Whey proteins such as lactoferrin are high value components in milk; however, under UHT conditions they can denature and lose their functionality and bioactivity. The heat stability of these proteins in water suspensions was investigated at different pH conditions using a pilot scale UHT plant. An RP-HPLC method was used to quantify β-lactoglobulin, α-lactalbumin and lactoferrin simultaneously. In a pH 3.0–5.0 lactoferrin-in-water suspension, at least 48% of undenatured lactoferrin remained after UHT. The highest level of undenatured lactoferrin was 81%, observed at pH 4.0; the iron binding capacity for this sample was 78%. In addition, mixtures of whey protein concentrate and lactoferrin in a lactose solution were investigated, which showed 84% and 70% nativity for whey at pH 3.0 and pH 3.5, respectively, and 78% nativity for lactoferrin at pH 3.5. These results provide useful information in product research and development to maintain bioactive protein nativity through UHT processing.

This study aimed to determine the interactive effects of casein, whey proteins and calcium ions on the rheological properties of calcium-induced skim milk gels. Skim milk blends with different casein to whey protein ratios (95:5, 93:7, 90:10, 85:15 and 80:20) were either not preheated or preheated at 90 degrees C for 10 min, and calcium chloride was added at 0-40 mmol L-1. Results suggested that the final gel strength (G ') was dependent on the concentration of added calcium chloride, ratio of casein to whey proteins, and whether the whey proteins were denatured by preheat treatment. Denatured whey proteins in preheated skim milk blends appeared to lead to two competing effects on the gel strength: reduced gel strength due to competition with casein micelles for calcium ion binding; and enhanced gel strength by participating in the gel structure through aggregation via calcium-bridging, hydrophobic interactions, and disulphide bonding. (C) 2020 Elsevier Ltd. All rights reserved.

The effects of holding temperature and ionic strength on the physicochemical and rheological properties of calcium-added skim milk were investigated. In skim milk with 10-20 mmol L-1 of added calcium chloride, increasing the temperature from 20 to 53 degrees C decreased the calcium ion activity and pH, but increased the particle size. Increasing the holding temperature from 70 to 90 degrees C increased the final G' of skim milk gels with 10-40 mmol L-1 of added calcium chloride. No gelation was observed when calcium chloride was replaced with sodium chloride to the equivalent ionic strength (60-120 mmol L-1). Furthermore, adding sodium chloride in addition to calcium chloride resulted in lower final G'. The results showed that the gel strength of calcium-induced skim milk gels was influenced by holding temperature, and the effect of ionic strength was dependent on the type of salt added (Ca(2+ )or Na+). (C) 2020 Elsevier Ltd. All rights reserved.

This study investigated the effects of heating skim milk with soluble calcium salts, calcium chloride, calcium lactate, calcium gluconate and calcium lactobionate, on the physico-chemical and rheological properties of milk. Regardless of the type of salt added, the amount of casein in milk serum decreased and the amount of calcium in centrifuge sediment increased along with the serum calcium. The amount of calcium salt required for gelation and the gel firmness (G') varied depending on the salt. The aCa2+ order in milk was calcium chloride>calcium lactate>calcium gluconate>calcium lactobionate. With the same amount of added calcium salt, the final G' of milk gel followed the same order. The findings from this study suggest that the different association constants and formation of an intermediate calcium complex with hydroxycarboylate groups affected the calcium ion activity, which may influence the gelation properties in milk.

Skim milk with added starch (waxy rice starch or potato starch at levels of 0-1.5 g/100 g) was either pressure-treated (500 MPa, 20 degrees C, 30 min) or heat-treated (80 degrees C, 30 min) and subsequently acidified (using glucono-delta-lactone) to form acid milk gels. In the second part of the study, the pH of the skim milk samples was adjusted from the natural condition (pH 6.64) to pH 6.5, 6.6 or 6.9 before the pressure or heat treatment and re-adjusted back to pH 6.64 after the respective treatment. The rheological properties of the samples during acidification and of the final acid gels were studied. The storage modulus. G' of the final acid milk gels increased as more waxy rice starch was added to milk before pressure or heat treatment. However, acid milk gels made from pressure-treated milk with added potato starch did not show significant changes in the G' of the final acid gels whereas those made from the heat-treated counterparts showed a marked increase in the final G' as the potato starch level increased. Waxy rice starch was gelatinised in milk by both pressure treatment and heat treatment whereas potato starch was gelatinised by heat treatment only. Increasing the pH of milk before pressure or heat treatment increased the final G' of the acid milk gel produced on subsequent acidification of the milk and the final G' was increased further by the addition of waxy rice starch before the pressure or heat treatment. (C) 2010 Elsevier Ltd. All rights reserved.

The gelatinisation of starch in skim milk required higher pressures than did the gelatinisation of starch in water. This study examined the effects of various milk components on the pressure-induced gelatinisation of waxy rice starch and normal rice starch, in order to understand the differences between the gelatinisation characteristics of starch in skim milk and starch in water. Gelatinisation was retarded in skim milk, which was attributed to the effects of soluble milk minerals and lactose. The presence of these components may reduce the plasticising ability of the suspension medium. Direct interactions between the milk components and starch molecules may also contribute to retarded gelatinisation. Milk proteins (casein and whey protein) did not affect the degree of pressure-induced gelatinisation at the concentrations of these components in skim milk, at 10% total solids.

The rheological properties of acid skim milk gels, prepared from milk with added potato starch and pH adjusted (pH 6.5–7.1) prior to heat treatment and acidification, were investigated. The storage modulus, G′, of the final acid gels was increased by heating the milk at higher pH and further increased by adding starch. The effect of pH at heating and addition of starch appeared to be additive and independent of each other up to a starch addition level of 1%. Above this starch level, the pH at heating had a lesser effect. This may have been due to the increased viscosity of the aqueous phase as a result of starch gelatinization or to direct contributions of the starch to the gel network structure. Confocal microscopy showed that milk proteins developed fewer but broader protein clusters at higher pH than at lower pH. Starch addition resulted in an increased density of the protein network.

Potato starch was added to skim milk at levels of 0–1.5%. The milks were heated and then acidified to form acid milk gels. The properties of the milks during acidification and the final properties of the acid gels were studied. The addition of starch resulted in a higher storage modulus, G′, in the final acid gels, and increasing the level of starch caused a linear increase in the final G′. Compared with acid gels prepared with no starch, the gelation time was reduced and the gelation pH was increased. However, the temperature and frequency dependences of the acid gels were not affected by the addition of starch. Furthermore, the breaking strain of the acid gels was not markedly affected by the addition of starch, whereas the breaking stress was dependent on the level of starch added. Confocal microscopy showed that the acid gels contained swollen starch granules embedded in a protein network, and that the protein network increased in density as the level of starch added increased.

This study investigated the high-pressure-induced gelatinization of different starches, namely normal rice, waxy rice, normal corn, waxy corn, tapioca and potato starches. The high-pressure-treated starch solutions were characterized by pasting behaviour, degree of swelling and changes in birefringence. Potato starch was found to be less affected by pressure treatment than the other starches, as it retained birefringence after a pressure treatment of 600MPa for 30min. Waxy and tapioca starches showed complete gelatinization after the same treatment, whereas normal starches were only partially gelatinized. The pasting curves of the normal starches showed an increase in the initial viscosity after pressure treatment, whereas the initial viscosities of the waxy starches after the 600MPa for 30min treatment were already equal to their respective peak viscosities.