Chapter 11
Development of a Standard Protocol for the
Processing of High Quality Sweetpotato Starch for Noodle Making
Kuakoon Piyachomkwan1 , Klanarong Sriroth2 , Kanjana Chinsamran2 ,
Kamlai Laohaphattanalert2 , and Christopher G. Oates 3

Introduction
Sweetpotato (Ipomoea batatas Lam) is one of the world’s most important food
crops with an annual production of about 120 million tons. The crop is mainly
cultivated in developing countries in Asia, Africa and Latin America with China
accounting for about 85 percent of total world production (Woolfe 1992; Yen 1982).
The storage root is the main part of the sweetpotato that is used for food. Like
other roots and tubers, sweetpotato has high moisture content and a relatively low dry
matter content of around 30 percent. Approximately 80-90 percent of its dry matter is
carbohydrate, mainly starch, which is a glucose polymer. This makes sweetpotato roots
a good raw material for the starch industry (Woolfe 1992; Wheatley and Bofu 2000).
Sweetpotato starch has unique characteristics and is mostly used by the food
industry as an ingredient in products such as cakes, breads, biscuits, cookies and
noodles. The starch is also processed into glucose syrups and various chemicals
through enzymatic, microbial and chemical processes.
Noodles, like bread, boiled rice, and pasta, have played an important role in the
human diet, especially in Asian countries such as Japan, China, Taiwan, Korea,
Vietnam and Thailand. There are many kinds of noodles which can be classified
according to raw material used, noodle size, manufacturing process or form of the
finished product (Table 1). Based on raw material, various types of noodles are
produced throughout the world. They can be classified into two types: the protein-based
and the starch-based noodles. Protein–based noodles are from wheat (Noda et al. 2001;
Janto et al. 1998), buckwheat or rice (Bhattacharya et al. 1999; Toh 1997; Kim 1998).
Starch–based noodles are from mungbean (Galvez et al. 1994), pigeon pea (Singh et al.
1989), red bean (Lii and Chang 1981), sweetpotato (Collado et al. 1997; 2001),
sorghum (Beta and Corke 2001), potato (Kim and Wiesenborn 1996; Peng et al. 1997)

and cassava (Kasemsuwan et al. 1998).
When wheat flour is used to make noodles, a gluten-containing dough is first
prepared and formed into sheets which are then cut into strings of different sizes. The

1

Senior Researcher, Cassava and Starch Technology Research Unit, National Center for Genetic Engineering and
Biotechnology, Bangkok, Thailand.
2
Associate Professor, Graduate Student, and Assistant Researcher, respectively, Department of Biotechnology,
Faculty of Agro-Industry, Kasetsart University, Bangkok, Thailand.
3
Food Technologist and Managing Director, Agro Food Resources (Thailand), Ltd., Bangkok, Thailand.

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protein matrix binds the product together in the early stage of production. Wheat flour
noodles are very popular in Japan and China. Unlike wheat flour noodles, starch
noodles are gluten- free, thus, have no protein matrix. The final product is created by a
matrix of starch polymers formed during processing when the granule starch structure
is destroyed. Before cooking, binding of starch mixture is usually done with a small
amount of partially gelatinized starch. This is added before mixing with the
ungelatinized starch, in order to function as a binder and to facilitate extrusion or
sheeting of the starch mixture to produce noodles. Both gluten- and gluten- free noodles
can be handmade or machine- made and sold fresh, boiled, dried, steamed and dried, or
steamed and fried as instant noodles to increase the product’s shelf life.
Asian noodles are generally starch noodles produced from purified starch.
Various starch types can be used to produce gluten-free noodles such as mungbean,
sweetpotato and canna. Around 28% of sweetpotato is processed to starch noodles

(Collado and Corke 1997). Sweetpotato noodles are white to off-white with transparent
appearance and soft texture. Thus, they are sometimes called “transparent noodle”
(Timmins et al. 1992) or “glass noodles” (Galvez et al. 1994). Sweetpotato starch is
commonly used in noodle production in China.
Demand for starch noodles rose as quality improved in recent years not only in
China but also in other parts of Asia such as Taiwan, Korea and Vietnam. Noodle
quality depends on several factors relating to the properties of the starch or the
production process. For noodle production, sweetpotato starch quality is considered
inferior relative to other starches such as mungbean. If starch properties important to
noodle quality could be identified and improved, the final noodle quality would be
improved. Thus, this paper evaluates sweetpotato starch quality with respect to noodle
production. The findings will be used as a basis for establishing a production protocol
of noodle- making starch.

Quality requirements of starch for noodle products
Noodle quality
A product’s quality and price are key factors influencing a buyer’s decision to
purchase the product. In term of food products, four quality attributes are important:
nutritional quality, phytosanitary quality, keeping quality and organoleptic quality.
Nutritional quality refers to types and amounts of beneficial nutrients in the product.
Phytosanitary quality is determined by the absence of microbial and other contaminants
such as insect, metal and dirt. Keeping quality indicates a product’s stability under
storage and handling after manufacture through the supply chain to final consumer.
These quality attributes are usually assessed by instrumental analysis because changes
may not be detected through mere sensory observations unless severe food
deterioration and spoilage has occurred. Organoleptic quality is more complex and
involves the interaction of several sensory properties of the noodle. These include
appearance (sight), texture (by hand and mouth) and flavor (taste and smell) which may
have interrelated effects. While there are trade standards on nutritional and


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phytosanitary quality that have to be complied with, organoleptic quality is not well
defined and is unique to a particular food and/or market. Meanwhile, organoleptic
attributes are influenced by an individual consumer’s preference and are often
regionally and culturally dependent.
Noodles for human consumption should be of a quality that is equivalent to or
better than standards governing food grade. In general, microbial content should be
minimal and there should be no pathogen. Processing and noodle type affect storage
quality of noodles. Generally, fresh noodles with high moisture content (≈35 percent)
have a short shelf life in terms of number of days, thus constraining prospects of market
expansion. This prompted the manufacture of dried noodles, which have a longer shelf
life and are easier to transport. An example is instant noodles made from wheat that has
been steamed and then fried to lower moisture content to 8 percent. This extends the
product shelf life to 5-6 months (Kim 1996).
Customers generally prefer noodles that have good color and texture when
cooked, have minimum cooking loss and high tolerance to overcooking.
Relationship between starch processing methods and noodle quality
Noodle color is important, as it is the first characteristic examined by
consumers. It is dependent on the color of the raw material and can be assessed visually
or instrumentally. Flavor, typically evaluated by trained panels, is unique to each
noodle type, and depends on raw material and other ingredients used. Texture is also an
important characteristic of noodles and can be determined through sensory evaluation
or instrumental analysis. High quality noodles should have the right firmness when
cooked, not too hard, nor too soft and sticky.
These properties depend on flour and starch quality. In protein-based noodles,
texture attributes are based on structures of interacting protein strands reinforced by
starch. For instance, firmness and elasticity of boiled Ramyon Chinese noodles increase
with increasing protein content and dough strength (Chung and Kim 1991; Miskelly

and Moss 1985). In contrast to wheat flour noodles, starch noodles are gluten-free
consisting mainly of purified starch that forms the structural network of the final
cooked product (Kim and Wiesenborn 1996). Thus, starch properties largely influence
noodle quality. Starch with high content of amylose (a linear glucose polymer) is
generally preferred for noodle production. This includes starches from mungbean with
amylose content of 27-30 percent (Galvez and Resurreccion 1993; Chotineeranat et al.
2000; Galvez et al. 1994; Singh et al. 1989) and canna with amylose content 26 percent
(Sriroth et al. 2001; Soni et al. 1990). Starch with a C-type pasting profile characterized
by the absence of a peak viscosity and a constant or increased viscosity during
continuous heating and shearing (good hot-paste stability) is claimed to be suitable for
noodle processing (Collado and Corke 1999). Some textural attributes of sweetpotato
noodles show high positive correlation with some starch paste properties as determined
by a Rapid Visco Analyzer (Collado and Corke 1997). Starch of high stability ratio,
i.e., the ratio of hot paste viscosity to peak viscosity, produces noodles with good
firmness and rehydration capacity or cooked weight. Peak viscosity of flour by
amylograph is positively correlated with smoothness of the cooked noodle. The

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optimum absorption of noodle dough increases with starch damage and fineness of
granulation (Lee et al. 1987). Further, smaller particle size improves the strength of
uncooked noodles without affecting the firmness of cooked noodles (Oh et al. 1985).

Baseline variability of the raw material
The variability of the properties of sweetpotato starch produced by micro and
small-scale processors in Shandong and Sichuan provinces, China was evaluated. These
are the two biggest sweetpotato producing areas in China accounting for 40 percent of
the 100 million tons total annual production of China and exceeding tha t of other nonChinese countries. Starch produced in these provinces is mainly used for noodle
production (Table 2). Sweetpotato production in these regions is mostly micro and

small-scale using similar technologies.
Starch properties including granule size, amylose content, paste clarity and
viscosity were analyzed in more than 100 sweetpotato starch samples to evaluate their
consistency. Starch samples collected from different processors in Sichuan and
Shandong have different properties. Starches, in ge neral, consist of medium-sized
granules with an average diameter of less than 30 microns. Some samples contained
granules that are slightly bigger with a diameter of 40 microns. In contrast, starch paste
and gel properties are less homogenous (Figures 1-4). Using a Rapid Visco Analyzer,
hot paste viscosity (HPV) (the pasting viscosity after the holding time at 95°C) ranges
from 100 to 236 for starch from Shandong and 125 to 270 RVU for samples from
Sichuan (Figure 1). A high variation in the paste properties of sweetpotato starch
samples from different processors in Shandong and Sichuan provinces is also evident.
This was observed in the peak viscosity (PV) and cold paste viscosity (CPV), the
pasting viscosity at the end of the hold time at 50°C (Table 3). This corresponds to the
variation in the stability ratio (HPV/PV) of sweetpotato starches (Figure 2), which has
been shown to be highly correlated with noodle firmness (r = 0.95, P < 0.01) and
rehydration upon cooking (r = -0.89, P< 0.05) (Collado 1997).
Unlike gluten-containing noodles, starch noodles are prepared by partially
gelatinizing a small portion of starch to serve as a binder and to facilitate extrusion or
the sheeting process. Starches with different gelatinization temperatures may undergo
different degrees of gelatinization when cooked under the same processing conditions.
As a result, noodle texture attributes are different. In this test, a significant variation in
pasting temperatures of starch samples was recorded. Sweetpotato starch collected from
Shandong has a narrower range (75-79°C) of pasting temperatures than the Sichuan
samples ( 65-75°C) (Figure 3). Starches with various properties can provide various
qualities of noodles as indicated by the texture analysis of starch gels (Figure 4).
Starches collected from Sichuan province with mainly small-scale processors are likely
to have more variability in their properties than those from Shandong.

Factors affecting starch quality

Raw material

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Root quality. The quality of sweetpotato roots, the raw material for starch production, is
the primary factor affecting starch properties. Quality of roots relates to the dry matter
content and to the way by which the starch is formed. Root quality is controlled by
internal (i.e., genetic variety) and external (i.e., environmental condition during
cultivation) factors. Starch content and properties vary in different sweetpotato
varieties. Starch content ranges from 40 to 80 percent (dry basis) in 18 cultivars
cultivated in Brazil and 33 to 73 percent (dry basis) in Filipino and American cultivars.
In fresh roots, starch content is about 18 percent on average, but this varies from 11 to
26 percent in 31 Indian cultivars, 7 to 22 percent in 292 Taiwanese cultivars, and 4 to
27 percent in 75 Thai cult ivars grown under similar conditions (Woolfe 1992).
Typically, sweetpotato starch has a type A pasting profile characterized by a high peak
viscosity followed by a high degree of shear-thinning. Starches of different varieties
show peak viscosity, shear-thinning, cold paste viscosity, pasting temperature and
stability ratio (Collado and Corke 2000). Other properties also differ among different
varieties (Collado 1997), as shown in Table 4.
Sweetpotato starches of the same variety but grown and harvested at different
times have varying properties. Starch extracted from older roots has a higher
gelatinization temperature and peak viscosity, but lower hot paste stability (Noda et al.
1995; 1997). Furthermore, environmental conditions at planting significantly influence
starch properties (Tian 1996). When soil temperature during sweetpotato tuber
development increases, amylose content, granule size, enzymatic digestibility,
gelatinization and pasting profile as well as amylopectin structure of produced starches
are altered (Noda et al. 2001).
Post-harvest handling. Functional properties of sweetpotato starch are affected by postharvest handling of roots prior to starch production. Sweetpotato roots have a high
moisture content (70-80 percent) and are therefore perishable. Extended storage time at

high temperature not only reduces starch content (Figure 5), but also alters starch
properties. Roots stored at temperatures above 25°C for more than 3 days produce
starches with lower paste viscosity (Table 5). Starch swelling properties are also
influenced by storage conditions (Figure 6).
Raw material form. Sweetpotato is a seasonal crop and during harvest season, a large
volume of roots is available for starch processing. However, the tubers are perishable
and if not stored properly, suffer high losses. Thus, simple processing technologies are
applied to avoid storage problems. Also, reducing water content converts the bulky
roots into a form more compact, easier to store and easier to transport such as in the
form of frozen cakes and dried chips. These materials can be used as raw material for
starch production but the preservation method used may cause some degradation of
starch properties (Table 6). For instance, the color quality of starch extracted from dried
chips is inferior to the starch extracted from fresh tubers and stored frozen cake.
Sweetpotato starch processing
Starch is the most commonly processed product of sweetpotato roots, produced
at a micro-scale (household), small-scale or large-scale (factory). Production of

144


sweetpotato starch at household and village levels involves three stages - extraction,
purification and final preparation (Figure 7). In rural areas, the process is not
standardized and the capacity is 100-2,000 kg roots per day. At the industrial level,
capacity is 10,000-100,000 kg roots per day. The operating process is similar to
household and village- level, but techniques used are different. The processes at the
large-scale level are more controlled.
Extraction. Extraction is the first stage in sweetpotato starch production wherein fresh
roots are washed and ground to produce a mash. Generally, for micro and small-scale
processors, extraction efficiency is poor, thus requiring improvements. In some
countries such as China, extraction rate of most processors is not more than 15 percent

(Wiersema et al. 1989). However, in Japan, some starch plants have a 28 percent
extraction rate (Woolfe 1992).
Root preparation. Fresh sweetpotato roots should be washed to remove
contaminating soil and dirt. Peeling of the skin can be used instead of washing to
remove dirt if clean water is not available (Gankonyo 1993). Peeling is sometimes
recommended to processors to improve starch quality, but this is time-consuming and
invariably causes losses of 10 to 20 percent. Thus, starch processing without skin
peeling is more widely practiced (Soekarto 1995).
Grinding. Typically washed roots are ground with or without water, using a pin
mill, hammer mill, or a traditional rasper. However, if the water is added during
grinding, the whiter starch can be obtained. When more water is used, extraction yield
increases. Type of milling depends on production capacity of starch processing
(Timmins et al. 1992). Grinding is usually carried out with a hammer mill where the
particle size is reduced to 60 – 80 mesh (Hal 2000).
Sieving. After grinding, the mash is sieved on a synthetic screen to remove
undesirable skin and fiber from starch (Woolfe 1992; Timmins et al. 1992). The mesh
size used in this step is very important for starch quality. Big aperture mesh can cause
contamination of starch with fibrous materials.
Sedimentation. After sieving, the starch slurry is allowed to sediment.
Processors often encounter a technical problem of slow sedimentation rate. To address
this problem, the unique process of adding sour liquid (an aqueous acidic fermented
extract from dried peas, faba or mungbeans) has long been applied by most processors
in China. Sour liquid is used to promote starch sedimentation that can be checked
visually. The rate of sedimentation depends on starch slurry concentration and the
amount of sour liquid used, which is inversely correlated to the slurry’s pH (Figure 8).
By using the same content of sour liquid, starch slurry with low starch concentration
sediments faster than that with high concentration. Sedimentation is further improved
when a higher amount of sour liquid is used, as indicated by a lower pH of starch
slurry. In starch processing, if the sedimentation rate is not satisfactory, more fresh sour
liquid is applied. Also, sour liquid must be added to the wet starch following the first

sedimentation to prevent discoloration of final starch products. However, the effect of
sour liquid on other starch properties has not been investiga ted. It is likely that sour
liquid may affect starch properties such as paste viscosity (Figure 9).

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Purification. This stage involves the separation of starch from other impurities that may
affect starch properties. Purification of sweetpotato starch is a difficult and complex
process. Due to the presence of polyphenolic compounds, ascorbic acid and carotene,
white starch is rarely obtained. Sweetpotato starch is frequently less pure and darker
than other commercial starches, presumably due to the contamination of “jalapin”- the
resin produced by the sweetpotato latificers, and polyphenolic compounds formed
during starch processing (Woolfe 1992). In some rural areas of China, fresh water is
used to purify sedimented starch prior to final recovery, but the starch products are
invariably discolored. Most processors employ the sour liquid method to enhance
separation and also starch quality (Timmins et al. 1992; Wheatley and Bofu 2000). In
Japan, purification is accomplished under alkaline conditions by using saturated lime
water (calcium hydroxide) to improve starch yield and whiteness. In the purification
stage, centrifugal separators are often used in Japan in place of the settling tanks
(Woolfe 1992).
Final preparation. After receiving purified starch, wet starch is subjected to a drying
process to remove water and prolong the product’s shelf life. In addition, drying also
affects starch properties, the extent of which depends on the drying protocol. Solar or
sun drying is the cheapest process since it is free and uses a non-polluting energy
source. However, the process depends on weather conditions thus, is difficult to
control. Product quality is relatively inferior and more likely to be contaminated with
microorganisms, dusts and insects. Therefore, drying machinery such as by cabinet and
tunnel dryers is preferred. The quality of starch is affected by drying temperature
(Figure 10). In general, starch dried at higher temperature has a lower peak viscosity,

but higher hot and cold paste viscosity. The hot paste stability can be improved when
dried at a higher temperature. Starch paste modification during the drying process is
presumably explained by heat- moisture phenomena (Collado et al. 2001). The last step
of starch process is milling of dried starch to reduce the particle size and then sieving as
size can affect the quality of starch-based product.
Conclusions
To produce sweetpotato noodles of good quality that are acceptable to
consumers, care should be taken in the use of good quality sweetpotato starch. Starch
quality highly depends on the root quality, the extraction process and processing
conditions. Fresh roots of different varieties provide starches with different pasting
characteristics and, therefore, influence the noodle quality. Even when the roots of the
same variety are used, poor handling after harvesting may result in starch of lower
paste quality. Fresh roots are recommended, however, this may not be practical given
the seasonal nature of the crop in China. Processing fresh roots into wet cake, rather
than using dried chip, is therefore recommended. During starch extraction, it is critical
to remove all dirt by adequate washing. Water should be used during milling as an
extracting medium to improve yield and starch quality. The n, sieving is done with 120
mesh to remove other fibrous impurities. Sometimes peeling of the roots is suggested
to obtain high quality starch. Otherwise, the use of sour liquid is necessary to improve
starch whiteness and purity. After purification, starch is then subjected to a drying
process. Drying starch cake with an intermediate moisture content (≈35-40%) at high

146


temperature can be applied to improve hot paste viscosity and the stability ratio of
starch, but the condition should be optimized to avoid starch gelatinization.
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Table 1. Classification of noodles based on various criteria and characteristics
Criteria
Raw material


Noodle size

Process

Product form

Class /Type

Characteristics

1. Soft wheat flour
- Japanese noodles (udon)
2. Hard wheat flour
- Chinese noodles (ra-men, chuka ramen, chuka-soba)
3. Buckwheat (mixed with wheat flour)
- Buckwheat noodles (soba)
4. Rice flour
- mien, bihon, beehon, bifun
- knanom-jeen
- vermicelli
5. Mungbean starch
- glass noodles
6. Sweetpotato starch
7. Other starches:
- Potato, Canna
1. Very thin - So-men
2. Thin - Hiya-mugi
3. Standard - Udon
4. Flat - Hira men
1. Type of binders

- Protein: wheat flour noodles
- Pregelatinized starch: starch noodles
2. Strand making
- Sheeting & cutting: So-men, Udon
- Extrusion: Rice noodles
3. Equipment
- Handmade: Tenobe so-men
- Machine-made: Udon, Hira-men
1. Fresh and uncooked
2. Cooked noodles (Boiled or steamed)
3. Frozen boiled noodles
4. Dried noodles
5. Instant noodles

White or creamy white in color and
soft texture
Light yellow in color and a little stiff
in texture
Light brown or gray in color with a
unique taste and flavor
White to yellow color and opaque
with tender texture

Transparent and firm texture
Transparent and elastic

1.0-1.2 mm strand width
1.3-1.7 mm strand width
2.0-3.8 mm strand width
5.0-7.5 mm strand width


Table 2. Sweetpotato production in Sichuan and Shandong provinces, China
Item
Sweetpotato production in 1993/94
(million tons)
Sweetpotato utilization
% processed into starch
Sweetpotato starch production
Use of sweetpotato starch
Sweetpotato starch market
Noodle market

Sichuan

Shandong

20

24

food, feed, processed
10-15
micro -scale
noodles
local

food, feed, processed
30-40
micro and small-scale
noodles

local and export
local, provincial, national and
export

local, provincial, national

Source: Wheatley and Bofu 2000

156


Table 3. Paste characteristics of sweetpotato starch samples from different processors in Shandong and
Sichuan provinces, China.
Starch from Shandong

Starch from Sichuan

Parameter

Minimum

Maximum

Average

Minimum

Maximum

Average


Mungbean
starch

Cassava
starch

Peak viscosity, PV, (RVU)

250

430

320

150

520

390

460

340

Hot paste viscosity, HPV,
(RVU)

100


240

150

130

270

190

250

100

Cold paste viscosity, CPV,
(RVU)

180

380

250

220

340

270

390


180

Stability ratio (HPV/PV)

0.39

0.77

0.48

0.33

0.83

0.48

0.54

0.29

Setback ratio (CPV/HPV)

1.40

1.82

1.65

1.26


2.25

1.45

1.58

1.81

Pasting temperature (°C)

75.0

79.0

76.5

66.0

76.0

73.0

71.5

68.5

Table 4. Minimum, maximum and average values for various starch properties of 44 sweetpotato varieties
Property


Range

Average

Amylose content (%)

12.9 to 29.7

19.1

Con A precipitation

Swelling volume

24.5 to 32.7

29.0

0.35 g (db) in 12.5 ml water at 92.5°C, 30 min

Solubility

12.4 to 24.1

16.9

Similar to swelling volume

Pasting property


Method of Analysis

Use of a Rapid Visco Analyzer at 7% starch
concentration

Peak viscosity (RVU)

66 to 132

108

Hot paste viscosity (RVU)

58 to 109

94

Cold paste viscosity (RVU)

82 to 186

149

Stability ratio

0.73 to 0.96

0.86

Setback ratio


0.73 to 0.96

1.59

Onset gelatinization temperature (°C)

61.3 to 70.0

64.6

Use of Differential Scanning Calorimeter

Gel hardness

15.8 to 37.1

22.7

Texture profile analysis

Gel adhesiveness

-52.3 to 3.0

-24.1

Texture profile analysis

157



Table 5.

Paste viscosity, as determined by a Rapid Visco Analyzer using 9.2% starch
content, of sweetpotato starch extracted from tubers stored at –20, 4 and
25 °C for 0, 3, 10, 14 and 22 days

Storage

Time

Pasting temp.

Hot paste
viscosity, HPV
(RVU)

Breakdown

(°C)

Peak
viscosity, P
(RVU)

Temperature

(days)


Fresh

0

80.35 ± 0.00

432 ± 6

207 ± 1

225 ± 6

272 ± 1

65 ± 1

–20 °C

3

80.58 ± 0.25

428 ± 2

220 ± 1

208 ± 1

282 ± 2


62 ± 2

10

80.95 ± 0.35

419 ± 8

232 ± 6

186 ± 2

299 ± 10

66 ± 4

14

80.18 ± 0.32

437 ± 2

228 ± 1

209 ± 1

295 ± 1

67 ± 1


22

80.35 ± 0.57

432 ± 3

218 ± 3

214 ± 1

280 ± 1

61 ± 3

3

80.58 ± 0.32

429 ± 9

221 ± 3

208 ± 6

290 ± 3

69 ± 1

10


79.95 ± 0.00

452 ± 23

217 ± 7

235 ± 16

294 ± 9

77 ± 1

14

79.98 ± 0.46

442 ± 1

221 ± 1

221 ± 1

293 ± 1

72 ± 2

22

80.15 ± 0.35


438 ± 1

206 ± 2

232 ± 3

276 ± 2

69 ± 4

3

80.93 ± 0.25

407 ± 1

199 ± 1

208 ± 1

267 ± 6

68 ± 4

10

80.50 ± 0.42

404 ± 4


201 ± 2

204 ± 2

266 ± 2

65 ± 1

14

80.50 ± 0.28

410 ± 1

200 ± 1

210 ± 1

265 ± 3

65 ± 3

22

79.58 ± 0.04

417 ± 3

188 ± 2


229 ± 1

247 ± 1

59 ± 1

(P-HPV,
RVU)

Cold paste
viscosity,
CPV(RVU)

Setback
(CPV-HPV,
RVU)

(°C)

4 °C

25 °C

Table 6.

Raw
material

Starch whiteness and paste characteristics as determined by a Rapid Visco Analyzer of
sweetpotato starches extracted from different raw materials.

Starch
whiteness
(Kett scale)

Paste clarity
(% light
transmittance)

Paste characteristics
Pasting
temperature
(°C)

Fresh
tuber

91 ± 0.4

Peak
viscosity
(RVU)

Trough
(RVU)

Final
viscosity
(RVU)

Stability

ratio

Setback
ratio

23.18 ± 0.59

78.4 ± 0.2

372 ± 8

207 ± 4

262 ± 3

0.56 ± 0.01

0.70 ± 0.01

Frozen cake stored for
10 days

92 ± 0.3

25.85 ± 1.11

77.0 ± 0.2

394 ± 15


194 ± 9

255 ± 8

0.49 ± 0.01

0.65 ± 0.00

20 days

91 ± 0.3

23.10 ± 0.57

77.8 ± 0.3

395 ± 3

204 ± 5

266 ± 8

0.52 ± 0.01

0.67 ± 0.02

30 days

92 ± 1.0


30.33 ± 0.79

78.3 ± 0.2

397 ± 7

200 ± 3

259 ± 7

0.50 ± 0.01

0.65 ± 0.02

Chip stored for
0 day

83 ± 0.4

23.08 ± 1.28

76.9 ± 0.0

380 ± 3

206 ± 8

270 ± 7

0.54 ± 0.02


0.71 ± 0.01

30 days

83 ± 2

21.45 ± 0.38

77.4 ± 1.2

390 ± 10

221 ± 6

277 ± 6

0.57 ± 0.01

0.72 ± 0.01

158


Shandong
-small roaad
of hillside area
Shandong
-small road
of plain area

Shandong
-main road
of plain area
Sichuan
-An Yue
County
Sichuan
-Qiong Lai
County
Sichuan
-Xi Yang
County
Sichuan
-Luo Jiang
County
Sichuan
-San Tai
County
Sichuan
-Others

260

Hot paste viscosity (RVU)

230

200

170


140

Cassava
110
Mungbean

80
0

5

10

15

20

25

30

35

40

45

50


55

60

65

70

75

80

85

90

95

100

105

110

115

120

Sample no.


Figure 1.

Hot paste viscosity (HPV) of sweetpotato starch samples from Shandong (close symbols) and
Sichuan (open symbols) provinces

Note: The line represents the average values of each province.
Source: Sriroth et al. 2001.

0.2

Shandong
-small roaad
of hillside area
Shandong
-small road
of plain area
Shandong
-main road
of plain area
Sichuan
-An Yue
County
Sichuan
-Qiong Lai
County
Sichuan
-Xi Yang
County
Sichuan
-Luo Jiang

County
Sichuan
-San Tai
County
Sichuan
-Others

0.1

Cassava

1
0.9
0.8

Stability ratio

0.7
0.6
0.5
0.4
0.3

0

Mungbean
0

5


10

15

20

25

30

35

40

45

50

55

60

65

70

75

80


85

90

95 100 105 110 115 120

Sample no.

Figure 2.

Stability ratio of sweetpotato starch samples from Shandong (close symbols) and Sichuan
(open symbols) provinces

Note: The line represents the average values of each province.
Source: Sriroth et al. 2001.

159


Shandong
-small roaad
of hillside area
Shandong
-small road
of plain area
Shandong
-main road
of plain area
Sichuan
-An Yue

County
Sichuan
-Qiong Lai
County
Sichuan
-Xi Yang
County
Sichuan
-Luo Jiang
County
Sichuan
-San Tai
County
Sichuan
-Others

79

Pasting temperature (oC)

77

75

73

71

69


Cassava

67

Mungbean

65
0

5

10

15

20

25

30

35

40

45

50

55


60

65

70

75

80

85

90

95

100

105

110

115

120

Sample no.

Figure 3. Pasting temperature of sweetpotato starch samples from Shandong (close symbols) and

Sichuan (open symbols)
Note: The line represents the average values of each province.

Shandong
-small roaad
of hillside area
Shandong
-small road
of plain area
Shandong
-main road
of plain area
Sichuan
-An Yue
County
Sichuan
-Qiong Lai
County
Sichuan
-Xi Yang
County
Sichuan
-Luo Jiang
County
Sichuan
-San Tai
County
Sichuan
-Others


5.500
5.000
4.500
4.000

Hardness 1 (N)

3.500
3.000
2.500
2.000
1.500
1.000

Cassava
0.500

Mungbean
0.000
0

5

10

15

20

25


30

35

40

45

50

55

60

65

70

75

80

85

90

95

100 1 0 5 1 1 0 1 1 5 1 2 0


Sample no.

Figure 4.

Hardness of the first cycle of sweetpotato starch samples from Shandong (close symbols)
and Sichuan (open symbols) provinces

Note: The line represents the average values of each province.

160


Saccharides content (%)

25
20

at -20 C

15

at 4 C
at 25 C

10
5
0
0


5

10

15

20

25

Time (days)

Figure 5. Contents of starch by a polarimetric method and water soluble non -starch
carbohydrates by a phenol -sulfuric method in sweetpotato roots when stored at –
20, 4 and 25 °C for 0, 3, 10, 14 and 22 days.
Note: Dash line = starch content, % wet basis
Solid line = water soluble non-starch carbohydrates (g/100g dry tuber).

161


Swelling power

40
35
30
25
20
15
10

5
40
0
35
30
25
20
15
10
5
40
0
35
30
25
20
15
10
5
0

0 day
3 days
10 days
14 days
22 days
25 OC

4 OC


-20 OC

60

65

70

75

80

85

90

Temp (oC)

Figure 6.

Swelling power of sweetpotato starch extracted from tubers stored at F–20, 4 and
25 °C for 0, 3, 10, 14 and 22 days.

162


Figure 7. Typical protocol of sweet potato starch production by the sour-liquid method.
Source: Timmins et al. 1992.

163



Sediment and liquid interface (ml)

80

20%

70

30%

60

40%

50
40
30
20
10
0
0

10

20

30


40
Time(min)

50

60

70

Sediment and liquid interface (ml)

(a)
4
4.5
5
distilledwater

80
70
60
50
40
30
20
10
0
0

10


20

30 Time (m
40in)

50

60

70

(b)

Figure 8.

Sedimentation rate of sweetpotato starch slurry using sour liquid:
(a) effect of starch slurry concentration (pH 4.0)
(b) effect of sour liquid content (using 20% starch slurry) on sedimentation rate.

Note: Sedimentation rate indicated by the volume mark between the sediment and liquid interface.

164


Figure 9.

Paste viscosity profiles of sweet potato starch processed without and with sour liquid
application.

Note: Method used was a Rapid Visco Analyzer (11% starch).


Figure 10.

Paste viscosity profile of sweetpotato starch obtained by drying cakes (45% moisture
content) at different temperatures (50, 70, 90 and 120°C).

Note: Method used was a Rapid Visco Analyzer (9.2% starch content)

165