Physicochemical properties of dehydrated apple pieces

Abstract

Royal Gala Apple pieces were dried utilising a number of dehydration regimes, including conventional hot air drying, freeze-drying, heat pump dehumidifier drying, modified atmosphere drying, and vacuum oven drying. The changes in physicochemical properties, such as colour, cell wall composition, cell wall polysaccharide mobility, rehydration parameters, and structure, were studied. Hunter L, a, and b values were recorded for the various dried apple samples, and browning was calculated as hue angle. Apple pieces were dried in three separate experiments, i) temperature of the drying medium was varied for air drying, ii) temperature of the drying medium was varied for modified atmosphere drying (nitrogen), and iii) oxygen concentration was varied whilst drying at 50°C. The experiments showed that for air drying, temperatures between 55 and 65°C resulted in the lowest level of browning, and that drying at a temperature of 75°C resulted in oxygen dependent non-enzymic browning. Modified atmosphere dehydration resulted in significantly less browning for each of the temperatures studied. This type of drying did not result in significant browning at 75°C, as hot air drying did. The oxygen concentration threshold where browning reactions begin to occur is around 0.3%v/v. The cell walls of fresh and dried apple pieces were isolated, fractionated, and studied by gas chromatography of alditol acetates, gas chromatography-mass spectrum of per- trimethylsilylated methyl glycosides, and colorimetric analyses. Cell wall changes which occur during drying are only slightly reversible upon rehydration. Dried products had cell walls which displayed very different fractionation characteristics to fresh tissue. Sugar residues normally associated with pectic polysaccharides were not. extracted as early in the fractionation sequence for dried products as they were for fresh tissue. The product dried by modified atmosphere dehydration demonstrated cell wall fractionation characteristics most similar to the fresh apple tissue. Thorough solid-state L’C NMR investigation was carried out, studying dried apple pieces, cell walls isolated from both fresh and dried apple pieces, and cell walls isolated from dried apple pieces after storage. The polysaccharides of the cell wall undergo hornification during dehydration. Directly after drying, and during storage, samples with higher water contents (13% moisture) undergo more changes than samples dried to very low water contents (3% moisture). Storage of dried apple pieces at higher water contents (13% moisture) allows water, which is relatively loosely bound compared to the more thoroughly dried product, to migrate in an attempt to reestablish equilibrium. Scanning Electron Microscopy was used to study the internal and external structure of the dried apple pieces. Interestingly, most apple pieces demonstrated a very good correlation between initial rehydration and particle density. Freeze-dried pieces did not show this correlation. The pore sizes of freeze-dried apple pieces were by far the largest {ca. 250pm) and this was thought to approximate a size where the surface tension of the rehydration solution barely allowed passage of moisture into the apple piece. The pores of the apple pieces dried by the remaining methods were smaller, trapping air, and acting as a barrier to the passage of moisture. Modified atmosphere dehydration resulted in a very good all round product. The product displayed very limited browning, few cell wall changes, high open pore porosity, and reasonable rehydration rate and efficiency. The industrial applicability of this technique is, however, likely to be quite limited due to the expenses involved.