The diluted fruit juice can be taken as Newtonian over a wide shear rate range. The concentrated juice or pulp, or the not so concentrated juice with added thickening agent, should follow a shear-thinning behaviour, with the apparent viscosity decreasing for with shear rate, except for either sufficiently high or sufficiently low shear rate, where the apparent viscosity becomes fairly independent of the shear rate. Alginate is often added to commercial juice based food products as thickener. Alginate gelling tends to be favoured by decreasing the temperature ― as seemingly occurred from 37 ºC to room temperature ― and by lowering the pH. Monovalent cations contributing for ionic strength, and/or higher pH or temperature, would tend to favour its thickening role. Divalent cations tend to expand the temperature gap between the gel-sol (higher) and sol-gel (lower) transition temperature, hence stabilizing the gel once it is formed ― nearly thermo-irreversible gels may form, for sufficiently high ion concentration. Aging at 37 ºC may possibly enhance the gelling trend of alginated thickened fruit juices by lowering their pH. It could be possible to counteract this effect by lowering the divalent cations content, e.g., by ion-exchange between Ca2+ and Na+. Syneresis, meaning water expulsion from recently formed gel flocs, may possibly help to explain the noted inhomogeneity. Such a syneresis effect, which results in deswelling of prior swelled gel flocs, hence reducing their size while increasing its density, often tends to be favoured by moderately increasing the temperature. Eventually, this effect may promote flocculation. 

Viscosity can also be reduced due to any residual enzyme activity or the pectin which is also responsible for its viscosity may degrade due to non-enzymatic reaction like acid hydrolysis and beta elimination.

The    very important  factor is  the  any  residual  enzyme  activity.