518 Produce Degradation: Reaction Pathways and their Prevention
Xerophilic fungi (e.g., Eurotium amstelodami, E. chevaliere, E. herbariorum,
and P. citrinum) have been associated with the formation of ketone odors on desic-
cated coconut. The ketones are a result of a modified β-oxidation of fatty acids that
produces 2-pentanone, 2-heptanone, 2-nonanone, 2-undecanone, 2-heptanol, and
2-nonanol. Desiccated coconut stored under high relatively humidity enables the
growth of Baciilus subtilis, which produces secondary metabolic products such as
2,3,5,6-tetramethylpyrazine and 2,3,5-trimethylpyrazine that give the product a pun-
gent aroma.
Acid-tolerant lactic acid bacteria can take part in fruit spoilage by the production
of volatile flavor compounds in products such as citrus juice. The formation of
neutral compounds (e.g., acetoin) is favored in fruit juices due to the presence of
fermentable substrates and high acidity. The low pH causes a switch from acid
production to that of neutral compounds, thereby preventing autoinhibition by lactate
or acetate accumulation.
Taints in fruit can be caused by the formation of halophenols such as 2, 6-dichlo-
rophenol. Halophenols are formed by the reaction of a phenolic precursor with hydrogen
peroxide catalyzed by haloperoxidase. Like many enzymes, haloperoxidase is widely
distributed in plants but is also present in the acidophile spore-forming bacterium
Alicyclobacillus acidoterrestris. The key difference of the bacterial enzyme is that it
does not require cofactors or halide ions for activity. There is debate on the significance
of the halophenols formed by Ali. acidoterrestris due to the presence of the reaction
pathway in fruits. Nevertheless, it has been demonstrated that fruit juice inoculated with
Ali. acidoterrestris accumulates 2,6-dibromophenol and 2,6-dichlorophenol, confirming
its potential role in spoilage (Jensen and Whitfield, 2003).
17.3.4 ANTIMICROBIAL AGENTS FROM MICROORGANISMS
The microbial metabolites that cause adverse quality changes in fruits and vegetables
are of key significance. However, it must be noted that not all associated microbes
are detrimental to plants. Microbial populations on plants can provide a defense
against phytopathogen attack through the production of antimicrobial compounds.
This has prompted the possibility of using such beneficial microbes as biocontrol
agents in pre- and postharvest operations as an alternative to chemical-based treat-
ments. In the following section the main antimicrobial agents produced by microbes
during their interaction with plants will be described. Although not directly related
to produce degradation, such compounds can indirectly improve quality.
17.3.4.1 Antibiotics
Antibiotics encompass a chemically heterogeneous group of organic, low-molecular-
weight compounds produced by microbes. Mass-produced antibiotics have been
used in horticulture for many years to control plant disease (Anjaiah et al., 1998;
McManus et al., 2002). However, it should be noted that many plant-associated
microbes can also produce antibiotics via secondary metabolism. For example,
numerous strains of antibiotic-producing Pseudomonas spp. have been isolated from
roots of various plants grown in soils from diverse geographical regions (Raaijmakers