Thermal Food Processing

578 Thermal Food Processing: New Technologies and Quality Issues
for B. licheniformis spores when acidifying from pH 7 to 4. A decrease from 14
to 11°C was observed for C. sporogenes PA3679, with a rather odd peak at pH 6.5.
Although the differences in the effect of pH on z values could be related to
the genetics of the microorganisms, i.e., different species would show different
behaviors, it has also been proposed that the temperature at which spores were
formed could play an important role in their behavior. 70 In their experiments,
Sala et al. 70 found that z values of a spore suspension of B. subtilis sporulated at
32°C did not change in the pH range 4 to 7, while those of other spore suspensions
of the same strain, but sporulated at 52°C, increased significantly when the heating
medium was acidified in the same pH range.
The sporulation temperature is known to play a key role in the heat resistance
of bacterial spores. 109 Spores sporulated at higher temperatures are more heat
resistant, 110–112 and Sala et al. 70 showed that sporulation temperature was also
related to the effect of pH on z values.
Also, the medium in which the spores are heat treated can be important for
z values. In this regard, Condón and Sala 67 found that the z value of B. subtilis
increased as the pH of the buffer was heated, but was approximately constant in
different foods.
Regarding vegetative cells, Steenstrup and Floros 103 also found an important
increase in z values of E. coli O157:H7 when apple cider was acidified with malic
acid. However, Condón et al. 81 did not find any change in z value of Aeromonas
hydrophila related to the pH of the menstruum in which the microorganism
was heated. Neither Pagán et al. 83 with Y. enterocolitica nor Mañas et al. 84 with
S. typhimurium found that pH caused any change in the heat resistance with
treatment temperature.
18.5 EFFECT OF TYPE OF ACID ON BACTERIAL
HEAT RESISTANCE
The type of acid used to decrease the pH also affects bacterial heat resistance.
Leguérinel and Mafart 113 found a relationship between the lowest pK a value of
nine weak organic acids (acetic, L-glutamic, adipic, citric, glucono-δ-lactone,
lactic, malic, malonic, and succinic acids) and their effects on the heat resistance
of B. cereus spores in tryptone salt broth. These authors measured the influence of
these acids by creating a new parameter, the z pH value, which is the distance of pH
from a reference pH (usually pH 7) that leads to a 10-fold reduction of the D value.
The relationship was fitted according to a linear regression with a high coefficient
of correlation (r 0) (z pH = –3.37pK a + 21.23; r 0 = 0.965). It was an inverse relationship
between the z pH and the lowest pK a value of the acid (in case of acids
with several acid groups), confirming that higher pK a led to a higher effect on
decreasing heat resistance. The sorted relation of acids (in terms of higher z pH
value) was as follows: L-glutamic > malonic > malic > citric > glucono-d-lactone >
lactic > adicpic > succinic > acetic.
These results are in agreement with those of other authors, 4,26,114,115 which
showed that lactic and acetic acids are slightly more effective in reducing heat