Apparent Viscosity of Milk and Cultured Yogurt Thermally Treated by UHT and Vat Systems

Abstract

Apparent viscosity of ultra-high-temperature (UHT) treated milk and of yogurt prepared from this milk was studied and compared to that of vat-treated milk and yogurt. UHT-treated milks (149 C, 3.3 sec) had an apparent viscosity of 2.3 to 2.7 cp, while the apparent viscosity of vat-treated milks (63 C and 82 C, 30 min) ranged from 1.9 to 2.0 cp. The apparent viscosity of unheated (raw) milk was 1.7 cp. The apparent viscosity of yogurt prepared from UHT-treated milk became nearly constant at 0.8 cp after 14 min of shearing, while the apparent viscosity of yogurts prepared from vat-treated milks ranged from 1.8 to 3.8 cp under the same conditions. All yogurts exhibited thixotrophic behavior.

Fermented milks and their future trends. Part II. Technological aspects

“…It can be observed that the physical properties of set yogurt only have been examined. In practice, the coagulum of stirred yogurt is broken mechanically before cooling and before packaging, but in all of these studies the coagulum of set yogurt was broken mechanically after refrigeration, which is contrary to the normal practice; hence, the physical properties of stirred yogurt, and in particular the thixotropic behaviour of the product, may not be similar to those reported by Labropoulos et al (1981Labropoulos et al ( , 1984, Parnell-Clunies et al (1986a, b) and Schmidt et al (1985). The method adopted for the production of yogurt by the last authors could be inconvenient for industrial application, because the production time is extended by one day.…”

“…heating milk at 98 °C for 1-87 min, represents the best process for industrial production. Schmidt et al (1985) prepared yogurt as described in Table 3, and they concluded that the effects of heating on yogurt texture were that the conventional yogurt exhibited higher firmness, viscosity and more syneresis that UHT yogurt; improved firmness and consistency of UHT yogurt was achieved at the longer holding time; and heating the milk at 138 °C rather than 149 °C was not, as reported by Labropoulos et al 1981Labropoulos et al , 1984, as destructive to the yogurt texture.…”

“…Heating of the milk increases the hydrophilic properties of the coagulum and the stability of the yogurt gel. Labropoulos et al (1981Labropoulos et al ( , 1984 concluded that the physical properties of yogurt manufactured from milk heated to 82 °C for 30 min, compared with 149 °C for 3-3 s, were best, and that the latter treatment is suitable only for the production of drinking yogurt or yogurt with thin consistency or low curd firmness. Similar observations were reported by Parnell-Clunies et al (1986a, 6) and Parnell-Clunies & Kakuda (1986) for whole milk heated at 85 °C for 10-40 min (vat process), at 98 °C for 0-5-1-87 min (high temperature short time -HTST) and at 140 °C for 2-8 s (ultra high temperature -UHT) for the manufacture of yogurt.…”

“…However, they concluded that: (a) yogurt firmness and viscosity was increased with Table 3. Milk processing for the manufacture of yogurt Labropoulos et al (1981Labropoulos et al ( , 1984 Raw whole milk the vat process v. HTST and UHT treatments, but the highest water holding capacity of the coagulum was observed with the HTST process followed by the UHT and vat treatments; (6) although structural changes of the yogurt coagulum are induced by heat, other parameters, i.e. hydrophobicity, denaturation of individual whey proteins rather than total denaturation and changes in disulphide and sulphydryl groups should not be overlooked; (c) yogurt made by the vat process exhibited syneresis and a grainy texture, and the application of UHT could be limited by the weak texture of the coagulum; (d) the HTST process, i.e.…”

Application of Renneted Skim Milk as a Fat Mimetics in Nonfat Yoghurt

“…Yoghurt is a non-Newtonian substance [31,32] and yoghurt viscosity is an indication of a network of casein-particle aggregation leading to gelation [33]. In the present study the apparent viscosity of curd yoghurt was measured at various spindle speeds (shear rate) and shear times to determine rheological characterization of yoghurt [23].…”