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1 Agric. Biol. Chem., 49 (4), , Retrogradation of Gelatinized Potato Starch Studied by Differential Scanning Calorimetry Fumiko Nakazawa, Shun Noguchi, Junko Takahashi and Masako Takada Kyoritsu Women's University, Hachioji-shi, Tokyo 193, Japan Received July 5, 1984 The retrogradation process of gelatinized potato starch was studied by DSCas a function of the aging temperature and water content. Differences in the retrogradation states dependent on these parameters were revealed by the pattern of the DSCcurves. Potato starch aged at a low temperature after gelatinization gave a low endothermic onset temperature. As the starch content increased, retrogradation proceeded over a wider temperature range. The retrogradation proceeded even at 50 C for a starch content of 50%. The gelatinization enthalpy for granular starch was found to be 4~4.5 cal/g of dry starch. By contrast, the maximumgelatinization enthalpy obtained for retrograded starch was found to be as low as 2.5cal/g of dry starch. Retrogradation of gelatinized starch is a serious problem offood storage. Several methods, such as those using enzymes,1 2) X-ray diffraction,3'4) pulsed NMR5)and precipitation6'7) have been applied in investigating this phenomenon. In a preceding paper,8) it was briefly shown that differential scanning calorimetry (DSC) was also successfully applicable to the study of starch retrogradation.8) In this report, the retrogradation of potato starch studied at various aging temperatures after gelatinization using DSC is described. The advantages of studying starch retrogradation by DSC are as follows: (1) it is applicable over a wide range of water content, (2) the heat of re-gelatinization after retrogradation can be determined directly, (3) no change of water content occurs over the course of aging due to perfect sealing of the sample cells, and (4) it is not time consuming and does not require any special technique. MATERIALS AND METHODS The patato starch used was a commercial product, dry starch fractions being used in this procedure. DSCwas carried out using a Daini-Seikosha 560U from 10 to 120 C, usually with a heating rate of2 C/min. Water as a reference and a sample of about 30mg (starch+water) were sealed in silver sample cells. After the first scan, the sample cells were kept at an appropriate constant temperature, 5 ~ 50 C, for aging. The second scan was then carried out at the same heating rate. The sample cells were sometimes held at 120 C after the first scan to allow perfect gelatinization to occur before aging. After correction, some of the DSCcurves are shown on the same flat baseline as they appeared in a previous paper9) for the convenience of comparing small curve differences. The series ofdsc curves shown on one figure was obtained for samples of almost the same weight of starch. RESULTS AND DISCUSSION Figure 1 shows the DSCcurves of potato starch-water mixtures of various starch content. The dry starch contents are shownon the graph. A single peak can be observed for samples with a low starch content and two peaks for those with a high starch content. This type of behavior is similar to that reported by Donovan.10) In this report, the retrogradations for a 30% and 50% starch content, which show respectively a single and a double peak in their DSC curves, will be discussed. When heating scans for 50% starch were successively run five times from about 13 to 120 C, only the first scan (on a reduced scale of 1/5) was markedly different from the others, as

2 954 F. Nakazawa et at. 55% Dry Starch Starting Temperature \ 1st scan 12.5 \ ~5" / 5 "^\S /// -^ UJ ^v \ // / ^^ x 30\^\y / 20 \\j/ ^^\ 5 \// I J0.14mcal/s 1"// ^V I / \ / 13 \ r^ / 12 \\ / x/ 13 \\\ 2nd I/ \\\\ /3rd s \\\ /^th \\ / 5th 20 A Fig. 1. DSCCurves of Starch-Water Mixtures Containing Various Amounts of Starch. Dry starch contents were 55, 50, 45, 35, 30 and 20% Heating rate was 2 C/min Fig. 2. Five Successive Heating Scans for 50% Starch. The curve for the first scan was plotted on a 1/5 reduced scale. Starting temperatures for each of the scans are shown in the figure. shown in Fig. 2. A few differences were observable among the four curves of the second to the fifth scans, depending on small differences between their temperatures at the start of the DSCmeasurements. It may safely be said that the first scan, with a heating rate of 2 C/min under the condition of complete sealing of the sample cell, gelatinized the starch perfectly, since the other scans, including the second one, were similar to each other. The second scans for 30%starch, aged for 7 days at 5 C after the first scan and after holding for 16hr at 120 C following the usual first scan, are shown in Fig. 3 in solid and dotted lines, respectively. Since their DSC curves are almost the same, it may also be concluded that the first DSC scan perfectly gelatinized the starch under the conditions reported here. Figure 4 shows the second series of scans for 50% starch after aging at 5 C for 12hr, and 2, 5, 10 and 30 days. The first scan is also shown on a reduced scale as a dotted line. The endothermic band around 65 C progressed faster than the band around 50 C. It is noticed that the bands grew even from 10 to 30 days after gelatinization. Figure 5 shows the same scans as shown in Fig. 4 after aging at 23 C for 1, 2, 5, 10 and 20 days. The thermograms are different from those aged at 5 C. There is no band around 50 C but there is one present at around 60 C. A band observed around 65 C as a faster growing band in the case of 5 C aging might also exist for 23 C aging, but the overlap of the 60 C band makes the band around 65 C obscure. Heats of transition of the second scans after aging at 5 and 23 C are shown for 50% starch in Fig. 6 as a function of aging time. It seems that retrogradation in 50%starch progressed faster for aging at 23 C than for aging at 5 C, contrary to the expected belief that the

3 Retrogradation of Gelatinized Potato Starch by DSC \ / 2 \ / u \ / l TEMPERATURE, C Fig. 3. Second Scan for 30% Starch after Aging for 7 Days at 5 C. The solid line shows the second scan after the usual first scan and the dotted line represents the curve obtained after holding for 16hr at 120 C following the usual first scan. e! \>r^^^\hr «ist scan #// LU l[ Fig. 4. Second Scans for 50% Starch after Aging at 5 C for 12hr, and 2, 5, 10 and 30 Days. The first scan is also shownas a reference spectrum on a reduced scale. ^ ^v \ist scan '''/(/ '^ I 0.056mcal/s \\-^V Fig. 5. Second Scans for 50% Starch after Aging at 23 C for 1, 2, 5, 10 and 20 Days.

4 956 F. Nakazawa et al. lower the aging temperature, the faster the retrogradation proceeds. Since the heat of gelatinization in the first scan was 4.4 mcal/mg on average, the heat in the second scan was almost half. Figure 7 shows the heats of transition of the second scan for 30% starch against aging time. Aging in 30% starch proceeds on the whole more slowly than it does in 50% starch, in agreement with the results for glutinous rice starch obtained by pulsed NMR.5) On comparing the aging at 5 C 3I 1 i. 8 f 8 fei- with that at 23 C in 30% starch, the former proceeds faster than the latter in contrast to the case of 50% starch as shown in Fig. 6. The heat of transition of the second scans after aging for more than 10 days, is about 2~2.5mcal/mg. This value is almost half that of the first scan, in agreement with the result for 50% starch. Figure 8 shows the series of second scans for 30% starch after aging for 5 days at 5, 10, 23 and 40 C. The curve of the first scan is also I 31 3 E o à" s.-s: t à" i/) < LxJ X AGING TIME, days Fig. 6. Heat of Transition of Second Scans for 50% Starch after"aging at 5 C (o) and 23 C (à"à") against the Number of Days Aged. AH=4.4mcal/mg in the first scan. I I AGING TIME, days Fig. 7. Heat of Transition Starch After Aging at 5 C (o.) of Second Scans for and 23 C (à" ) against 30% the Number of Days Aged. zl//=4.6mcal/mg in the first scan. i : / r i / 1st scan ; «' *l... V TEMPERATURE, C Fig. 8. Second Scans for 30% Starch after Aging for 5 Days at 5, 10, 23 and 40 C. The spectrum of a first scan is shown on a reduced scale as a reference.

5 Retrogradation of Gelatinized Potato Starch by DSC 957 i io\a scalx"'//^ 1 ^ v=::::=;4l3c^o c ill n^ ^^^^ Z3C "I à" 1 1 i i i TEMPERATURE Fig. 9. Second Scans for 50% Starch after Aging for 5 Days at 5, 10, 23, 40 and 50 C. The spectrum of a first scan is shownon a reduced scale., C shown on a reduced scale for reference. The spectra of the second scans shifted to a higher temperature as the aging temperature was increased. In comparison with the aging at 5 and 10 C, the heat of transition for aging at 23CCwas lower and that at 40 C became very small. The second scans for 50% starch after aging for 5 days at 5, 10, 23, 40 and 50 C are shown in Fig. 9. The characteristics of the thermograms are qualitatively similar to that for 30% starch. However, there are two differences: the heat of transition for aging at 23 C was larger than those at 5 and 10 C, and there were apparent endotherms after aging as high as 40 and 50 C. In the case ofa low water content such as that in 50% starch, retrogradation at a lower temperature proceeded slowly because of the lower mobility of the starch molecules. The dependence of the heat of transition on aging temperature shownin Figs. 8 and 9 after aging for 5 days was temporary, since retrogradation proceeded even after 5 days. The process of retrogradation at different temperatures of starch-water mixtures with various degrees of water content are clearly shown as thermal spectra by DSCcurves. It has been shownthat the molecular state after retrogradation differed according to the water content and the temperature at which the mixtures were stored, since the thermal spectra after aging are different in each case. Acknowledgment. We thank Mrs. Y. Tamura and M. Yajima for their assistance in our experiment. REFERENCES 1) K. Kainuma, A. Matsunaga, M. Itagawa and S. Kobayashi, J. Jpn. Soc. Starch ScL, 28, 235 (1981). 2) K. Sakai, S. Hizukuri and I. Maeda, Nippon Nogeikagaku Kaishi, 50, 191 (1976). 3) U. Matsukura, A. Matsunaga and K. Kainuma, J. Jpn. Soc. Starch ScL, 30, 106 (1983). 4) S. Hizukuri, K. Itoh, I. Maeda and J. Nikuni, /. Jpn. Soc. Starch Sci., 19, 70 (1972). 5) F. Nakazawa, S. Noguchi, J. Takahashi and M. Takada, Kaseigaku Zasshi, 34, 566 (1983). 6) S. Kitamura, S. Yoneda and T. Kuge, Carbohydr. Polym., 4, 127 (1984). 7) R. Collison, "Starch and its Derivatives," ed. by J. A. Radley, Chapman & Hall, 1968, p ) F. Nakazawa, S. Noguchi, J. Takahashi and M. Takada, Agric. Biol. Chem., 48, 201 (1984). 9) F. Nakazawa, S. Noguchi, J. Takahashi and M. Takada, Agric. Biol. Chem., 48, 2647 (1984). 10) J. W. Donovan, Biopolymers, 18, 263 (1979).

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