546 Part V: Fruits, Vegetables, and Cereals
acid to the juice during comminution improves the
consistency of the juice (Miers et al. 1970, Becker et
al. 1972).
Extraction Extraction is a process of separating
the seeds and skins from the juice. There are two
types of extractors: screw type and paddle type. The
screw-type presses tomatoes between a screw and a
screen with openings of about 0.5–0.8 mm (Lopez
1987). Paddle-type extractors beat the tomato against
screens. The extraction yields 3% seeds and skins
and 97% juice. Aiming for lower yield of 70–80%
results in a better quality juice and a high quality
residue for other purposes.
Deaeration The deaeration process improves the
color, flavor, and ascorbic acid content of the juice.
It is done immediately after the extraction by a vac-
uum deaerator. Removal of oxygen inhibits oxida-
tive processes.
Homogenization A stable juice is one in which
the solid and liquid phases do not separate during
storage for a long period of time. The ability of a
juice to separate depends on many factors, including
the serum viscosity, the gross viscosity, the pH of
the medium, the size and shape of the suspended
solids, and others. Homogenization is a process of
forcing the juice through narrow orifices at a pres-
sure of 6.9–9.7 MPa and a temperature of about
65°C to finely break up the suspended particles
(Lopez 1987).
Salting Sodium chloride is sometimes added to
the juice at a rate of 0.5–1.25% by weight in order to
improve the taste of the tomato juice. Citric acid is
added to improve the color, flavor, and taste. As well,
citric acid inhibits polyphenol oxidase by removing
copper, reducing browning reactions, and improving
color.
Quality Attributes of Tomatoes
The major quality attribute of ripe tomato is its red
color, which is due to the lycopene content of the
fruit. Other important physicochemical parameters,
which determine the quality of tomato are Brix, acid-
ity, pH, vitamin C, ash, dry matter, firmness, fruit
weight, and flavor volatiles. For processed tomato
products, the required quality attributes are precipi-
tate weight ratio, serum viscosity, total viscosity
(Brookfield), and lycopene content. Several quality
attributes of tomato and tomato products can be
improved by genetic modification of tomatoes (Oke
et al. 2003). These comparisons were made between
fruits obtained from untransformed and genetically
transformed tomato plants carrying antisense phos-
pholipase D cDNA, and between juices prepared
from these fruits. The levels and activity of phospho-
lipase D, the key enzyme involved in membrane lipid
degradation, was considerably reduced by antisense
transformation. These changes potentially resulted in
increased membrane stability and function that also
improved several quality parameters. The flavor pro-
files of blended antisense tomato fruits were different
from those of the controls, being enriched in volatile
aldehydes such as pentenal and hexenal. Increased
membrane stability in transgenic fruits potentially
resulted in lowered degradation of unsaturated fatty
acids such as linoleic and linolenic acids and may
have contributed to increased substrate availability
for lipoxygenase pathway enzymes (lipoxygenase,
hydroperoxide lyase, etc.) during blending, with an
increased evolution of volatile aldehydes. Table 23.6
provides a comparison of various quality parameters
between a genetically modified tomato and a control;
it shows improvements in several quality parameters
due to the transformation.Physicochemical Stability of JuicesKinetic Stability There are two categories of
juices: clear and comminuted. Clear juice such as
apple juice contains no visible vegetal particles,
whereas a comminuted juice such as tomato juice
contains mostly vegetal particles suspended in a liq-
uid. To be stable, a clear juice needs to remain clear
(without sediment) during its shelf life. On the other
hand, a comminuted juice may not separate into dis-
tinct phases during the shelf life of the product. An
important quality attribute of juices such as tomato
juice is the stability of their dispersion system. In
order to be stable, a juice needs to be kinetically and
physically stable.
The ability of a polydisperse system containing
suspended particles to maintain its homogenous dis-
tribution without agglomeration is called kinetic
stabilityor sedimentation stability. Kinetic stability
depends on many factors, the most important of
which include the size of suspended vegetable parti-