Gelatinization and Liquefaction of Starch with a Heat Stable clt-Amylase

Y.C. LEE and K.T. KIM

ABSTRACT Sweet potato and corn starches were. gelatinized and liquefied to dex- trose equivalent (DE) 10 in a steam-jacketed kettle or in a twin-screw extrusion cooker in a single step process with a heat stable a-amylase. The time required to gelatinize and liquefy 20% starch slurries in a kettle at 95°C was 45 to 50 sec. Starch powder (10 kg/hr) and water (2-6 kg/hr) were fed to an extrusion cooker to adjust the moisture contents of the starch from 20 to 55% (wet basis) and gelatinized and liquefied to DE 10 at a barrel temperature of 120°C or above with 2 to 3% (w/w) a-amylase added. The optimum conditions for the op- eration were to feed starch with water to maintain 50% moisture and 2% heat stable cr-amylase at a barrel temperature of 120°C.

INTRODUCTION

GELATINIZATION of starch is required as a pretreatment to convert raw starch to various processed products, such as mal- todextrin, glucose and alcohol. It can be generally achieved by heating a starch slurry to the gelatinization temperature but consumes a large amount of energy (i.e., 30% total energy needed for alcohol fermentation, Ueda, 1981). Attempts have been made to reduce energy consumption for gelatinization of starch by using microorganisms or enzymes which can hydro- lyze raw starch (Svendshy et al., 1981) and acids or alkalies which lower gelatinization temperature (Bhattacharya and Hanna, 1987), but there are problems to overcome for practical appli- cations.

High-temperature short-time (HTST) extrusion cooking has been used for the production of various convenience foods and modified food ingredients (Williams et al., 1977), and starch can be efficiently gelatinized by extrusion cooking (Williams, 1977). Linko (1982) reported the possibilities of using a twin screw HTST - extrusion cooker for continuous gelatinization and enzymatic hydrolysis of starch with a heat stable cx-amy- lase. Brooks and Griffin (1987) and Chen and Chang (1984) indicated that a rice flour slurry could be hydrolyzed in a single step process utilizing a heat stable a-amylase. The objectives of this research were to investigate optimum conditions for gelatinization and liquefaction of starch slurries and powders in a single step process using a heat stable cx-amylase and to evaluate some of the quality factors of the liquefied starch products.

MATERIALS & METHODS

SWEET POTATO and corn starch powders used for the experiment were supplied by Sunil Glucose Co. (Inchon, Korea); the heat stable cY-amylase selected was obtained from Novo Industri A/S(Dcnmark). The manufacturer indicated that the enzyme preparation had an activ- ity of 120 kilo Novo oc-amylasc units (KNU), with one KNU being defined as the amount of enzyme ncccssary to break down 5.2 g starch per hour to a non-iodine staining polymer under Novo’s standard con- ditions (Anonymous, 1985).

Authors Lee and Kim are with the Dept. of Food Science & Technology, Chung-Ang Univ; Heuksuck-Dong, Seoul, Korea.

Degree of gelatinization and liquefaction

Degree of gelatinization of samples was determined by the Wootton et al. (1980) method and expressed as percent by comparing it with a completely gelatinized control. Gelatinized samples were dried, and 2g powder were mixed with 10 mL water for 1 min. The mixture was centrifuged at 2,000 x g for 10 min. One mL of the supcrnatant, 10 mL of water and 1 mL of O.SN-HCI were mixed in a test tube, and then 0.1 mL of iodine solution (KI 4% and I* 1%) was added. The absorbance of solution was measured at 600 nm(X1).

To prepare a completely gelatinized starch sample, 2 g of starch were mixed with 95 mL water for 1 min and then 5 mL of lOM-KOH added. After mixing for 5 min, the solution was centrifuged at 2,000 g for 10 min. Ten mL water and 0.1 mL of iodine solution were added to the 1 mL portion of supernatant, and the absorbance of the solution was measured at 600 nm (x2). The degree of gelatinization was then calculated as the ratio of Xl to X2.

Liquefied samples were analyzed for glucose content by the dini- trosalicylic acid (DNS) method (Miller, 1959), and the degree of liquefaction was expressed as dextrose equivalent (DE). One gram of liquefied sample and 3 mL of DNS-glucose reagent were mixed in a flask and heated in a boiling water bath for 5 min. After 200 mL of water was added to the mixture, the absorbance was measured at 540 nm using Spectronic 70 spectrophotometer (Baush & Lomb, Roch- ester, NY).

Gelatinization and liquefaction of starch slurry

A 20% (w/w) starch slurry with 1% or-amylase solution(w/w) was heated at 95°C and pH 6.5 in a steam-jacketed kettle for gelatinization and liquefaction in a single step process. The slurry was agitated during heating; time required for the slurry to reach DE 10 was defined as the liquefaction time. Since Chung and Chang (1985) suggested that optimum DE value of the liquefied starch for saccharification was 8 - 12, DE 10 was chosen as a criterion for liquefaction of starch in this study.

Gelatinization and liquefaction of starch powder

Sweet potato starch and corn starch were gelatinized and liquefied with a Werner and Pfleider twin-screw extruder model ZSK 30 (Ram- sey, NJ) containing two co-rotating and intcrmeshing screws in a high shear configuration. The extruder barrel consisted of 5 sections, with a 14:l length to diameter barrel and a 4mm diameter die.

Starch powder was fed into the extrusion cooker at 10 kg/hr and water at 2-6 kg/hr to adjust the moisture contents of starches to 20, 30, 40,50 and 55%(w/w). The inner barrel temperature of the extruder was adjusted to llo”C, 12o”C, 130°C and 135°C. cu-Amylasc was added to the starch at 1, 2, 3, and 4% concentration(w/w), and the screw speed was held at 180 rpm. The extrudate was dried in a forced convection oven at 60°C for 24 hr and ground to obtain powder which passed through a 60 mesh sieve. The powdcrcd sample was kept in sealed jars at room temperature for further analysis.

Consistency of liquefied starch solution

Consistency of the liquefied starch solution was determined at 25°C by a Brookfield Viscometer (model LVT, Brookficld Eng. Inc.). Con- sistency data obtained were converted to ccntipoisc by the conversion tables of the Brookfield Viscometcr.

RESULTS & DISCUSSION Gelatinization and liquefaction of starch slurry

The liquefaction time of a starch slurry to DE 10 by 1% heat stable cY-amylase at 95°C was 45 set for sweet potato

Volume 55, No. 5, 1990-JOURNAL OF FOOD SCIENCE- 1365

HEAT STABLE CPAMYLASE STARCH. . .

Table 1 -Liquefaction time and consistency of 20% starch slurries liq- uefied to DE IO with 1% heat-stable eamylase

Sweet potato starch Corn starch

Liquefaction 45 50 time(sec)

Consistency (cP) 3.6 3.8

Table 2-Effects of barrel temperature and moisture content on percent gelatinization of sweet potato starch powder

Moisture

Barrel (% wet basis

temp (“C) 20 30 40 50 55

110 80.4 81.1 83.5 86.8 87.3 120 100. 100. 100. 100. 100. 125 100. 100. 100. 100. 100. 130 100. 100. 100. 100. 100. 135 100. 100. 100. 100. 100.

Table 3-Effects of barrel temperature and moisture content on percent gelatinization of corn starch powder

Moisture

Barrel temp (“C)

110 120 125 130 135

20 90.0

100. 100. 100. 100.

(% wet basis)

30 40 50 55 89.8 85.7 86.0 84.5

100. 100. 100. 100. 100. 100. 100. 100. 100. 100. 100. 100. 100. 100. 100. 100.

Table 4-DE values of liquefied sweet potato starch as affected by barrel temperature and cu-amylase

fx-amylase (% w/w)

Barrel temp 1°C) 1 2 3 4

120 11.4 12.3 14.4 13.3 125 11.2 12.4 12.5 12.5 135 10.1 10.1 9.8 10.9

Table 5-DE values of liquefied corn starch as affected by barrel tem- perature and rr-amylase

Barrel temp (“C)

120 125 135

1

8.0 6.4 5.9

cu-amylase (% w/w) 2 3

10.2 10.2 9.8 9.9 7.9 7.9

4

10.0 9.4 8.4

Table 6-DE values of simultaneously gelatinized and liquefied sweet potato and corn starches extrusion cooked at 120°C

Moisture content (% w/w)

30 40 50

cu-Amylase Sweet Sweet Sweet (%)

2 3

potato 9.3 9.5

Corn 7.3 7.5

potato 9.8

10.0

Corn 8.4 8.5

potato 10.6 10.7

Corn

10.1 10.0

starch and 50 set for corn starch (Table 1). Using a short grain rice (var. Nortai), Griffin and Brooks (1989) reported that the liquefaction time of 20% rice flour to DE 8.1 at 95°C by 0.01% heat stable ol-amylase was 75 min. The big difference between the two sets of data appeared to be due to the fact that high enzyme concentration and refined starch, instead of cereal flour, were used in this study, and liquefying time was counted from the point when the solution reached 95°C. Consistency of li- quified 20% sweet potato starch was 3.6 CP and that of liq- uefied 20% corn starch was 3.8 CP (Table 1).

Gelatinization and liquefaction of starch powder The effects of starch moisture content and extruder barrel

temperature on the degree of gelatinization are shown in Tables 2 and 3. The results revealed that % gelatinization of sweet potato and corn starches were 80.4-89.8% at llO”C, regardless of starch moisture content. Gelatinization of starch was com- plete at barrel temperatures of 120°C or above in the range of moisture contents tested. Harper (1981) showed that wheat flour with 21% moisture, extruded at 110°C and 140 rpm, resulted in 80% gelatinization; corn starch with 35% moisture extruded at 105°C by Mapimpianti single screw extrusion cooker had 98.6% gelatinization (Guidolin et al., 1984).

The DE values of starch liquefied at different barrel tem- peratures and cY-amylase concentrations added to starch with 50% moisture are shown in Tables 4 and 5. Higher DE values (9.8 - 14.4) of liquefied sweet potato starch suggested that sweet potato starch could be gelatinized and liquefied in the extruder more easily than corn starch. DE values of liquefied starch increased with an increase of cY-amylase added, and the trend was more obvious at lower barrel temperatures. This could be due to the fact that the heat stable cx-amylase used was denaturated by heat at the higher rate as the barrel tem- perature was raised. These results indicated that optimum bar- rel temperature and enzyme concentration for gelatinization and liquefaction in a single step process were 120°C and 2- 3% ol-amylase, respectively.

DE values of liquefied starch as affected by starch moisture content and or-amylase at 120°C are given at Table 6. DE values of liquefied starch increased with an increase in starch moisture content. There was no noticeable difference in DE values between starch liquefied by 2% and 3% ol-amylase. Linko et al, (1984) also found that the DE values of liquefied barley starch increased with an increase in moisture content during extrusion cooking. Starch moisture content over 50% was not attempted, because the extruder could not process the samples efficiently. The results of this study showed that op- timum conditions for gelatinization and liquefaction of starch powder in a single step process were to feed starch with 50% moisture and 2% heat stable cy-amylase and to maintain the barrel temperature at 120°C.

Based on the results of this study, it appeared possible to simplify the gelatinization and liquefaction processes of starch slurries as well as starch powders by using a heat stable CY- amylase in a single step high temperature short time process. This process could be applied to gelatinization and liquefaction of starch materials for alcohol fermentation, glucose and mal- todextrin production and other processed products with in- creased productivity, reduced manpower reuqirement and reduced energy consumption (Bengera et al., 1984).

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corn by roll and extrusion cooking. Cereal Sci. Today 14(1):4. Anonymous, 1965. Termamyl Product Data Sheet. Now Laboratories, Inc.

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cooking of starches and flours. In Tl~ermal Processing and Quality of Foods, (Ed.) Zeuthen, P., Cheftel, J.C., Eriksson, C., Jul, M., Leniger. H., Linko, P., Varela, G., and Vos, G., p. 236. Elsevier Applied Sci. Pub- lishers, London and New York.

Bhattacharya, M. and Hanna, M.A. 1987. Kinetices of starch gelatiniza- tion during extrusion cooking. J. Food Sci. 52: 764.

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rice maltodextrins hydrolyzed from milled rice flour using heat stable alpha-amylase. J. Food Sci. 54: 190.

Guidolin, E., Sangiovanni, V., Virtucio, L., and Papotto, G. 1984. Evalu- ation of the behavior of different types of starches and flours after pre- cooking in a high shear or in a low shear extruder-cooker In Z’/wr.ermal Processing and Quality of Foods, (Ed.) Zeuthen, P., Cheftel, J.C., Eriks- son, C., Jul, M., Leniger, H., Linko, P., Varela, G., and Vos, G., p, 195. Elsevier Applied Sci. Publishers, London and New York.

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