2.1. Moisture sorption isotherm and glass transition of breakfastcerealsCommercial breakfast cereals (Corn flakes) were purchased in alocal supermarket in Valencia (Spain). According to the producer’sspecifications, the composition of corn flakes was (g/100 g): 84 gof carbohydrates, 7 g of proteins, 0.9 g of lipids, 3 g of fiber, 0.5 g ofsodium and 1.3 g of salt. Likewise, the ingredients specified on thelabel were maize, sugar, barley malt flavoring, salt, niacin, iron, vitaminB6, vitamin B12, riboflavin (B2), thiamine (B1), folic acid, andvitamin D. The cereals were conditioned immediately after openingat different relative humidities. Sorption experiments were carriedout by triplicate at 20 C. For moisture conditioning, the cereals(approximately 10 g) were placed at 20 C in hermetic chamberscontaining saturated salt solutions with different relative humidities(LiCl: 0.113, CH3COOK: 0.230, MgCl2: 0.330, K2CO3: 0.430,Mg(NO3)2: 0.520, CuCl2: 0.680, NaCl: 0.755 and KCl: 0.845) (Greenspan,1977). The sample weights were controlled till a constant valueassumed as equilibrium was reached (Dweight < ±0.0005 g)(Spiess and Wolf, 1983). At this moment, the water activity of eachsample was assumed to be equal to the corresponding relativehumidity of environment. Initial moisture content of cereals wasdetermined by triplicate using a vacuum oven (Vaciotem, J.P. Selecta,Barcelona, Spain) (60 ± 1 C and at a pressure <100 mm Hg). Ineach equilibrated sample, the final moisture content was calculatedusing the initial moisture content and the final registered weight.These values were used to construct the sorption isotherms. In orderto predict the water sorption behavior of the samples, the GAB (Guggenheim,Anderson and de Boer) (Van den Berg and Bruin, 1981)model was used (Eq. 1).we ¼woCKawð1 KawÞð1 þ ðC 1ÞKawÞð1Þwhere we, water content at equilibrium (g water/g dry solids);aw, water activity; wo, monolayer value (g water/g dry solids); C,constant related to monolayer sorption heat and K, constant relatedto multilayer sorption heat.
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