Sweetpotato - Bioenergy Crop Plants - page 738

limit the growth of foliage (Yong 1961). Moisture has a vital influence on its growth and production.
The water content of the leaf is 86%, stem 88.4%, and storage root 70.6% depending on the cultivar.
Moist soils and 80% relative humidity favor the development of the vegetative part of the plant. It is
important to have moist soils at planting to achieve good establishment. The soil must be kept moist
during the 60-to 120-day growth period, although the humidity must be low at harvesting to prevent
rotting of the storage root (Carballo 1979). Sweetpotato can be cultivated in a wide range of soils.
Soil that is friable with a depth of more than 25 cm and has a good superficial and internal drainage
system is ideal. It also prefers lightly acid or neutral soils, with an optimal pH between 5.5 and 6.5.
Excessively acid or alkaline soils often encourage bacterial infections and negatively influence yields
(Cairo 1980). Sweetpotato is multiplied by sexual and asexual means, although the former is only of
interest to geneticists and plant breeders. Asexual reproduction using tubers and stems is the quickest
and most economic and therefore the form of production most commonly used (Ramirez 1992).
The chemical composition (Table 31.1) of sweetpotato varies greatly according to genetic and
environmental factors (FAO 1998). Storage roots may have a smooth or irregular surface, and the
skin and the flesh may range from almost pure white through cream, yellow, orange, and pink to a
very deep purple (Onwueme 1978).

31.2 sWeetPotato as Feedstock For ethanol
Most agricultural biomass containing starch can be used as a potential substrate for the production
of gaseous or liquid fuels (Nigam and Singh 1995). Two characteristics of a crop suitable for energy
use include high yield per unit area and high dry matter content (Bonelli et al. 2007). A high yield
reduces land requirements and lowers the cost of producing energy from biomass (Demirbas 2007).
Sweetpotato satisfies these requirements, making it very suitable for bioethanol production. Ziska
et al. (2009) evaluated cassava, sweetpotato, and field corn as potential carbohydrate sources for
bioethanol production in Alabama and Maryland. They found that sweetpotato yielded 1.5 times
as much carbohydrate as corn in Alabama and 2.3 times as much in Maryland. Sweetpotato also
yielded the highest concentration of root carbohydrate: 9.4 mg/ha in Alabama and 12.7 mg/ha
in Maryland. Corn is the primary feedstock for bioethanol production in the United States, by a
production process that consumes 75-90% as much energy as is available from the fuel (Marris

taBle 31.1
General composition of sweetpotato storage roots

constituent Percent or (mg/100 g)

Moisture 50-81%

Protein 1.0-2.4%

Fat 1.8-6.4%

Starch 8.0-29%

Nonstarch carbohydrates 0.5-7.5%

Reducing sugar 0.5-7.5%

Ash 0.9-1.4%

Carotene (average) 4 mg/100 g

Thiamine 0.10 mg/100 g

Ascorbic acid 25 g/100 g

Riboflavin 0.06 mg/100 g

Source: Adapted from FAO, Storage and Processing of Roots and Tubers
in the Tropics , Food and Agriculture Organization of the United
Nations, 1998, available at http://www.fao.org/DOCREP/X5415E/
x5415e01.htm (accessed June 2010).