Microbial Metabolites in Fruits and Vegetables 515
(e.g., Methylobacterium) have specifically been identified as enhancing the flavor
volatiles in fruit. Due to their characteristic pink colonies they are^ sometimes referred
to as PPFMs (pink-pigmented facultative methylotrophs).^ A common feature of
methylotrophs is their ability to grow on methanol and^ methylamine as well as on
a variety of C 2 , C 3 , and C 4 compounds. Methylotrophs can readily become estab-
lished on plants by utilizing the methanol that accumulates on leaf surfaces as a
consequence of plant metabolism.
Although common inhabitants of plants, they were previously considered to be
neutral, having no adverse or positive effects. However, it has been established that
some Methylobacterium^ strains can produce plant regulators such as cytokinins,
PQQ, urease, and polyhydroxybutyrate that stimulate plant development. In addition,
because of their relatively good growth on leaf surfaces, their role in suppressing
phytopathogens through competitive inhibition has been observed (Romanovskaya
et al., 2001). Interestingly, the interaction of Methylobacterium extorquens with
growing strawberry plants also has a positive impact on fruit flavor. The characteristic
flavor of strawberries is produced in part by the concentration of 2,5-dimethyl
4-hydroxy 2H-furan 3-one (DMHF). This compound is synthesized by the plant
from 1,2-propanediol. It has been demonstrated that the activity of Methylobacterium
extorquens significantly increases the concentration of DMHF in the subsequent
fruit. Although not fully investigated, it is thought that the bacterium oxidizes
1,2-propanediol to form DMHF, in addition to other precursors, that the plant’s
biosynthetic apparatus converts to other flavor furanines (Zabetakis, 1997; Zabetakis
et al., 1999). Therefore, enhancing fruit’s flavor by introducing methylotrophs to
growing plants would be a simple, environmentally acceptable means of improving
produce quality.
The odorous secondary metabolites produced by a diverse range of fungi are
known to include esters, terpens, alkanes, and fatty acids. Most provide fruity odors
that are perceived as beneficial for fruit fermentation. For example, Saccharomyces
cerevisiae and Kluyveromyces marxianus both produce low levels of 2-phenylethanol
and terpenes that enhance fruity odors during fermentation. Interest in deriving
bioflavors and fragrances from microbial metabolism has been expressed (reviewed
by Vadamme and Soetaert, 2002), but its impact on fresh produce quality remains
obscure.
The products derived from vegetable and fruit fermentation is a further example
of how produce quality can be enhanced (Steinkraus, 1996). Favorable fermentation
of vegetables and fruits are performed by yeasts (alcoholic fermentations) and lactic
acid bacteria (Table 17.1). One of the most popular fermented vegetable products
is sauerkraut, produced by the fermentation of cabbage. Typically a combination of
Leuconsotoc mesenteroides, L. plantarium, L. cucumeris, and L. pentoaceticus is
used to produce desirable product flavor. The end products of sauerkraut fermentation
are lactic acid along with smaller amounts of acetic and propionic acids and carbon
dioxide, in addition to a mixture of aromatic esters. The acids, in combination with
alcohol, form esters, which contribute to the characteristic flavor of sauerkraut.
Vegetables can also be fermented by Bacillus (B. pumilus, B. licheniformis), but
alkalization, rather than acidification, occurs. The alkaline pH is produced by the
deamination of amino acids by the action of the Bacillus protease enzyme. Due to