Based on a model of a suspension of interacting particles in a pseudoplastic solvent, a rheological equation was developed which could describe the flow properties of concentrated orange juice. The function was obtained by modifying Casson’s equation and has the following form: The magnitude of the parameter ko is affected by the concentration of the suspended particles, the concentration of soluble pectin and by conditions favorable for pectin gel formation. The other two parameters, k and m, are determined mainly by the solvent properties. The mechanism responsible for the flow behaviour of the concentrated orange juice under various conditions could be predicted and explained by the properties of the components in the system.

Orange juice is a suspension of heterogeneous particles in a clear serum. The size of the particles varies between 0·05 μm and a few hundred μm. The particles with a size below 2 μm constitute the stable ‘cloud’. This fraction consists of needle‐like crystals of hesperidin, chromoplastids, amorphous (rag) particles and oil globules attached to some of these particles. The adsorption of oil globules on the rag particles enhances their stability in suspension, by decreasing their density. The hesperidin crystals are formed partly by crystallisation immediately after juice extraction. All the cloud particles exhibit a negative charge, which decreases with decreasing pH. However, it seems that hydration rather than electrical charge is responsible for the stabilisation of the cloud. Heat treatment of the juice causes an increase in the number of fine particles at the expense of coarser ones. In this process some extraction of pectin into the serum also takes place but this has little significance on the cloudiness and cloud stability of the juice.

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Orange pectinesterase was isolated by ammonium sulfate fractionation followed by ion exchange on CM-Sephadex column. Two pectinesterase peaks were observed, The same pH and temperature optima (pH 7.5, temp 6O"C), almost equal Michaelis constants (0.029%, 0.031%) and energy of activation values (5,600 cal mol-', 5,740 cal mol-I) were obtained for the two pectinesterase fractions. Both enzyme fractions were inhibited by EDTA and polygalacturonic acid and were found to cause clarification of orange juice in a pattern similar to that obtained with natural juice. It can be assumed that the two pectinesterase chromatographic entities have similar catalytic properties.

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