Optimal temperature and additives improving stability ofapple powder

Abstract: In order to improve the storage stability and to optimize the storage conditions, the effect of temperature and additives on the moisture adsorption isotherms and glass transition temperature of spray drying apple powders were investigated based on the water activity concept and the theory of glass transition temperature. The water activity concept and the glass transition temperature theory can be used to predict the stability of food. Monolayer water content is another functional value that relates to the amount of bound water in a solid material. This water normally varies between 0.04-0.11g/g dry solids for several dried foods. This low water content does not support microbial reactions, and hence contributes to food product stability. Food materials are fairly stable below their glass transition temperature but when the temperature rises above glass transition temperature, a solid structure is transformed to a supercooled liquid state with time-dependent flow. The use of state diagrams that indicate the material's physical state, in combination with the sorption isotherms, helps in the prediction of food stability. Thus, the objective of this work was to provide experimental data of the state diagram and moisture absorption isotherm of spray dried apple powder produced with different temperature and additives, in order to obtain useful information about powders stability. Apple powders were produced by spray drying using different additives: β-cyclodextrin, corn starch, maltodextrin, soluble starch. Additives were then added to the filtered pulp, in a concentration of 30% (w/w), under magnetic agitation, until complete dissolution. This concentration was selected in a preliminary study, as the minimum concentration at which it was possible to dry the juice without excessive powder stickiness on the chamber wall. Spray drying process was performed in a laboratory scale spray dryer LPG5 (XianFeng, JiangSu, China). The feed flow rate used was 20 mL/min, inlet and outlet air temperature was 180℃ and 80℃. Static weighing method was used to obtain the data of sorption isotherms at different temperature (5, 25 and 45℃). Eight saturated salt solutions were prepared (LiCl, MgCl2, Mg(NO3)2, NaBr, NaNO3, NaCl, KCl and KNO3) in order to provide relative humidity. Triplicate samples of 1 g of apple powder were weighed into aluminum vials and equilibrated over the saturated solutions in desiccators at 5℃, 25℃, 40℃ and 45℃. The time used for equilibrium was one to two weeks based on weight changes of the samples not exceed 0.1%. Five models available in the literature (Lewicki, GAB, Smith, Henderson and Peleg) were evaluated by determining the best fit to the experimental data. Differential scanning calorimetry (DSC) was used to identify the glass transition temperature of apple powders with the addition of β-cyclodextrin, soluble starch, maltodextrin and corn starch at 40 oC. The Khalloufi, El-Maslouhi, Ratti equation was used to fit the experimental data of glass transition temperature of apple powders. The results showed that the adsorption curves of apple powders can be best fitted by GAB model. The GAB model provided a satisfactory description of the sorption isotherms with the highest values of Adj.R2(0.9921-0.9978) for all tested powders. The equilibrium moisture content increases with increasing water activity. When the critical water activity <0.76, the equilibrium moisture content declined with increasing in temperature; when the water activity >0.76, the equilibrium moisture content increased with increasing in temperature. The adsorption curves of apple powder with the addition of β-cyclodextrin, soluble starch and corn starch followed a typical type II (sigmoid), the powder with the addition of maltodextrin followed a typical type III. The glass transition temperature decreased with an increasing in equilibrium moisture content. All of four additives can increase the glass transition temperature when storage at 40℃. Their ability to increase the glass transition temperature from high to low was: β-cyclodextrin, corn starch, maltodextrin, soluble starch. At 40 oC, the critical water content of β-cyclodextrin, corn starch, maltodextrin was 0.0745, 0.0137, 0.0032 g/g, respectively; while the critical water activity of β-cyclodextrin, corn starch, and maltodextrin was 0.1795, 0.0159, 0.0106, respectively. The storage temperature of apple powder with the addition of soluble starch should not be more than 17 oC. The apple powder produced with β-cyclodextrin was the most stable one with the highest critical water activity of 0.1795. This meant that when the powder was stored at 40℃, the maximum relative humidity to which it can be exposed was 17.95% and its moisture content was 7.45%. However, when stored at a relative humidity higher than 17.95% (at 40℃), or at a higher temperature (at water activity was 0.1795), the powder would suffer physical transformations such as collapse, stickiness and caking. The results provide valuable information for selecting drying methods and establishing optimum storage conditions of apple powder.