The water vapour permeability constants o f three flexible films (LDPE, PET and a laminate of both films) were determined at 20,30 and 40 "C and from 55 t o 90% relative humidities. The relationship between permeability and temperature followed the Arrhenius model for the three films. The relative humidity also influenced the permeability o f the films. A mathematical model describing permeance (P/Xthe permeability o f laminates or films) as a function of external relative humidity and temperature was developed. The model can be used t o predict the permeance o f the three films a t different temperatures and relative humidities.
Lemon juice at concentrations of Y, 20", 30", 40" and 5O"Brix was stored at lo", 20" and 36°C for 16 weeks and sampled regularly for total soluble solids (TSS), pH, titratable acidity and ascorbic acid. No signifcant differences were found in the first two of these factors as a function of storage time. There was a small but signifcant decrease in citric acid concentration over 16 weeks. Ascorbic acid loss was greater at higher temperatures; at a constant temperature, the loss was smaller as TSS increased. Ascorbic acid degradation data fitted zero-, first-and second-order models equally well at all five TSS. Rate constants in 9"Brix juice were signgcantly higher than those for the other four concentrations at all three temperatures. E, values of 47.8 and 24.1 W mol-' were calculated for ascorbic acid degradation in 9" and 20"Brix juices. The effect of temperature far outweighed the effect of TSS on ascorbic acid degradation. Over the 16-week storage period, maximum retention of ascorbic acid (95.7%) was obtained in the 50"Brix lemon juice concentrate stored at 10°C.
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