little salt (o2%) and the product softens unacceptably, too much salt
( 4 3%) and the correct microbial sequence is not obtained. The salt
serves a number of purposes:
(i) it extracts moisture from the shredded cabbage by osmosis to
form the brine in which the fermentation will take place;
(ii) it helps to inhibit some of the natural microflora of the cabbage
such as pseudomonads which would otherwise cause spoilage
and helps to select for the lactic acid bacteria;
(iii) it helps maintain the crisp texture of the cabbage by withdrawing
water and inhibiting endogenous pectolytic enzymes which cause
the product to soften;
(iv) finally, salt contributes to the flavour of the product.Traditionally, fermentation vats have been made of wood but nowa-
days are more often of concrete with a synthetic polymer lining to protect
from attack by the acid brine. The tanks are sealed by covering the salted
cabbage with plastic sheeting. They are then filled with brine to press the
sheeting on to the cabbage expelling the entrapped air.
Although commercial starter cultures for sauerkraut fermentation are
available, they are used less often than in other food fermentations. The
time course of a typical sauerkraut fermentation is shown in Figure 9.10
and shows how strongly selective the process is. At the start, lactic acid
bacteria (LAB) comprise only about 1% of the total microflora, but
many of the non-lactics fail to grow and two days later LAB account for
more than 90% of the total microflora. During this time, they produce
sufficient acid to decrease the pH to below 4 further inhibiting the
competing microflora. Underlying this overall dominance by LAB is a
natural succession of different species which contribute to the charac-
teristic flavour of sauerkraut. The fermentation is initiated by
Leuconostoc mesenteroideswhich is among the less acid- and salt-tolerant
LAB but grows fastest during these early stages. As a heterofermenter it
produces CO 2 which replaces entrapped air and helps establish anaerobic
conditions within the product and prevent the oxidation of vitamin C
and loss of colour. Since fructose is present as an alternative electron
acceptor, it also produces appreciable amounts of acetic (ethanoic) acid
from acetyl-CoA which is a major contributor to sauerkraut flavour.
Reduction of fructose leads to the accumulation of mannitol. As the pH
drops due to acid production in a weakly buffered medium so the
Leuconostocis inhibited and replaced, first by heterofermentative lacto-
bacilli, and then by more acid-tolerant homofermentative lactobacilli
such as Lactobacillus plantarum. Acid accumulation continues in the
form of lactic acid although the pH stabilizes somewhere around 3.8 (the
pKaof lactic acid). At the end of fermentation which can last from 4–8
338 Fermented and Microbial Foods