Protection of Foods by Drying
453
increased slightly, whereas in the adsorption IM system at the same a w , the cells died off slowly during
the first three weeks and thereafter, more rapidly. At all a w values below 0.88, the organisms died
off, with the death rate considerably higher at 0.73 than at higher values.
27 Findings similar to these
have been reported by Haas et al.,
15 who found that an inoculum of 10
5 staphylococci in a meat–sugar
system at an a w of 0.80 decreased to 3 × 10
3 after 6 days and to 3 × 10
2 after 1 month. Although
growth of S. aureus has been reported to occur at an a w of 0.83, enterotoxin is not produced below an
a w of 0.86.
34 It appears that enterotoxin A is produced at lower values of a w than enterotoxin B.
35
Using the model IM Hennican at pH 5.6 and a w 0.91, Boylan et al.
5 showed that the effectiveness
of the IM system against S. aureus F265 was a function of both pH and a w . Adsorption systems are
more destructive to microorganisms than desorption systems. Labuza et al.
22 found that the reported
minimum a w s apply in IMF systems where desorption systems are involved, but that growth minima
are much higher if the food is prepared by an adsorption method. S. aureus was inhibited at a w 0.9
in adsorption, whereas values between 0.75 and 0.84 were required for desorption systems. A similar
effect was noted for molds, yeasts, and pseudomonads.
With regard to the effect of IMF systems on the heat destruction of bacteria, heat resistance increases
as a w is lowered and the degree of resistance is dependent on the compounds employed to control a w
(see Table 17–3). In a study of the death rate of salmonellae and staphylococci in the IM range of
about 0.8 at pasteurization temperatures (50–65
◦ C), it has been found that cell death occurs under firstorder kinetics.
18 These investigators confirmed the findings of many others that the heat destruction
of vegetative cells is at a minimum in the IM range, especially when a solid menstrum is employed.
Some D values for the thermal destruction of Salmonella Senftenberg 775W at various a w values are
given in Table 17–3.
With respect to molds in IMF systems, these products would be made quite stable if a w were
reduced to around 0.70, but a dry-type product would then result. A large number of molds are capable
of growth in the 0.80 range, and the shelf life of IM pet foods is generally limited by the growth
of these organisms. The interaction of various IM parameters on the inhibition of molds was shown
by Acott et al.
2 In their evaluation of seven chemical inhibitors used alone and in combination to
inhibit Aspergillus niger and A. glaucus inocula, propylene glycol was the only approved agent that
was effective alone. None of the agents tested could inhibit alone at a w 0.88, but in combination,
the product was made shelf stable. All inhibitors were found to be more effective at pH 5.4 and
a w 0.85 than at pH 6.3. Growth of the two fungi occurred in 2 weeks in the a w 0.85 formulation
without inhibitors, but did not occur until 25 weeks when potassium sorbate and calcium propionate
were added (Table 18–7). Growth of Staphylococcus epidermidis was inhibited by both fungistats,
with inhibition being greater at a w 0.85 than at 0.88. This is probably an example of the combined
effects of pH, a w , and other growth parameters on the growth inhibition of microorganisms in IMF
systems.
Storage Stability of IMF
The undesirable chemical changes that occur in dried foods occur also in IMF. Lipid oxidation
and Maillard browning are at their optima in the general IMF ranges of a w and percentage moisture.
However, there are indications that the maximum rate for Maillard browning occurs in the 0.4a w –0.5a w
range, especially when glycerol is used as the humectant.
36
The storage of IMFs under the proper conditions of humidity is imperative in preventing moldiness and for overall shelf stability. The measurement of equilibrium relative humidity (ERH) is of
453
increased slightly, whereas in the adsorption IM system at the same a w , the cells died off slowly during
the first three weeks and thereafter, more rapidly. At all a w values below 0.88, the organisms died
off, with the death rate considerably higher at 0.73 than at higher values.
27 Findings similar to these
have been reported by Haas et al.,
15 who found that an inoculum of 10
5 staphylococci in a meat–sugar
system at an a w of 0.80 decreased to 3 × 10
3 after 6 days and to 3 × 10
2 after 1 month. Although
growth of S. aureus has been reported to occur at an a w of 0.83, enterotoxin is not produced below an
a w of 0.86.
34 It appears that enterotoxin A is produced at lower values of a w than enterotoxin B.
35
Using the model IM Hennican at pH 5.6 and a w 0.91, Boylan et al.
5 showed that the effectiveness
of the IM system against S. aureus F265 was a function of both pH and a w . Adsorption systems are
more destructive to microorganisms than desorption systems. Labuza et al.
22 found that the reported
minimum a w s apply in IMF systems where desorption systems are involved, but that growth minima
are much higher if the food is prepared by an adsorption method. S. aureus was inhibited at a w 0.9
in adsorption, whereas values between 0.75 and 0.84 were required for desorption systems. A similar
effect was noted for molds, yeasts, and pseudomonads.
With regard to the effect of IMF systems on the heat destruction of bacteria, heat resistance increases
as a w is lowered and the degree of resistance is dependent on the compounds employed to control a w
(see Table 17–3). In a study of the death rate of salmonellae and staphylococci in the IM range of
about 0.8 at pasteurization temperatures (50–65
◦ C), it has been found that cell death occurs under firstorder kinetics.
18 These investigators confirmed the findings of many others that the heat destruction
of vegetative cells is at a minimum in the IM range, especially when a solid menstrum is employed.
Some D values for the thermal destruction of Salmonella Senftenberg 775W at various a w values are
given in Table 17–3.
With respect to molds in IMF systems, these products would be made quite stable if a w were
reduced to around 0.70, but a dry-type product would then result. A large number of molds are capable
of growth in the 0.80 range, and the shelf life of IM pet foods is generally limited by the growth
of these organisms. The interaction of various IM parameters on the inhibition of molds was shown
by Acott et al.
2 In their evaluation of seven chemical inhibitors used alone and in combination to
inhibit Aspergillus niger and A. glaucus inocula, propylene glycol was the only approved agent that
was effective alone. None of the agents tested could inhibit alone at a w 0.88, but in combination,
the product was made shelf stable. All inhibitors were found to be more effective at pH 5.4 and
a w 0.85 than at pH 6.3. Growth of the two fungi occurred in 2 weeks in the a w 0.85 formulation
without inhibitors, but did not occur until 25 weeks when potassium sorbate and calcium propionate
were added (Table 18–7). Growth of Staphylococcus epidermidis was inhibited by both fungistats,
with inhibition being greater at a w 0.85 than at 0.88. This is probably an example of the combined
effects of pH, a w , and other growth parameters on the growth inhibition of microorganisms in IMF
systems.
Storage Stability of IMF
The undesirable chemical changes that occur in dried foods occur also in IMF. Lipid oxidation
and Maillard browning are at their optima in the general IMF ranges of a w and percentage moisture.
However, there are indications that the maximum rate for Maillard browning occurs in the 0.4a w –0.5a w
range, especially when glycerol is used as the humectant.
36
The storage of IMFs under the proper conditions of humidity is imperative in preventing moldiness and for overall shelf stability. The measurement of equilibrium relative humidity (ERH) is of