Bound Water in Fruit Products by the Freezing Method

SUMMARY— An insulated heat‐sink containing Freon‐12® (N.B.P. −21.6°F) provides a reproducible system for measurement of thermal properties of fruit products. The difference in time required to remove the latent heat of fusion of the “eutectic” mixture in comparison with distilled water measures the “bound” water. The 34‐37%“bound” water in papaya (var. solo) pulp is unaffected by varying pH in the range 3.0–6.0. Less than 10% water is bound in guava, passion fruit and pineapple juice products with up to 35% sucrose added. Thermal conductivity of the solid phase “eutectic” mixture in the 5–50% soluble solids range fits the regression Y = AB x , Y = (3.69) (0.96) x where x = percent total soluble solids (TSS) and Y = thermal conductivity (Cal x 10 3 /(cm 2 ) (sec) (°C/cm)). The relationship of freezing point to TSS values as 1.10 + 0.64 (TSS) in the test system is significant (P < 0.01) in the range 5–50% TSS. The time for fusion (θ w ) of sucrose/water in the range 5–50% TSS fits the regression: log θ w = (−0.8325) (−0.0165) (TSS) or θ w = (0.984) (0.164) TSS sec. Addition of solutes such as sucrose will inhibit gelation without affecting “bound” water values. Pectin gel structures apparently are dependent on secondary bonding and independent of “bound” water per se.