a period of time.
- 3 -
Mixed strain lactic starter cultures can be stored in frozen state for
several months with only a minor loss of activity. These cultures should
be handled as follows: (1) transfer a culture several times to determine activity,
flavor and aroma and freedom from contamination, (2) on the last transfer make
sufficient starter for freezing and ripen it to 0.80 to 0.85% acidity, (3) dilute
the starter 1-10 with 10% reconstituted NDM which has been steamed for one hour,
or preferably autoclaved at 1210 C. for 10 minutes, (4) place in suitable closed
sterile containers, freeze and store at -100F. to -150 F; (5) thaw these samples
in water at 400 F, (6) either use the diluted tha-ed culture and inoculate at ten
times the usual rate (equivalent to the undiluted culture) or incubate the thawed
culture until the desired acidity is reached and then use it as undiluted starter,
at the normal rate depending on the fermented product being produced.