Microbial Metabolites in Fruits and Vegetables 513
Although most, if not all, bacterial and fungal species produce α-amylase,
Bacillus cereus, B. subtilis, B. stearothermophilus, Lactobacillus cellobiosus, Strep-
tococcus spp., Clostridium butiricum, Cl. thermosaccharolyticum, Aspergillus niger,
A. oryzae, Fusarium oxysporum, Candida japonica, and Pichia polymorpha produce
the enzyme in significant amounts (Antranikian, 1992). Industrial α-amylase is
usually obtained from Bacillus spp. (Pariza and Johnson, 2001). It should be noted
that the optimum temperature for α-amylase activity is usually higher than the
optimum temperature for growth of the microorganism that produces the enzymes.
Thus, some bacterial α-amylases are active at temperatures as high as 90°C (Asther
and Meunier, 1990; Maesmans et al., 1994). Generally, the optimum temperature
for bacterial α-amylases ranges from 20 to 90°C, while α-amylases from fungal
species require only 25 to 50°C for optimal activity.
Inhibitors of α-amylase have been extracted from white kidney beans (Phaseou-
lus vulgaris) and roselle (Hibiscus sabctarifla). Their optimum temperature was
determined to be 40°C. Interestingly, the inhibitors are effective only towards animal
α-amylase and not toward enzymes of plant or microbial origin (Hansawasdi et al.,
2000; Chun et al., 2001).
Similarly to α-amylase, β-amylase (EC 3.2.1.2; α-1,4-glucan maltohydrolase)
hydrolyzes only α-1,4 bonds, but this enzyme eliminates one maltose molecule at
a time starting from the nonreducing end of amylase or amylopectin. It is commonly
found in plants and some bacteria, but it is relatively rare in fungi. Good bacterial
producers of β-amylase are Bacillus spp. and Cl. thermosulfurogenes (Wang and Yu,
1994; Huang and Allen, 1997; Mohan-Reddy et al., 1999); the optimum temperature
for their enzymatic activity ranges from 30 to 75°C.
Another starch-degrading exoenzyme, glucoamylase (EC 3.2.1.3; α-1,4-D-glu-
can glucohydrolase; amyloglucosidase), liberates glucose molecules from the non-
reducing ends of starch molecules. The reaction is fast on α-1,4 linkages but sig-
nificantly slower on α-1,6 bonds. Glucoamylase is often detected in fungi, especially
in Aspergillus spp., and yeast such as Pichia, Candida, and Saccharomyces. Glu-
coamylase is, on the other hand, rare in bacteria but has been detected in B. stearo-
thermophylus and Cl. therosaccharolyticum. Similarly, α-glucosidase (EC 3.2.1.20;
α-D-glucoside glucohydrolase; maltase) releases one glucose molecule from the
nonreducing end, with a higher affinity towards oligosaccharides than towards high-
molecular-weight molecules. Contrary to glucoamylase, α-glucosidase is widely
distributed not only in fungi and yeasts but in bacteria as well. Both glucoamylase
and α-glucosidase have optimum temperatures above 40°C, with extremes of 70 and
115°C for Cl. thermohydrosulfuricum and Pyrococcus furiosus, respectively (Cos-
tantino et al., 1990; Antranikian, 1992). However, contrary to α- and β-amylases,
which exhibit their highest activity under neutral pH conditions, glucoamylase and
α-glucosidase require acidic pH.
The starch disbranching enzyme pullulanase (EC 3.2.1.41; α-dextrin-6-glucono
hydrolase) hydrolyzes only amylopectin’s α-1,6 bonds, resulting in the accumulation
of oligomers of various lengths. This enzyme has been detected in bacteria and
plants (Warner and Knutsen, 1991; Stathopoulos et al., 2000; Schindler et al., 2001).
B. acidopullulyticus has a characteristically high pullulanase activity and is used
for large-scale production of the enzyme (Kusano et al., 1990).