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6 Fruits and fruit products
I Introduction
A Definitions
Fruits are defined in general terms
as
'the portions
of
plants which bear seeds'. Such a definition
includes true fruits such as citrus, false fruits such
as
apples and pears, and compound fruits such as
berries. The definition includes tomatoes, olives and cucurbits such
as
cucumbers and melons, although
these are often incorrectly regarded
as
vegetables.
B Important properties
Most fruits are high in organic acids, and hence are
of
low pH (Table 6.1). The principal acid in citrus
fruits and berries is citric acid, in pome and stone fruits, malic acid and in grapes, tartaric and malic
acids.
Fruits are
of
substantial nutritional value, as a major source
of
vitamin
C.
Some provide useful levels
of
potassium, calcium and magnesium, and contain significant levels
of
vitamin
A,
thiamine and niacin
(Holland et ai., 1992).
C Methods
of
processing
Because
of
their low pH, most fruits are more susceptible to damage from fungi rather than from bacte-
ria. Most fruit-based products require only pasteurization to be microbially stable. Exceptions include
those from olives and cucumbers, and in some countries tomatoes.
Fruits may be processed by canning, freezing, sun-drying or dehydration. The pH
of
tomatoes is
sometimes reduced to below 4.5 by adding acids during processing, while olives and cucumbers are
Table
6.1 Representative pH values for fresh fruita
Fruit pH range Fruit pH range
Passionfruit 1.9-2.2 Orange 3.0-4.0
Lemon 2.2-2.6 Apricot 3.3-4.4
Grapefruit 2.9-3.4 Pineapple 3.4-3.7
Raspberry 2.9-3.5 Pear 3.4-4.7
Apple 2.9-3.6 Mango 3.8-4.7
Strawberry 3.0-3.6 Fig 4.8-5.0
Grape 3.0-4.5 Tomato 4.0-4.5
Blackberry 3.0-4.2 Olive 3.6-3.8
Cherry 3.2-4.0 Melon 6.2-6.7
aAdapted from Splittstoesser, 1987; CRC, 1990.
ICMSF, Micro-Organisms in Foods 6
© ICMSF 1998
FRUITS
AND
FRUIT
PRODUCTS
253
often pickled or fermented with lactic acid bacteria to produce microbiologically stable products without
the need for a low-acid canning process.
D Types
affinal
products
Fresh fruits. These are commonly sold after minimal processing and packaging treatments which may
include washing. dipping, or wrapping
in
paper impregnated with preservatives against moulds.
In
some
countries certain fruits may be held for several months under refrigeration or controlled atmosphere stor-
age, and then sold as fresh produce.
Fruits are also frequently sold
as
canned, frozen or dried products. Moistened dried fruit packaged
with a preservative added have become increasingly popular. Dried fruits are also used
in
a variety
of
other products, e.g. confectionery bars, biscuits, chocolates, breads, mueslis and other cereal-based prod-
ucts. The microbiology
of
these products usually differs little from that
of
unprocessed dried fruit and
will not be considered further here.
Chopped fresh, frozen or canned fruit may be sold in fruit salads and related products, or incorpo-
rated in dairy products such
as
yoghurts.
Tomatoes. These are canned
as
whole, peeled or diced fruits with or without added juice, as juice, or
concentrated to puree, paste or soup; dried as whole or halved fruit, or
as
a powder, or formulated into
products such
as
tomato sauce (catsup or ketchup) preserved with vinegar. or in the form
of
chilli
sauce.
Stone fruit. These are also sometimes infused with glucose syrups and partially dried
to
produce glace
confectionery.
II
Initial microflora
The initial microflora
of
fresh fruits comes from the field, and from harvesting and transportation equip-
ment. Field sources include soil, insects, air and fruit exudates. Soil
is
the source
of
heat-resistant fungal
ascospores, especially Byssochlamys species. Insects carry a variety
of
microbes. and
in
particular are
responsible for inoculation
of
figs with yeasts and sometimes AsperRillus fluvu.l'. Fruit exudates provide
nutrients for yeasts, especially pigmented Basidiomycetous yeasts such as Rhodotoruia species. Some
fungal pathogens
of
fruit, including Lasiodiplodiu theohromae, may
be
present systemically in trees and
invade developing fruit through the stems (Johnson et al., 1991, 1992). Bacteria are usually present only
in low numbers.
III Primary processing
A Effects
of
processing on microorganisms
The processes
of
harvesting, cleaning, sorting, packing and initial storage
of
fruit usually have little
effect on the initial microt1ora. Some microorganisms are removed
by
dry or wet cleaning processes.
