2009_Book_FoodChemistry

18.2 Fruit Products 851
cherries, plums, grapes and many subtropical or
tropical fruits are of low suitability.
Rapid chilling is important (air temperature
≤−30 ◦ C, freezing time about 3 h) to avoid
microbial growth, large concentration shifts in
fruit tissues, and formation of large ice crystals
which damage tissue structure. A blanching step
prior to freezing is commonly used only for
few fruits, such as pears, and occasionally for
apples, apricots and peaches. Some fruits are
covered, prior to freezing, with a 30–50% sugar
solution or with solid granulated sugar (1 part
per 4–10 parts by weight) and are left to stand
until the sap separates. In both instances oxygen
is eliminated, enzymatic browning is prevented,
and the texture and aroma of the fruit are better
preserved. Addition of ascorbic acid or citric acid
is also common.
Frozen fruit which is stored at −18 to 24 ◦ Cis
stable for two to four years.
a shiny, glossy appearance, are exposed to steam
for a short time.
18.2.5 Fruit Pulps and Slurries
Fruit pulp is not suitable for direct consumption.
The pulp is in the form of slurried fresh fruit or
pieces of fruit either split or whole, and, when
necessary, stabilized by chemical preservatives.
The minimum dry matter content of various pulps
is 7–11%. For pulp production the fruit, which
has been washed in special machines, is lightly
steamed in steam conduits or precooking retorts.
The fruit slurry is an intermediary product, also
not suitable for direct consumption. The production
steps are similar to those for pulp. However,
there is an additional step: slurrying and straining,
i.e. passing the slurry through sieves. Both
the pulp and the slurry can be stored frozen.
18.2.4 Rum Fruits, Fruits in Sugar Syrup,
etc.
Rum fruits are produced by steeping the fruit in
dilute spirits in the presence of sufficient sugar.
Fruits preserved in vinegar, mostly pears and
plums, are prepared by poaching in wine vinegar
sweetened with sugar and spiced with cinnamon
and cloves.
Fruits in sugar syrup are prepared by treating
raw or precooked fruits or fruit portions (may be
precooked under a vacuum) with highly concentrated
sucrose solutions which also contain starch
syrup. The latter is added to enhance translucency,
smoothness and tractability of the product.
Candied lemon or orange peels are products of
this kind.
Other varieties provide intermediary products
processed further into fruit confections: glazed
fruits (these are washed fruits treated with
a sugar solution containing gum arabic and then
subsequently dried at 30–35 ◦ C) or candied fruits
in which the dried, glazed fruit is also immersed
in a concentrated sugar solution and then dried
to form a candied hull. Another product is
crystallized fruit in which the dried, glazed
fruits are rolled over icing or granulated sugar
(sucrose), then dried aditionally and, to achieve
18.2.6 Marmalades, Jams and Jellies
18.2.6.1 Marmalades
Marmalade is a spreadable preparation made
from pulp, slurry, juice, aqueous extracts or peels
of citrus fruits and sugars. The product (1 kg) has
to contain at least 200 g of citrus fruit (of which
75 g endocarp) and 60% by weight of soluble
solids. The addition of fruit pectin and starch
syrup are customary.
For the production of marmalade, the fresh fruits
or intermediary products, such as fruit pulps or
slurries, are boiled in an open kettle at atmospheric
pressure (T up to 105 ◦ C) or in a closed
vacuum boiler at reduced pressure (T: 65–80 ◦ C)
with the addition of sugar (usually added in two
batches). In general, the latter process is used industrially.
The aroma substances are recovered
from the vapor and returned in concentrated form
before filling. The solids content and pH value are
usually controlled automatically during boiling.
Other ingredients (gelling agents, starch syrup
and acids) are added before the thickening is completed
by boiling. The end of boiling is determined
by refractometer readings (the total boiling
time is usually 15–30 min). The hot (70–75 ◦ C)