205
FREEZE—DRYING
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_
This has been observed before (Heller, 1941 : Greaves, 1960) in the course of work with“
pure cultures and attempts to stabilize these by lyophilisation. Leach et al., (1959) have
also shown that temperature during drying is important, and that there are a number of '
interactions between the s01utes present, the drying temperatures, andthe specic microbes.
It would be of interest if further work were done with food systems of differing complexity
and under controlled conditions.
'
_
However, while it is obvious from the above cited 1964 work that platen temperatures -
do inuence the survival rate of organisms, from a pragmatic point of view, the parameters
and limitations of platen temperature programming are governed by the economics of
various rates of drying and the resultant product quality; and the effects on the microbial
.…
ora in the
are incidental.
‘
Storagè«of the Dehydrated Foodstu'
,
The ability of Organisms
dehydrated foodstuffs to survive during storage is dependent
on the-water activity
during storage, and the nature of the gaseous
atmosphere. In addition, the
organism is, a priori, a factor.
Scott (1958), Christian
the matter of water activity of
dehydrated
and the survival of test miè£bbfgamsxs;
observed an
Ï
optimum
work
eta1., (1966)_no
optimal was noted.
…
°Ï
;
-
Silverman and Goldblith (1965) inoculatéd‘“ S tyjæhz‘inuiüm into beef and egg which
showed better survival When stored under nitrogen than under air (Figure 3).
Thus,
'
although. organisms are destroyedmorasin inthe_presence of oxygen than in its absence
during} storage of organisms, the practical faCt is that an inert atmosphere is necessary
for maintenance of quality of freeze—dried foods. The greater sensitivity of bacteria in
the presence of ( oXygen (in comparison with oxygen being
been noted With
'
heat energy and With ionizing energy.
1
‘
and
From ‘a public health point of view,
most important considerations, in thegcase
of froZçri foods as well as freeze—dried
the surviüng Œgaùisms, particularly
those of public health signicance, grow aî1d iultiply
particularly
if the food is mishandled. With this in mind, May
and
'
(1966) have studied
shrinäp and chicken.
'
As W1thfrozen foods, the
the
'
growthrates
°C, with shrimp, the storage
time'jÊlôhger than at 20 °C.
;' W1th both chicken and
With
stdrage
'
'iSî’fneceSSary
rehydratlonand,1fÂhandled
of
of
,
quahtyfromarmorobwlogrcalpointofvrewasfrozenfoodsLeftoverrehydratedlyoph1
_
appears” -
tobeaboutthesame aÎSWlîfroz‘m>ÎÏ1&W6dprodIm’csstoredatthesàm‘e temperature.
.
,
*aw=watervaporpressuæofrhefood
4
_
.
_
…
,
C :
1;’
,[
}-…_‘ "
.
-
—'
…