Plums freezing

State Diagram for Freeze-Dried Plum and Glass Transitions of Plum Skin and Pulp... 

The plum is a worldwide-distributed fruit and can be consumed either fresh or dried, and also as an ingredient of many prepared foods. Studies on drying kinetics (Sabarez et al., 1997 Cabas et al., 2002) and sorption isotherms (Cabas et al., 2000) of plums may be found in literature, but no works were found concerning freezing or freeze-drying processes. 

The objective of this work was to study the state transitions of freeze-dried plum and to draw the corresponding state diagram for this material. Class transitions of separated plum skin and pulp were also investigated, aiming to clarify the effect of each of these fractions on the behavior of the whole fruit. 

Black Diamond plums Prunus domestica) were obtained at the local market. Analysis of the fruits resulted in 16.65 0.89 g of total sugars/100 g moist sample, including 5.20 0.24 g of reducing sugars/100 g moist sample, and 6.21 0.44 g of pectin/100 g dry matter. The fruits had their stone removed and were cut in small pieces, being immediately frozen by immersion in liquid nitrogen and freeze-dried in a Heto HDl (Heto Lab Equipment). The separated skin and pulp of some fruits were also frozen and freeze-dried. 

The results of DSC analyses of freeze-dried plum (skin and pulp at the natural proportion) presented different behaviors for each domain. At Uy, 0.75, two glass transitions (Tg) were visible (Figure 58.1a) as a deviation in base line and shifted toward lower temperatures with increasing moisture content and caused by the plasticizing effect of water (Slade and Levine, 1991). The first one, clearly visible at lower temperatures, was attributed to the glass transition of a matrix formed by sugars and water. The second one, less visible and less plasticized by water, was probably caused by macromolecules of the fruit pulp. Two Tg are normally visible in systems formed by blends of polymers (Verghoogt et al., 1994) and in edible films (Sobral et al., 2002) caused by phase separation between polymers and between proteins and plasticizers, respectively. However, Sobral et al. (2001) and Telis and Sobral (2002) also observed two Tg for persimmon and tomato, respectively, at low domain. 

DSC traces of freeze-dried plum (skin/pulp at the natural proportion) at 0.11 0.90. 

DSC traces for fhe separated fractions of plum, as well as a trace for the mixture of skin and pulp at the natural proportion, freeze-dried and without subsequent conditioning, are shown in Figure 58.2. The thermogram of pure skin presented only an endothermic peak, without second-order transitions. However, thermograms of pure pulp showed the two glass transitions already observed in samples of pulp and skin at the natural proportion. These results eliminated the possibility that the second glass transition observed at higher temperatures in samples of whole fruit could be caused by components of plum skin. 

A state diagram for freeze-dried plum was obtained and the Gordon-Taylor model could adequately represent the sugar matrix glass-transition curve. 

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