S WEET P OTATO S TARCH

S. N. Moorthy

^

and S. Shanavas

Central Tuber Crops Research Institute, Sreekariyam, Thiruvanathapyram- 695 017,

Kerala, India

A

BSTRACT

Sweet potato is an important food crop in the tropical countries and the roots are rich in starch. The starch has very desirable physicochemical and functional properties and therefore can have applications in food and industries. This chapter discusses the different characteristics of the starch in comparison with other root and tuber starches and the potential applications. The starch granule size ranges from 4-43 µm, with ‗A‘ type XRD pattern and an amylose content of around 20%. The swelling power and solubility are similar to other root starches. The viscosity and pasting temperature are almost in the same range as cassava. The enzyme digestibility, water binding capacity and rheological properties have also been described. The conditions for the liquefaction and saccharification of sweet potato starch for possible application in the production of ethyl alcohol are given in detail.

A

BBREVIATIONS

BU Branbender units;

DP Degree of polymerization;

DSC Differential Scanning Colorimetry;

HPAEC High Performance Anion Exchange Chromatography;

FTIR Fourier Transform Infra Red;

PV Peak viscosity;

RVA Rapid Visco Analyzer;

SEM Scanning Electron Microscopy

 Corresponding author: E-Mail: S.N.Moorthy48@gmail.com Tel: 91-471-2598551; Fax: 91-471-2590063

I

NTRODUCTION

Starch is one of the major biochemical components in the plant kingdom, especially in the root and tuber crops. The starch content varies from 10-30% and the different starches have different functional properties. The application of the starch in food depends on the starch content and starch properties. Next to cassava, sweet potato has the highest starch content among the root crops and the extraction process is comparatively simple.

In developing countries, sweet potatoes are processed into starch, noodles, candy, flour and desserts. In China for example, sweet potato starch production has become an important cottage industry. Moreover, China is the largest grower of sweet potatoes, providing about 80% of the worlds supply. There are more than 2000 varieties of sweet potatoes in China which can be roughly divided into ‗general type‘, ‗high starch type‘ and ‗food consumption type‘ (Liu, 2004). The major content of the dry matter in sweet potato is mainly starch which can be extracted from the roots. The uses of sweet potato starch are primarily determined by its physicochemical properties like starch granule shape and size, amylose content, molecular starch structure and pasting properties, retrogradation tendency, etc. A number of studies on the distinctive properties of sweet potato starch have been undertaken in different laboratories in the last decade. Crystalline structure, gelatinisation, pasting behaviour and retrogradation have been investigated (Takeda, 1986; Noda et al., 1992, 1996; Collado and Corke, 1997;

Garcia and Walter, 1998; Katayama et al., 2002; Katayama et al., 2004). The objective of this chapter is to bring together the present knowledge on starch derived from this crop.

S

TARCH

E

XTRACTION

The properties of sweet potato starch are very similar to cassava starch. However, though the extraction of starch from cassava is widely practiced, starch extraction from sweet potato is not so widely prevalent. The main reasons attributed are that the settling of starch is slow such that the longer residence time can lead to microbial growth and thereby lower the quality; this reduces the price of starch. For getting optimum yield and quality of starch, the correct time of maturity, methodology used for extraction and processing machinery are important. If roots are harvested late, the starch may get converted to sugar and fibre and thus affect yield and quality. Delays in processing sweet potatoes can result in the accumulation of sucrose and reducing sugars (Heinze and Appleman, 1943). Delays between shredding and starch extraction in sweet potato or the roots may lead to the synthesis of toxic compounds such as the alkaloid ipomeamarone, and the derived starch may become inedible and hazardous (Jain et al., 1951). The method of starch isolation (Figure 1) can affect both the physicochemical properties of the starch and the level of non-starch components, which in turn may also affect the physico-chemical properties of the starch indirectly (Lii and Chang, 1978; Takeda et al., 1986). The recovery of starch from sweet potato roots increased by more than 20% by using pectinase and cellulase enzymes. These enzymes act by breaking the pectin-cellulosic matrix of cell membranes resulting in the release of the starch granules. The treatment up to 0.05% concentration of enzyme gives higher yield without affecting its starch properties (Kallabinski and Balagopalan, 1991; Moorthy, 1999). Other methods used to

improve yield of starch is use of lime (Radley, 1976a) and dilute acetic and lactic acid during extraction.

Figure 1. Course starch production by farming households (Liu, 2004).

B

IOCHEMICAL

C

HARACTERISTICS

Though the starch appears to be in the pure form free from other components, thorough investigation of extracted starch have revealed that it is invariably contaminated by various other components (Table 1), i.e., fibre, lipids, proteins and minerals depending on a number of factors such as method of extraction, age of the crop, environmental conditions, etc. Some of these impart desirable qualities to the starch, while others have a detrimental effect on quality.

Table 1. Proximate composition of isolated starch

Parameters (g/kg) Range Reference

Moisture 139-150 Lii and Chang, 1978

Ash 0.8-1

2.6-5.1 Delpeuch et al., 1978,1979 Lii and Chang, 1978

Fibre 0.7-1.8

0.5-1 Delpeuch et al.1978,1979

Lii and Chang, 1978 Crude protein 4.8-5.4

1.3-2 Delpeuch et al. 1978,1979

Lii and Chang, 1978

Crude lipid 0.6-6 Delpeuch et al. 1978,1979

Phosphorus 0.19 Lii and Chang, 1978

Starch 980-988 Delpeuch et al., 1978,1979

The starch content in the extracted starch is nearly more than 95% but this depends on maturity. The moisture content suggested for safe storage of starch is 13% (ISI, 1970; Radley, 1976a), but among tuber and root starches large variation have been found (Kay, 1987;

Takeda et al., 1986; Soni et al., 1990; Melo et al., 1994). The root starches contain much

smaller quantities of native lipids in them, hence the addition of lipids or surfactants was found to enhance the properties of starch quality and it was found that there is no hindrance for the root starches to complex with surfactants or lipids added externally. The phosphorous content in sweet potato is similar to that in cassava starch (Rickard et al., 1991) but both of these are less than that in Irish potato (Hizukuri, 1969). Phosphate is believed to be an important factor in determining the granular strength by forming cross linkages.

G

RANULAR

C

HARACTERISTICS

Size and Shape

The granule size varies from less than 1µm to more than 100µm. Sweet potato starch granules have been reported as round, oval an polygonal shapes with size ranging between 3 and 28µm(Chen et al., 2003).The size and shape of starch granules from sweet potato are given in Table 2. Madamba et al. (1975) found significant differences among all sweet potato varieties studied (Figure 2). Sweet potato granules are of a similar size to those of cassava and maize but are smaller than those of potato which also have a larger range of granular size (Dreher and Berry, 1983).

Starch grains are of variable shape (oval, round, faceted round and polygonal) and are normally non-aggregated. Granule size ranges from 4-43µ, depending on the cultivar. The mean size of the granule ranges between 12.3-21.5µ. The granule size is reported to affect some functional properties like swelling, solubility and digestibility. Bowkamp (1985) reported negative correlation between particle size and susceptibility to amylase and acid degradation for sweet potato cultivars. According to Rasper (1971), particle size including size distribution, is one of the characteristics that most markedly affects the functional properties of starch granules. Smaller granules are reported to have both high solubility and water absorption capacity (Georing and Dehaas, 1972). Earlier studies revealed that sweet potato starch is polygonal or nearly round in shape (Tian et al. 1991; Woolfe, 1992; Shin and Ahn, 1983; Bouwkamp, 1985) and has a centric distinct hilum. Polarisation crosses are comparatively less distinct.

Table 2. Size, shape and X-ray diffraction pattern of sweet potato starches Sl.no Size (μm) Shape X-ray pattern Reference

1 14-34 Round polygonal Ca Shin and Ahn, 1983

2 3-42 Round polygonal - Seog et al., 1987

3 4-40 - Ca Delpeuch et al., 1978

4 10-14 - - Lii and Chang, 1978

5 4-43 Polygonal oval round

non-aggregated - Bouwkamp, 1985

6 9-38 Non-aggregated, oval

polygonal - Madamba et al., 1975

Figure 2. Frequency curves of the granules sizes of starch of six sweet potato varieties grown in Philippines (Madamba et al., 1975).