514 Produce Degradation: Reaction Pathways and their Prevention
Starch-degrading enzymes of microbial origin have an important role in the
production of corn syrups, bakery products, alcohol, and alcoholic beverages such
as beer and whiskey. In addition to food industry applications, thermoresistant
α-amylase is a common ingredient in laundry detergents.
17.2.6 ENZYME TRANSPORT IN MICROBES
Polymeric materials of plants (e.g., starch and pectin) must be degraded into transport-
able units in order to be assimilated by the microbial cell. To achieve this the microbes
release enzymes (exoenzymes) to attack the plant cell walls, etc. It is interesting to note
that more than 50% of the protein synthesized within microbial cells is exported. This
underscores the significance of enzyme secretion in cells, which, if controlled, could
provide a novel method of enhancing the shelf life of produce.
Within bacteria there are three secretory systems, commonly termed Types I, II,
and III. The Type I system is characterized by transferring preformed enzymes
(e.g., protease) in a single step from the cytoplasm of the cell.
Type II secretion systems are relevant to plant degradation; the enzymes pecti-
nase and cellulase are transported via this system. In contrast to the Type I system,
the Type II system involves a two-step mechanism. The protein within the cytoplasm
is transported to the periplasm via a sec-dependent protein transporter from which
it passes through a cytoplasmic membrane spanning structure to the outside of the
cell (reviewed by Sandkvist, 2003).
The enzymes are synthesized as precursors with additional amino-terminal
sequences, called signal sequences, that designate the protein for export. These signal
sequences are eventually cleaved via a peptidase at the sec transporter. The protein
is transported into the periplasmic space, where it may undergo further modifications
such as subunit assembly, before it is translocated across the outer membrane. Type II
transport is an energetic process involving ATP hydrolysis via a sec ATPase.
The Type III secretion system can be best described as a tubelike structure
(200 nm in length) that spans both the cytoplasmic membrane of Gram-negative
phytopathogens and cell walls of plants. Interestingly, a very similar system is used
in animal pathogens, which suggests a common evolutionary link (reviewed by Innes,
2003). Similar to the mechanism in animal pathogens, the main role of Type III
secretion systems is to interface with the host cell so that effector molecules and
enzymes (e.g., proteases) can be released into the plant to alter cellular function.
For example Pseudomonas syringae pv. tomato DC3000 transfers proteins into plants
that inhibit the hypersensitive response that would otherwise result in an attack on
the invading pathogen. Other proteins also nullify plant defense systems, enabling
the invading pathogen to become established (Buttner and Bonas, 2003).
17.3 MICROBIAL EFFECTS ON PRODUCE FLAVOR
17.3.1 MICROBIAL METABOLITES THAT ENHANCE PRODUCE QUALITY
Although many microbial metabolites can be considered detrimental to the quality
of fruits and vegetables, there are examples of positive effects. Methylotrophs