2009_Book_FoodChemistry

18.2 Fruit Products 855
18.2.10.1 Evaporation
Concentration by evaporation is the preferred industrial
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 evaporators
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 residence
time is 3–8 min at an evaporating temperature
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 concentrating
highly viscous products such as fruit
slurries.
18.2.10.2 Freeze Concentration
Concentration of juice by freezing is less economical
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
(M r < 500, e. g., salts, sugars, aroma compounds),
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 glucose
solution as affected by soluble dry matter (DM)
(according to Schobinger, 1978)
18.2.11 Fruit Syrups
Fruit syrups are thick, fluid preparations made
by boiling one kind of fruit with an excess of
sugar. They are sometimes prepared without heating
by directly treating fresh fruit or fruit juice
with sugar, occasionally also using small amounts
of tartaric or lactic acids. Fruit syrups from citrus
fruits often contain small amounts of peel aromas.
Fruit syrups are rapidly cooled to avoid aroma
losses and caramelization of sugar. The boiling
process partially inverts sucrose, preventing subsequent
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 ◦ Cfinaltemperature)
in order to retain the aroma. Syrup production
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