2009_Book_FoodChemistry

854 18 Fruits and Fruit Products
late in the presence of Ca 2+ ions. However, since
heat treatment damages fruit aroma, the use of
polygalacturonase is preferred. This enzyme degrades
the pectic acid to such an extent that flocculation
does not occur in the presence of divalent
cations.
18.2.8.4 Preservation
Finally, the fruit juice preservation step involves
pasteurization, preservation by freezing, storage
under an inert atmosphere, or concentration
(cf. 18.2.10) and drying (cf. 18.2.12).
Pasteurization kills the microflora and inactivates
the enzymes, particularly the phenol oxidases.
Since a longer heating time is detrimental to the
quality, a short, high-temperature heat treatment
is the preferred process, using plate heat exchangers
(clear juices 85–92 ◦ C, 10–15 s; fruit slurries
up to 105 ◦ C and up to 30 s) with subsequent rapid
cooling. The juice is stored in germ-free tanks.
Filling operations for the retail market can lead to
reinfection, hence a second pasteurization is required.
It is achieved by filling preheated containers
with the heated juice, or by heating the filled
and sealed containers in chambers or tunnel pasteurizers.
Preservation by freezing generally involves transforming
the juice or juice concentrate into an ice
slurry (at −2.5 ◦ Cto−6.5 ◦ C),thenpackingand
cooling to the retail market storage temperature.
The product is stable for 5–10 months in a temperature
range of −18 ◦ Cto−23 ◦ C.
Storage in an inert atmosphere makes use of the
fact that filtered, sterilized juices are microbiologically
stable at temperatures below 10 ◦ Cand
in an atmosphere of more than 14.6gCO 2 /l. To
attain such a concentration of CO 2 , the filled storage
tank has to be at a pressure of 0.59 MPa at
10 ◦ C, or 0.47 MPa at 5 ◦ C.
Fruit juices are poured into retail containers, i.e.
glass bottles, synthetic polyethylene pouches,
aluminum cans, or aluminum-lined cardboard
containers.
18.2.8.5 Side Products
Pomace is the residue from the production of fruit
juices. Citrus fruits and apple pomace are used for
the recovery of pectins. Other fruit residues are
used as animal feed, as organic fertilizer, or are
incinerated.
18.2.9 Fruit Nectars
Fruit nectars are produced from fruit slurries or
whole fruits by homogenization in the presence
of sugar, water and, when necessary, citric and
ascorbic acids. The fruit content (as fresh weight)
is 25–50% and is regulated in most countries, as is
the minimum total acid content. Apricots, pears,
strawberries, peaches and sour cherries are suitable
for nectar production. The fruits are washed,
rinsed, disintegrated and heated to inactivate the
enzymes present. The fruit mash is then treated
with a suitable mixture of pectinolytic and cellulolytic
enzymes. The treatment degrades protopectin
and, thus, separates the tissue into its individual
intact cells (“maceration”).
High molecular weight and highly esterified
pectin formed from protopectin provides the high
viscosity and the required turbidity for the nectar.
Finally, the disintegrated product is filtered
hot, then saturated with the usual additives,
homogenized and pasteurized.
Fruit products from citrus fruits (comminuted
bases) are obtained by autoclaving (2–3 min at
0.3 MPa) and then straining the fruits through
sieves, followed by homogenization. Fruit nectars
also include juices or juice concentrates from
berries or stone fruits, adjusted by addition of
water and sugar.
18.2.10 Fruit Juice Concentrates
Fruit juice concentrates are chemically and microbiologically
more stable than fruit juices and
their storage and transport costs are lower. The
solid content (dry matter) of the concentrates is
60–75%. Intermediary products, less stable concentrates
with a dry matter content of 36–48%,
are also produced. These semiconcentrates are
pasteurized at 87 ◦ C. Fruit juice concentration is
achieved by evaporation, freezing, or by a process
involving high pressure filtration. Initially,
the pectin is degraded to avoid high viscosity and
gel setting (undesired properties).