Rheological properties of dairy desserts prepared in an indirect UHT pilot plant

The Viscolab (Tetra Pak), an indirect ultra-high treatment (UHT) pilot plant, was used to prepare dairy desserts containing kappa-carrageenan, skimmed milk powder (SMP), adipate cross-linked acetyl-substituted waxy maize starch, sucrose and water. Effects of varying concentrations of carrageenan, milk powder and starch on the dessert rheology were studied, while the sucrose and water content were kept constant. The rheological measurements included both small amplitude oscillation tests and large deformation penetration tests. To investigate the influence of shear during the process, the scraped-surface heat exchangers of the UHT pilot plant were run at two rotation speeds. At a low rotation speed of 70 rpm, both the complex modulus and gelation temperature were found to increase with increasing carrageenan content and upon substitution of SMP by starch. These findings are in line with the previously reported exclusion effect of starch, demonstrated by the authors for lab-scale and sterilized dairy desserts. However, more intense shearing at 350 rpm mainly affects the starch granules, which results in a lower complex modulus and gelation temperature of dairy desserts containing high amounts of starch. Where starch granules seem to considerably contribute to small oscillatory deformation measurements, they appear to contribute little to the gel strength. Compared to earlier reported lab-scale experiments, the dessert preparation in an UHT pilot plant involves a more intense heat treatment. As a result, a more extensive whey protein denaturation and subsequent complexation with casein micelles is believed to contribute to the rheological properties of the UHT desserts.