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

Erwee, Anton (2003-12)

Thesis (MSc Food Sc )--Stellenbosch University, 2003.

Thesis

ENGLISH ABSTRACT: Dried fruit is a well-known food product that has been produced for many years. The product characteristics have remained constant throughout this time with a moisture content of ca. 18 - 26% (mlm). However, in recent times there has been a definite trend towards a fmal dried fruit product in the intermediate moisture range with a moisture 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 therefore requires a subsequent pasteurisation step to ensure a safe product. A further consequence of the increased moisture content, higher aw and the temperature of the pasteurisation 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 is required to enable product export. Moisture sorption isotherms may be used to depict the relationship between moisture content and aw. Moisture sorption isotherms were thus determined for Royal type apricots and nectarines at 25° and 40°C. Samples, equilibrated at relative humidities in the range of 11-97%, were obtained using saturated salt solutions and a static 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 model predicted the aw of both apricots and nectarines the best at 25° and 40°C with the Henderson equation second best in all instances. The binding energy, as a function of moisture content, was calculated to determine energy requirements for drying. At low moisture contents « 20%) an increase of energy was required for drying. Discolouration of macerated dried Royal type apricots and nectarines during accelerated storage as affected by moisture (32, 36, and 40%, mlm) and sulphur dioxide (S02) content (2500, 3000 and 3300 mg.kg" for apricots; 1800, 2200 and 2600 mg.kg" for nectarines) was investigated. The macerated samples were stored at 30°, 40°, 50° and 60°C. Colour was quantified in terms of the L* value of the CIELab system (used throughout the study). Moisture and S02 contents affected both the 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 highest S02 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 and thus shelf life was described in terms of the QIO value. QIO and aw values of apricots ranged 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 as affected by rehydration method, moisture content, packaging atmosphere and pasteurisation method, was investigated. The fruit halves were rehydrated using three different methods to obtain moisture contents of 36 and 40%, respectively. Dry heat and steam pasteurisation techniques were used to render a microbiologically safe product. Commercial packaging material was used and the atmosphere was modified with CO2 to lower the O2 concentration in the headspace. A two-step rehydration at 45°C, steam pasteurisation at 90°C for 150 minutes and packaging under a high CO2 atmosphere rendered a product with the best colour retention under accelerated storage conditions of 40°C for eight weeks. To confirm the results obtained with accelerated storage at temperatures that the 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 as affected by rehydration method, moisture content, packaging atmosphere and pasteurisation method was investigated. The methodology for rehydrating, pasteurising and packaging the high moisture dried fruit developed in this study was compared against the standard method used by the industry. The new processing method increased shelf life. Samples were stored for a period of 30 weeks and were tested every five weeks to determine CO2 concentration in headspace, colour retention and S02 concentration of the fruit. Both apricots and nectarines achieved a shelf life of 30 weeks at both storage temperatures and an extrapolated shelf life of 89 weeks at 5°C, but only 32 weeks at 25°C.

AFRIKAANSE OPSOMMING: Droë vrugte is 'n welbekende voedselproduk en word reeds vir baie jare vervaardig. Die produkeienskappe het konstant gebly gedurende hierdie tydperk met 'n produkvoginhoud van ca. 18 - 26% (mlm). Daar is egter 'n tendens die afgelope tyd na 'n finale produk in die intermediêre voggebied met 'n voginhoud van ca. 36% en 'n water aktiwiteit (aw) van ca. 0.85. Hierdie verandering in voginhoud en aw maak die produk vatbaar vir mikrobiologiese bederf, en gevolglik word pasteurisasie benodig om dit te preserveer. 'n Verdere gevolg van die verhoogde voginhoud en aw en die hoë temperature van pasteurisasie, is die verhoogde tempo van nieensiematiese verbruiningsreaksies. Huidig is die produk se rakleeftyd 15 weke terwyl 'n minimum van 30 weke benodig word om hierdie produk suksesvol uit te voer. Vogsorpsie-isoterme kan gebruik word om die verwantskap tussen voginhoud en aw uit te beeld. Vogsorpsie-isoterme van Royal tipe applekose en nektariens is gevolglik bepaal by 25° en 40°C. Monsters, geëkwilibreer by relatiewe humiditeite van 11 - 97%, is verkry deur gebruik te maak van versadigde soutoplossings en 'n statiese gravimetriese metode. Tipe I isoterme, wat tipies van droëvrugte is, is verkry. Ses wiskundige modelle naamlik; BET, GAB, Iglesias en Chirife, Halsey, Henderson, en Chung en Pfost, is gepas op die data. Die GAB model het die aw van beide appelkose en nektariens by 25° en 40°C die beste voorspel en die Henderson model die tweede beste in al die gevalle. Die bindingsenergie as 'n funksie van voginhoud is bereken om die energie vereistes van droging te bepaal. By lae voginhoude (<20%) is 'n skerp styging in benodigde energie waargeneem. Die verkleuring van gemaalde gedroogde Royal tipe appelkose en nektariens gedurende versnelde opberging en die invloed van voginhoud (32, 36, en 40%, mlm) en swaweldioksied (S02) konsentrasie (2500, 3000 en 3300 mg.kg' vir appelkose; 1800, 2200 en 2600 mg.kg" vir nektariens) is ondersoek. Die gemaalde monsters is gestoor by 30°, 40°, 50° en 60°C. Kleur is gekwantifiseer in terme van L* waardes van die CIELab sisteem (ook gebruik vir daaropvolgende ondersoeke). Vog en S02 het albei die aanvanklike kleur asook die tempo van verbruining beïnvloed. Die hoogste L* waardes, d.i. die ligste kleur, is verkry vir die monsters met 40% voginhoud en die hoogte S02 vlakke. Verhoogde temperature tydens opberging het aanleiding gegee tot verhoogde verliese van vog en S02. Die invloed van 'n 10°C verhoging in opbergingstemperatuur op die tempo van verbruining en dus rakleeftyd, word beskryf in terme van QIO waardes. QIO en aw waardes van die appelkose het gestrek van 1.96 - 2.47 en 0.833 - 0.890, onderskeidelik, terwyl QIO waardes van 1.50 - 4.61 en aw waardes van 0.844 tot 0.890 verkry is vir die nektariens. Verkleuring van gedroogde nektarien halwes gedurende versnelde opberging by 40°C en die invloed van rehidrasie metode, voginhoud, verpakkingsatmosfeer en pasteurisasiemetode is ondersoek. Die vrughalwes is gerehidreer deur middel van drie metodes om die voginhoud te verhoog tot 36 en 40%, onderskeidelik. Droë hitte en stoompasteurisasie metodes is gebruik om 'n mikrobiologiese veilige produk daar te stel. Kommersiële verpakkingsmateriaal is gebruik en die CO2 konsentrasie van die atmosfeer in die verpakking is verhoog om die invloed daarvan te bepaal. 'n Twee-stap-rehidrasie by 45°C, stoompasteurisasie by 90°C vir 150 minute en 'n hoë C02 atmofeer het aanleiding gegee tot die monster met die beste kleurbehoud tydens versnelde opberging by 40°C vir agt weke. Om die resultate, verkry met die versnelde rakleeftyd studie, te bevestig by temperature waarby die produk normaalweg blootgestel sal word tydens kleinhandel, is 'n rakleeftyd studie uitgevoer by 5° en 25°C. Verkleuring van heel gedroogde Royal tipe appelkoos en nektarien halwes, die invloed van rehidrasie metode, voginhoud, verpakkingsatmosfeer en pasteurisasiemetode is ondersoek. Die metodiek vir die rehidrasie, pasteurisasie en verpakking van hoë vog droëvrugte ontwikkel in hierdie studie, is getoets teen die standaardmetode wat deur die industrie gebruik word. Die nuwe prosesseringsmetode het aanleiding gegee tot 'n langer rakleeftyd. Monsters is opgeberg vir 'n tydperk van 30 weke om die rakleeftyd te bepaal. Die CO2 konsentrasie in die pakkie, kleurbehoud en S02 konsentrasie van die vrugte is elke vyf weke getoets. Beide appelkose en nektariens het 'n rakleeftyd van 30 weke by albei opbergingstemperature behaal, terwyl 'n ekstra-gepoleerde rakleeftyd van 89 weke by 5°C en 32 weke by 25°C behaal is.

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