an ever larger scale and many are stored for significant periods after
harvest and may be transported from one part of the world to another.
Microbiological problems may occur at all stages in the production of
plant products. During growth in the field there is a wide range of plant
pathogens to contend with and, although these are dominated by the
fungi, there are a significant number of bacteria and viruses. A further
range of micro-organisms may cause post-harvest spoilage although
there cannot be an absolutely clear boundary between plant pathogens
and spoilage organisms because many plant products are made up of
living plant tissue even after harvest.
Plants have evolved many mechanisms to prevent microbial invasion
of their tissues. The outer surface is usually protected by a tough,
resistant cuticle although the need for gas exchange requires specialized
openings in parts of the leaf surface, the stomata and lenticels, which
may provide access by some micro-organisms to plant tissue. Plant
tissues may contain antimicrobial agents which are frequently phenolic
metabolites, indeed the complex polyphenolic polymer known as lignin is
especially resistant to microbial degradation. Many plants produce a
special group of antimicrobial agents, the phytoalexins, in response to
the initiation of microbial invasion. The low pH of the tissues of many
fruits provides considerable protection against bacteria and the spoilage
of these commodities is almost entirely by fungi. In contrast, many
vegetables have somewhat higher pH and may be susceptible to bacterial
spoilage (Table 5.8).
Another physical factor influencing the pattern of spoilage is the
availability of water. Cereals, pulses, nuts and oilseeds are usually dried
post harvest and the low water activity should restrict the spoilage flora
to xerophilic and xerotolerant fungi. These three groups of plant prod-
ucts,i.e.fruits, vegetables and cerealsetc., are sufficiently distinct that
they will now be considered separately.
Table 5.8 pH values of some fruits and vegetables
Fruits pH Vegetables pH
Apples 2.9–3.3 Asparagus 5.4–5.8
Apricots 3.3–4.4 Broccoli 5.2–6.5
Bananas 4.5–5.2 Cabbage 5.2–6.3
Cherries 3.2–4.7 Carrots 4.9–6.3
Grapefruit 3.0 Cauliflower 6.0–6.7
Grapes 3.4–4.5 Celery 5.6–6.0
Limes 2.0–2.4 Lettuce 6.0–6.4
Melons 6.2–6.7 Parsnip 5.3
Oranges 3.3–4.3 Rhubarb 3.1–3.4
Pears 3.4–4.7 Runner beans 4.6
Plums 2.8–4.6 Spinach 5.1–6.8
Raspberries 2.9–3.5 Sweet potato 5.3–5.6
Tomatoes 3.4–4.9 Turnips 5.2–5.6
146 Microbiology of Primary Food Commodities