18.2 Fruit Products 85518.2.10.1 Evaporation
Concentration by evaporation is the preferred in-
dustrial process. Since the process leads to losses
in volatile aroma constituents, it is combined with
an aroma recovery step. The aroma of the juice
is enriched 100 to 200 times by a counter-current
distillation. This aroma is stored and recombined
with the juice only at the dilution stage. In order
to maintain quality, the residence time in evapora-
tors is as short as possible. In a high-temperature,
short-time heating installation, e. g., in a 3- and
4-fold stepwise gradient-type evaporator, the resi-
dence time is 3–8 min at an evaporating tempera-
ture of 100◦C in the first step and about 40◦C
in the fourth step. The concentrate is then cooled
to 10◦C. Recovery of the aroma is achieved by
rectifying the condensate of the first evaporation
stage. A short-time treatment of juices is also
possible in thin-layer falling film evaporators.
These are particularly suitable for concen-
trating highly viscous products such as fruit
slurries.
18.2.10.2 Freeze Concentration
Concentration of juice by freezing is less eco-
nomical than evaporation. Hence, it is utilized
mostly for products containing sensitive aroma
constituents, e. g., orange juice. The juice is
cooled continuously below its freezing point
in a scraper-type cooler. The ice crystals are
separated from the resultant ice slurry by pressing
or by centrifugation. The obtainable solid content
of the end product is 40–50%. This content is
a function of freezing temperature, as illustrated
with apple juice in Fig. 18.11.
18.2.10.3 Membrane Filtration
Concentration of juice by filtration using
semipermeable membranes and high pressure
(0.1–1 MPa) is known as ultrafiltration. When
the membrane is permeable for water and only
to a limited extent for other small molecules
(Mr<500, e. g., salts, sugars, aroma com-
pounds), the process is called reverse osmosis.
Concentration of juice is possible only to about
25% dry matter content.
Fig. 18.11.Freezing temperature of apple juice and glu-
cose solution as affected by soluble dry matter (DM)
(according toSchobinger, 1978)18.2.11 Fruit SyrupsFruit syrups are thick, fluid preparations made
by boiling one kind of fruit with an excess of
sugar. They are sometimes prepared without heat-
ing by directly treating fresh fruit or fruit juice
with sugar, occasionally also using small amounts
of tartaric or lactic acids. Fruit syrups from cit-
rus fruits often contain small amounts of peel aro-
mas.
Fruit syrups are rapidly cooled to avoid aroma
losses and caramelization of sugar. The boiling
process partially inverts sucrose, preventing sub-
sequent sucrose crystallization. Low-acid fruits
are treated with tartaric or lactic acid. Boiling
in closed kettles permits recovery of vaporized
aroma compounds which can be added back to
the end product. As in marmalade production,
the boiling is occasionally done under vacuum
(50◦C starting temperature, 65–70◦Cfinaltem-
perature) in order to retain the aroma. Syrup pro-
duction by a cold process is particularly gentle.
The raw juice flows over the granulated sucrose
in the cold until the required sugar concentration
has been achieved. Aroma-sensitive syrups which
contain turbidity-causing substances, e. g., citrus
fruit syrups, are made by adding sugar to the
mother liquor with vigorous stirring. Fruit syrup
can contain at most 68% of sugar (calculated as