Studying the Effects of Heat and Cold Shock on Cell wall Microstructure and Survival of Some LAB in Milk

World Applied Sciences Journal 4 (2): 191-194, 2008
ISSN 1818-4952
© IDOSI Publications, 2008

Studying the Effects of Heat and Cold Shock on
Cell wall Microstructure and Survival of Some LAB in Milk

F. Tabatabaie and A. Mortazavi

Department of Food Science, Faculty of Agriculture,
Ferdowsi University of Mashhad, P.O. Box 91775-1163, Iran

Abstract: In this study we investigated the effect of heat shock (42-45°C for 5, 10, 15, 20, 25, 30 minutes)
cold shock (4, 0, -5, -10 and -20°C for 2h) on cell wall Microstructure of LAB. Cell wall Changes and
micro -damage behavior of LAB under the impact of heat and cold shock was studied by optical microscope
and Transmission Electron Microscope (TEM). Treatment of LAB with heat and cold shock had significant
effects on permeability properties of LAB just in long heat exposures and very cold temperatures. The
survival of LAB was just affected by the heat shock.

Key words: Micro structural properties • lab • heat shock • cold shock • survival

INTRODUCTION
other internal compounds is very important for different

aspects of dairy processing technology as reducing the

Probiotic bacteria are defined as living
yoghurt coagulation time.
micro -orginsms, which upon ingestion in certain

numbers; exert health benefits beyond in herent basic
MATERIAL AND METHODS
nutrition. Probiotic’s ability to grow well in the product

and also their survial in the final product is of great

Milk with 2/5% fat and 10/5% dry weight was
importance to show their health benefits. Increasing
supplied from local market. Pure lyophlised cultures of
the entymatic activity of probiotics without any
Lactobacillus acidophilus (strain, LAI), Lactobacillus
negative effect on their survival is noteworthy [1].
casei (strains AB) Lactococcus lactis.spp cremoris and
The secretive enzymes of probiotic bacteria improve
lactococus lactis.spp lactis were supplied from local
their survival [2]. Every kind of shock can influence
industries.
the functionality of the cell wall of bacteria and

Packages of bacteria were prepared according to
improve or damage their normal physiological
the company’s instructions and were added (5%, v/w)
and vital activities [3, 4]. Cold shock changes the
to the milk aseptically, then the milk was distributed
lipid phase in the membrane and develops
into tubes with screwing caps to perform the shocks.
hydrophobic holes in it. The result is the enhanced
Cold and freezing shock for 2 hours in 4/0, -5, -10 and
permeability of cell membrane and release of
-20°C. And Heat shock for 5, 10, 15, 20, 25 and
internal compounds [5, 6].
30 minutes in 42-45°C were performed on the samples.

Freezing shocks in temperatures below -20°C in
Then samples were diluted serially and plated on MRS
comparison to-10°C have less impact on cell wall of the
agar. Then the samples were prepared to observe under
bacteria [7]. The reduction of bacterial population is
the optical and TEM Microscope [9-11].
very fast in temperatures near the freezing points

especially -2°C [8].
RESULTS AND DISCUSSION

Heat shock also causes holes in the membrane and

inactivates the sensitive enzymes and ribosoms, the
Freezing and cold shock: Figure 1-7 show the effect
final result is the reduction of biological activities of the
of cold and freezing shock on probiotic bacteria.
bacteria or their death.

According to the results obtained by optical

The main purpose of this study was to evaluate the
microscope, there was a population balance both in
potential of ultrasound cold, freezing and heat shock on
control and treated samples. As discussed before, by
cell wall of the bacteria and increasing the cell wall
decreasing the temperatures, the streptobacillus or
permeability of some probiotic bacteria by TEM and
filaments are formed. Micro cracks, micro voids and
optical microscope. The release of internal enzymes and
raptures (less than -20°C) are also observed.
Corresponding Author: Dr. F. Tabatabaie, Department of Food Science, Faculty of Agriculturel, Ferdowsi University of
Mashhad, P.O.BOX 91775-1163, Iran
191

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World Appl. Sci. J., 4 (2): 191-194, 2008





Fig. 1: Microscopic picture of probiotic bacteria before

freezing shock
Fig. 6: Microscopic picture of probiotic bacteria after

freezing shock (-20°C)




Fig. 2: Microscopic picture of probiotic bacteria after

freezing shock (4°C)
Fig. 7: Microscopic picture of probiotic bacteria (cell

wall damages are not obvious)



Fig. 3: Microscopic picture of probiotic bacteria after
freezing shock (0°C)



Fig. 8: Microscopic picture of probiotic bacteria
before heat shock.The number of Lactococcus
and lactobacillus are the same and all of
bacteria are safe



Fig. 4: Microscopic picture of probiotic bacteria after
freezing shock (-5°C )



Fig. 9: Microscopic picture of probiotic bacteria after 5
minutes exposure to heat shock



Fig. 5: Microscopic picture of probiotic bacteria after
freezing shock (-10°C)

Heat shock: Figure 8-12 show the effect of heat shock
on the probiotic bacteria. Studies showed that,
increasing the exposure time, does not change the
population to special group of bacteria, other

characteristics of bacteria as the shape of colony, length

and diameter of cell, was unchanged after heat
Fig. 10: Microscopic picture of probiotic bacteria before
treatment [10, 11].
heat shock

192

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World Appl. Sci. J., 4 (2): 191-194, 2008

Table 1: Changes of pH, acidity and total count of bacteria after the exposure to heat shock

0 min
5 min
10 min
15 min
20 min
25 min
30 min
pH
6.40
6.06
6.033
6.00
6.00
6.00
6.00
Acidity
0.20
0.30
0.280
0.27
0.28
0.28
0.27
Total count
5.98 CFU
5.80 CFU
5.730
5.55
5.32
5.23
5.22
Dry matter
12.27
12.23
12.230
12.00
12.07
12.00
12.07

Table 2: Changes of pH, acidity and total count of bacteria after the exposure to cold shock

Control
4°C
0°C
-5°C
-10°C
-20°C
pH
6.40
6.16
6.16
6.13
6.03
6.06
Acidity
0.20
0.20
0.21
0.28
0.28
0.29
Total count
5.98
5.95
5.89
4.96
4.94
4.94
Dry matter
12.27
12.03
12.03
12.00
12.03
12.03

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