oligosporus, although otherRhizopusandMucorspecies are often iso-
lated. Over two days incubation at ambient temperature (30–35 1 C), the
mycelium develops throughout the mass of beans knitting it together.
During fermentation the pH rises to around 7, fungal proteases increase
the free amino acid content of the product and lipases hydrolyse over a
third of the neutral fat present to free fatty acids.
Unlike many fermented foods, tempeh production is not a means of
improving the shelf-life of its raw material which is in any case inherently
quite stable. Tempeh contains antioxidants which retard the develop-
ment of rancidity but will keep for only one to two days as sporulation of
the mould discolours the product and a rich ammoniacal odour develops
as proteolysis proceeds.
Tempeh production does however improve the acceptability of an
otherwise rather unappealing food. Fresh tempeh has a pleasant nutty
odour and flavour and can be consumed in a variety of ways, usually
after frying in oil.
In addition to improving acceptability, fermentation also improves
the nutritional quality of soya beans. In part this stems from the
reduction or removal of various anti-nutritional factors at different
stages in the processing. Destruction of the trypsin inhibitor and lectins
during cooking of the beans has already been mentioned and levels of
phytic acid, which can interfere with mineral nutrition, are also reduced
by about a third in the course of processing. The notorious ability of
beans to produce flatulence is also regarded as an anti-nutritional
property and flatulence-inducing oligosaccharides such as stachyose
and raffinose are partially leached out of the beans during the soaking
stage.
Despite the extensive proteolytic changes which occur during fermen-
tation, studies have failed to show that the protein in tempeh is more
easily digested. With the exception of thiamine which decreases, other
vitamins increase to varying degrees during fermentation. Vitamin B 12 ,
the anti-pernicious anaemia factor, shows the most marked increase and
this is associated with the growth of the bacteriumKlebsiella pneumoniae
during fermentation. The usual source of this vitamin in the diet is
animal products and it has been suggested that tempeh could be an
important source of B 12 for people subsisting on a largely vegetarian
diet.
Tempeh can be made from a number of different plant materials
including other legumes, cereals and agricultural by-products. One vari-
ety that has achieved some notoriety is tempeh bongkrek which is made
in central Java using the presscake remaining after extraction of coconut
oil. Tempeh bongkrek has been associated with occasional serious out-
breaks of food poisoning due to the bacteriumBurkholderia cocovenenans
growing in the product and elaborating the toxins bongkrekic acid and
364 Fermented and Microbial Foods