Agriculture Reference
In-Depth Information
18.1
Basic Tuber Composition
to cereals. In one study, potatoes were second
only to soybean as a source of vegetable protein
in the USA (Johnson and Lay, 1974). However,
unlike cereals, potatoes have a high lysine con-
tent and provide a high-quality protein that is
one of the few vegetable proteins that rival that
of egg and other animal protein in quality
(Woolfe, 1987; Lisinska and Leszczynski,
1989). A combination of potato and egg pro-
tein rates among the best nutritional quality
proteins of all food proteins, including meat and
milk. The major deficiency of potato protein is
that it does not contain high amounts of me-
thionine. The soluble proteins of potatoes are
primarily of proteinase inhibitor proteins (Pou-
vreau
et al
., 2001) that function in defense
against insects and small animals, but when
cooked are high-quality proteins. The protease
inhibitor proteins include protease inhibitors
I and II, cysteine protease, aspartate protease
inhibitor, potato Kunitz-type protease inhibitor,
and potato carboxypeptidase inhibitor. In add-
ition to pest defense, these protease inhibitors
may be involved in regulating tuber protein
content. Potato proteins include the patatin
glycoprotein family (Paiva
et al
., 1983) that are
the major storage proteins (Shewry, 2003). In
tubers over
200
g, patatin accounts for ~40% of
the protein (Paiva
et al
., 1983).
The protein efficiency ratio (PER) based on
weight gain per unit protein fed (Desborough
et al
., 1981), averages about 2.3 compared to
1.76, 1.43, 0.77, and 1.3-1.9 respectively, for rice,
maize, wheat, and soybean (Leiner, 1977; Jewell
et al
., 1980).
Solanum
group Phureja and Andige-
na genotypes have been identified with higher
amounts of protein, but yields were poor when
crossed with
Solanum tuberosum
(Desborough,
1985). Transgenic potatoes overexpressing the
protein amaranth albumin
1
had a 60% increase
in total protein (Chakraborty
et al
., 2010).
Starch and fiber
Tubers are typically 20% solids and 80% water,
although the dry matter can vary from
14
to
37% (Burton, 1966). Dry matter content can
decrease along a stem- to bud-end gradient.
Starch, a complex carbohydrate, accounts for
70-
80% of the dry matter, with an amylopect-
in:amylose ratio of ~
3:1
(Woolfe, 1987). The
amylopectin:amylose ratio has a large effect on
texture. Amylopectin is highly branched, digested
more quickly, holds together better during cook-
ing and forms less dense starch granules, whereas
amylose is a straight chain molecule that is not as
easily digested and separates easily when cooked.
In general, food containing more amylose has a
more floury texture when cooked is not digested
as quickly and has lower glycemic index values.
Higher-starch potatoes (lower moisture,
higher specific gravity) such as the Russets have
greater proportions of amylose, reduced pro-
cessing costs, lower oil absorption, and are used
for French fries. The fluffier texture is also pre-
ferred by most for baking and mashed potatoes.
Waxy potatoes have lower starch content, lower
specific gravity, and higher amylopectin:amyl-
ose ratios that lessen the amount of sloughing
during boiling, making these potatoes better for
boiling. The FAO and World Health Organization
(WHO) recommend that
55-
75% of daily calories
be consumed in the form of complex carbohy-
drates. Potatoes also contain dietarily desirable
resistant starch that acts like fiber. Resistant
starch increases if cooked potatoes are chilled.
Potatoes contain fiber, most of which is insol-
uble. Eating a potato without the skin will re-
duce the amount of dietary fiber by over 30%.
According to the USDA nutrition database, a
large baked potato with skin provides
300
calor-
ies and
7
g of fiber, about 28% of the recom-
mended dietary allowance (RDA). Potato starch
and fiber content is affected by both genotype
and environment (Bach
et al
., 2012, 2013).
18.2
Potatoes and Health
While potatoes have been well regarded as a
healthful, important food for hundreds of years
in the West and thousands of years in the Andes,
over the last decade the role of potatoes in a
healthy diet has been disputed by some nutri-
tionists in the USA. Whereas most nutritionists
Protein
Potatoes contain ~2% protein on a fresh weight
basis, and on a dry weight basis are comparable
