Chemistry Reference
In-Depth Information
matter of plants, for example, is a biopolymer that consists of many thou-
sands of molecules of glucose (the monomer) interconnected in a linear
arrangement (see Fig. 66). The nucleic acids, which occur in animals and
plants, are also
polynucleotides
made up of
nucleotides
, themselves
biopolymers made up of three different monomers:
sugar
,
nitrogen bases
,
and
phosphate groups
(see Fig. 74). Many natural fibers, such as cotton,
wool, and silk, consist of linear biopolymers. Cotton, composed of cellu-
lose, for example, consists of polymerized carbohydrate monomers; wool
and silk are proteins composed of polymerized amino acids.
Since the early twentieth century, a vast and ever-increasing range of
artificial polymers have been synthesized. Nylon, polythene, styrofoam,
and plexiglass are just a few of the myriad artificial polymers that have
greatly enlarged the range of materials available to humankind, serving
as structural materials, plastics, rubbers, surface finishes and textile fibers
(Nigam and Prasad 1992).
The
nondrying oils
, which are comestible, do not polymerize and there-
fore, do not sdidify when exposed to the atmosphere. On exposure to the air
for long periods of time, however, they combine with oxygen and water and
become
rancid
, develop an offensive odor and acquiring a bad taste.
Olive oil
is a good example of a nondrying, comestible oil. Pressing the pulp of the
fruit of the olive tree (
Olea europea
) expresses the yellow-green viscous oil
that has been used since antiquity as food. In the Mediterranean Sea area, to
which olive trees are indigenous, olive oil was used not only as a foodstuff
but also as a cleansing agent, a fuel for illumination, and an ingredient of
ointments employed for cosmetic and ritual purposes (Frankel et al. 1993).
Other nondrying oils widely used as foodstuffs are listed in Table 79a.
Fats
The animal
fats
are greasy solids, insoluble in water, that melt at a low tem-
perature, generally below 100°C (see Table 79b). They are obtained either by
rendering
(the process of separating fat from fatty tissues, such as muscle
or skin, by melting) or by
churning
(vigorously agitating milk).
Lard
, the
fat along the back and underneath the skin of the hog,
suet
, the hard fat
around the kidneys and loins of cattle and sheep, and
tallow
, the overall fat
of cattle and sheep, are made by rendering fatty tissues;
butter
, a mixture of
water and fat is obtained by churning milk (Baer and Indictor 1973; Jamieson
1932).
