Effects of freeze drying, refractance window drying and hot-air drying on the quality parameters of açaí

Açaí is a dark purple berry native to the Amazonian region in Brazil and widely consumed in South America. The fruit is a major component of the diet and significant to the local economy. Recently, more attention has been paid to açaí due to its high antioxidant properties. However, açaí is a highly perishable fruit and should be processed quickly after harvesting in order to preserve its nutritional and sensory properties as well as its bioactive compounds. Drying is one of the processing techniques used to preserve the fruit. Dehydrated products present high stability and are easy to transport and store. Dried açaí is, currently, mainly produced by freeze drying. Even though this method results in high quality products, the high cost associated with capital investment and operation is its major disadvantage. Thus, the search for alternative and more economic drying methods is of interest. Refractance Window drying is a novel and promising drying technique, and it has been demonstrated to retain food quality. Hot-air convective drying is an ancient and inexpensive process. It is the most widely used drying method in the food industry, although care should be taken to ensure product quality. The objective of this research was to evaluate the impact of three drying methods - freeze drying, refractance window drying and hot-air drying - on the quality parameters of açaí after drying and over a 3 month storage period at 25°C. To examine the potential quality changes, the following analyses were performed: moisture content, water activity, moisture working isotherm, glass transition temperature, color, antioxidant capacity, anthocyanin quantification, and flavor analysis. The moisture contents of the dried samples were all below the monolayer values. The water activity values of the açaí immediately after drying and during storage were also low, indicating relatively good product stability. The Brunauer-Emmett-Teller (BET) and Guggenhein-Anderson-de Boer (GAB) models showed a good fit to the isotherm data. Differential Scanning Calorimetry (DSC) thermograms suggested that there exists a subtle glass transition between 50-60°C. DSC analysis also revealed that the lipids present in açaí are liquid at room temperature. Drying affected both the color and flavor of açaí juice. Dried samples were featured by a lighter color and lower hue angle and chroma when compared to the juice. Storage was marked by a decrease in lightness, most likely because of the formation of brown pigments by Maillard reaction and polymerization of oxidized phenolic compounds. For flavor analysis, the compounds (E,Z) 2,6-nonadienal, α-ionone and 2-phenylethanol/β-ionone were the most important odorants identified in açaí samples. The dried açaí had a higher concentration of Strecker aldehydes and compounds derived from lipid oxidation, which indicates the occurrence of those reactions during drying. During storage, lipid oxidation was the most important reaction that took place in the dried samples. Freeze drying proved to be superior to the other drying methods, as shown by a best retention of the anthocyanins and antioxidant capacity (ORAC) of the fruit. Refractance Window dried açaí also exhibited a good retention of anthocyanin content similar to that of freeze-dried samples and an ORAC value higher than that of the hot-air dried powders. Anthocyanin content and antioxidant capacity, within each treatment, did not change over the 3 months of study