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2. Materials and methodsThe methods of hydration and DSC procedures are amodification of those previously described. (Takigami,Takigami, & Phillips, 1993, 1994). A Perkin?Elmer DSCII equipped with cooling apparatus was used to measure thephase transitions of the sorbed water in the samples whichwere cooled from 25 to 2908C at 108C/min and subsequentlyheated to 258C at a rate of 108C/min. The temperaturescale and heats of fusion were calibrated using distilledwater and indium as the standard materials.After DSC measurements, the sample pans were prickedwith a pin to remove water from the samples, which were,thereafter, dried under reduced pressure for 30 min at1108C, then left overnight at room temperature. Afterweighing the water content was determined and Wc definedasWc.g=g. .Weight of water .g. in polysaccharideWeight of dry polysaccharide .g.It is possible to distinguish between non-freezing water(Wnf), freezing-bound water (Wfb) and free water (Wf)from the individual transitions or shape of the transitions,and these are connected by the relationshipWc . Wnf 1 Wfb 1 Wf3. Results and discussionA representative set of DSC heating curves for kappacarrageenan(E407) is shown in Fig. 1, and for PES(E407a) in Fig. 2. The Wc values are listed in the figure.The systems are first cooled to 2808C and slowly warmed.At Wc 0.35 for kappa-carrageenan and Wc 0.22 for PES, notransition is observed, indicating that at this water contentthe water is bound to the hydrophilic groups and is non-Food Hydrocolloids 14 (2000) 609?6130268-005X/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.PII: S0268-005X(00)00040-0www.elsevier.com/locate/foodhyd* Corresponding author.610 S. Takigami et al. / Food Hydrocolloids 14 (2000) 609?613Fig. 1. DSC heating curves for water?kappa-carrageenan (E407) systems.Fig. 2. DSC heating curves for water?PES (E407a) systems.freezing. As the value of Wc is increased, both systems showthe presence of at least two broad transitions, due to themelting of free water (at temperature Tf) and freezingboundwater (at temperature Tfb). As Wc is increased Tfbshifts towards Tf and eventually overlaps in each of thesystems. We have used this value where overlap occurs ofthe bound water melting temperature with that of free wateras an indication of the relative abilities of individual polysaccharidesto bind water (Takigami et al., 1994). It isimmediately evident by direct observation of Figs. 1 and 2that kappa-carrageenan retains water in a bound formmore effectively than PES. The task is to quantify thisdifference.Immediately following the first heating and cooling cycle,the procedure was repeated, but we did not observe anysignificant change in the transitions, indicating that bothsystems can readily form stable and reversible thermodynamictraps for water. The melting of the freezing-boundwater is an endothermic process. The freezing processhowever is exothermic. Fig. 3 shows the typical behaviourof both systems on cooling. More than one transition areevident both at water contents 0.58 and 0.6, which aresubsequently masked as the water content is increased. Itwould appear, therefore, that within the broad transitionalS. Takigami et al. / Food Hydrocolloids 14 (2000) 609?613 611Fig. 3. DSC cooling curves for water?PES (E407a) systems.Fig. 4. The relationship of melting temperature with water content for PES(E407a) and kappa-carrageenan (E407). (W) For bound water for E407a;(A) free water for E407a; (K) overlapped water for E407a; (X) bound waterfor E407; (B) free water for E407: (O) overlapped water E407.envelope, there are several metastable states of water, due tothe water interacting and being ‘bound’ within various siteswithin the polysaccharide structure. The unsymmetricalcurves, even at high water contents, show that the freeandbound water exist alongside each other.The overall behaviour can more readily be identified inFig. 4, which directly compares the behaviour of the twocarrageenan systems. Whereas the temperature of freezingof the free water is constant with an increase in Wc (the valueof Tfb moves up, to approach that of free water. Followingthe sorption of the non-freezing water at the hydrophilicgroups of the framework polysaccharide structure, as Wcincreases, the freezing-bound water builds up as a structuredentity and retains its distinctive character from free water.The enthalpy of melting of this structured water remains lessthan that of free water throughout (due to defects introducedinto the freezing-bound ice. The free water, on the otherhand, melts near 08C in the form of hexagonal ice, becausein the free form there is no interference by the carrageenanstructure.As was previously shown, it is possible to calculate theamounts of the various types of water at increasing Wcvalues (Takigami et al., 1993, 1994), as shown in Figs. 5and 6. In comparison with other polysaccharides we havestudied, the ability of PES to strongly bind water is low.There is an initial build-up of non-freezing water to 0.5 gwater per gram PES, which does not change with watercontent. This value represents the complete hydration ofthe sugar skeleton and corresponds to ,13 moles ofwater in the non-frozen state per disaccharide unit. Thiswater is bound tightly by the OH and other ionic sites.The amount of bound water, while increasing with watercontent remains low particularly for PES. At Wc 1.5, forexample, kappa-carrageenan bind at least twice the amountof water than PES, and continues to increase beyond Wc2.8, whereas PES is saturated at Wc 1.5. Table 1 shows acomparison of kappa-carrgeenan and PES with other polymersystems.The gel?sol transition has been compared for kappacarrageenanand PES (Tanaka et al., 1996), and demonstratesthat the presence of cellulose in PES leads to alower gel?sol transition temperatures. More heat absorption612 S. Takigami et al. / Food Hydrocolloids 14 (2000) 609?613Fig. 5. The amounts of various types of water as a function of water contentfor PES (E407a). (X) Non-freezing water; (A) bound water; (K) free water;(W) total freezing water.Fig. 6. The amounts of various types of water as a function of water contentfor kappa-carrageenan (E407). (X) Non-freezing water; (A) bound water;(K) free water; (W) total freezing water.
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好好努力,迎接新的开始回复: 可以帮忙翻译一下吗?楼主你不是在开玩笑吧。。。。
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白天看中国股市,晚上看中国足球!回复: 可以帮忙翻译一下吗?楼主先翻出来,让大家帮忙改如何?。。。
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回复: 可以帮忙翻译一下吗?不开玩笑是我的弟弟让我帮忙翻译的.我汗,姐姐英语一蹋糊涂.他真是看走了眼睛.我今天如何交代啊??难受
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好好努力,迎接新的开始回复: 可以帮忙翻译一下吗?有好心人帮我翻译一段也可以我可以有所交代啊
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好好努力,迎接新的开始回复: 可以帮忙翻译一下吗?在做毕设?。。。
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回复: 可以帮忙翻译一下吗?专业性有些强。。。翻出来还不如不翻呢。。。强要非专业的翻,至少也给个word list吧。。。还是找找专业论坛吧。。。
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回复: 可以帮忙翻译一下吗?在做毕设?。。。点击展开... 对的,.聪明是毕设.他说就简单翻译一下就行了
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好好努力,迎接新的开始回复: 可以帮忙翻译一下吗?这个没法简单翻译,专业英语简单翻译就是错的翻译。这段1000多字,拿去外边翻译公司估计至少400块,汗。。。我是看得一头雾水,类似专业的朋友帮帮忙吧。
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