Development of a Standard Protocol for the Processing of High Quality Sweetpotato Starch for Noodle Making

Chapter 11 Development of a Standard Protocol for the Processing of High Quality Sweetpotato Starch for Noodle Making Kuakoon Piyachomkwan1, Klanarong Sriroth2, Kanjana Chinsamran2, Kam

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 Oates3

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

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 Vario us 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

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

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 non-Chinese 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

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 harvest 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)

post-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

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)

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

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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