Biomedical Engineering Reference
In-Depth Information
one of the most important canned vegetables. It is either canned fresh or soaked in
1.67% (w/v) brine for a period of about 18 h. This vegetable contains a mucilage
which is a viscoelastic substance exhibiting both elastic and viscous properties.
47
Such mucilage is extracted into the brine of the canned product and results in a
slimmy texture undesirable to the consumer. The normal pH range of okra is 5.8 to
6.4. Thus unfermented, canned okra requires a high thermal processing which causes
squashing and changes of its natural color. Roukas and Kotzekidou
47
overcame these
problems associated with the canning of okra. They used fermentation with starter
cultures in order to decrease the pH value and eliminate thermal processing and also
remove as much of the mucilage as possible and retain the natural color. The strains
used were
Lactobacillus plantarum
,
L. brevis
,
L. cellobiosus
, and a commercially
available starter culture (Vege-Start) (Chr. Hansen's Laboratorium A/S, Copenhagen,
Denmark).
D
ESIGN OF A
S
PECIAL
C
OMPOSITION
Plants represent the major renewable resource of complex carbohydrate polymers
such as fiber and starches. Plants will find applications as bioreactors for producing
new varieties of complex carbohydrate polymers because they have a great advantage
over all other organisms, which is their mass potential. Only plants are able to produce
millions of tons of, for example, carbohydrates which are needed for technical pro-
cesses.
12
With respect to starch composition, one of the priorities is to produce plants
that would only produce one sort of starch (i.e., either amylopectin or amylose
48,49
);
this goal has been reached in transgenic potatoes. By expressing a gene under the
control of the 35S RNA promoter, which encoded the anti-sense RNA of the granule-
bound starch synthase (the enzyme responsible for amylose synthesis), transgenic
potatoes were created devoid of amylose and containing only amylopectin.
50
Van der Meer et al.
51
induced fructan accumulation in normally non-fructan-
storing plants and analyzed the metabolic and physiological properties of such plants.
Fructans are polyfructose molecules functioning as nonstructural storage carbohy-
drates in several plant species that are important crops. The normally non-fructan-
storing potato plant was modified by introducing the microbial fructosyltransferase
genes so that it could accumulate fructans. Constructs were created in order that the
fructosyltransferase genes of either
Bacillus subtilus
(
sac B
) or
Streptococcus mutans
(
ftf
) were fused to the vacuolar targeting sequence of the yeast carboxypeptidase Y
(
cpy
) gene. These constructs were placed under the control of the constitutive
cauliflower mosaic virus 35S promoter and introduced into potato tissue. The regen-
erated plants accumulated high molecular mass (>5
10
6
D) fructan molecules in
which the degree of polymerization of fructose units exceeded 25,000. This modi-
fication affected photosynthate partitioning in microtubers and leaves and increased
the nonstructural carbohydrate content in leaves. Unlike starch, which is insoluble,
vacuolar fructans are soluble and can contribute to the osmotic potential of this
compartment. Next to their role as a plant carbohydrate reserve, fructans may have
other functions, including involvement in dry and cold tolerance.
52
On the other hand, researchers at the University of California, Berkeley, have
successfully transferred a gene into tomatoes and lettuce which manufactures a
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