Development of technology for the production of stable high moisture dried fruit

[摘要] ENGLISH ABSTRACT:Dried fruit is a well-known food product that has been produced for many years. Theproduct characteristics have remained constant throughout this time with a moisturecontent of ca. 18 - 26% (mlm). However, in recent times there has been a definitetrend towards a fmal dried fruit product in the intermediate moisture range with amoisture content of ca. 36%. The high water activity (aw) of the product (ca. 0.85)makes the product susceptible to microbiological spoilage and the product thereforerequires a subsequent pasteurisation step to ensure a safe product. A furtherconsequence of the increased moisture content, higher aw and the temperature of thepasteurisation step, is the increased rate of non-enzymatic browning reactions.Currently the shelf life is only 15 weeks while a minimum shelf life of 30 weeks isrequired to enable product export.Moisture sorption isotherms may be used to depict the relationship betweenmoisture content and aw. Moisture sorption isotherms were thus determined for Royaltype apricots and nectarines at 25° and 40°C. Samples, equilibrated at relativehumidities in the range of 11-97%, were obtained using saturated salt solutions and astatic gravimetric method. Isotherms were found to be of type I, typical of dried fruit.Six mathematical models namely; BET, GAB, Iglesias and Chirife, Halsey,Henderson, and Chung and Pfost, were fitted to experimental data. The GAB modelpredicted the aw of both apricots and nectarines the best at 25° and 40°C with theHenderson equation second best in all instances. The binding energy, as a function ofmoisture content, was calculated to determine energy requirements for drying. At lowmoisture contents « 20%) an increase of energy was required for drying.Discolouration of macerated dried Royal type apricots and nectarines duringaccelerated storage as affected by moisture (32, 36, and 40%, mlm) and sulphurdioxide (S02) content (2500, 3000 and 3300 mg.kg for apricots; 1800, 2200 and2600 mg.kg for nectarines) was investigated. The macerated samples were stored at30°, 40°, 50° and 60°C. Colour was quantified in terms of the L* value of theCIELab system (used throughout the study). Moisture and S02 contents affected boththe initial fruit colour and the rate of discolouration. The highest L* values, i.e.lightest fruit colour, were obtained for fruit at 40% moisture content and the highestS02 levels. Increasing storage temperature accelerated the loss of moisture and S02.The influence of a 10°C increase in storage temperature on the rate of browning andthus shelf life was described in terms of the QIO value. QIO and aw values of apricotsranged from 1.96 - 2.47 and 0.833 - 0.890, respectively, while QIO values of 1.50 -4.61 and aw values ofO.844 to 0.890 were obtained for nectarines.Discolouration of dried nectarine halves during accelerated storage at 40°C asaffected by rehydration method, moisture content, packaging atmosphere andpasteurisation method, was investigated. The fruit halves were rehydrated using threedifferent methods to obtain moisture contents of 36 and 40%, respectively. Dry heatand steam pasteurisation techniques were used to render a microbiologically safeproduct. Commercial packaging material was used and the atmosphere was modifiedwith CO2 to lower the O2 concentration in the headspace. A two-step rehydration at45°C, steam pasteurisation at 90°C for 150 minutes and packaging under a high CO2atmosphere rendered a product with the best colour retention under acceleratedstorage conditions of 40°C for eight weeks.To confirm the results obtained with accelerated storage at temperatures thatthe product would normally be retailed at, shelf life tests were also performed at 5°and 25°C. Discolouration of whole dried Royal type apricot and nectarine halves asaffected by rehydration method, moisture content, packaging atmosphere andpasteurisation method was investigated. The methodology for rehydrating,pasteurising and packaging the high moisture dried fruit developed in this study wascompared against the standard method used by the industry. The new processingmethod increased shelf life. Samples were stored for a period of 30 weeks and weretested every five weeks to determine CO2 concentration in headspace, colour retentionand S02 concentration of the fruit. Both apricots and nectarines achieved a shelf lifeof 30 weeks at both storage temperatures and an extrapolated shelf life of 89 weeks at5°C, but only 32 weeks at 25°C.

[发布机构] Stellenbosch University