Ultrasonic Processing for Dairy Applications: Recent Advances

The application of ultrasound to conventional dairy processes has the potential to provide significant benefits for the dairy industry such as energy savings and improved product properties. In recent years, the physical and chemical effects of high-intensity ultrasound in liquid and solid media have been extensively studied. Specific dairy processing applications such as emulsification, crystallisation, inactivation of microbes, functionality modifications and fat separation that harness the physical forces of ultrasound are highlighted in the present review

Effect of concentration, pH and added chelating agents on the colloidal properties of heated reconstituted skim milk

The thermal processing of milk changes the composition and surface properties of the colloidal particles present and alters the physical properties of the milk. Whilst some changes such as those used to improve the texture of products such as yoghurt and are desirable, others such as gel formation during the manufacture of Ultra-High Temperature milk are highly undesirable. This work aims to characterize the effects of milk composition and pH on the chemical and physical changes that occur when milk is heated in order to understand and control the effect of thermal processing on the functional properties of the milk. Particularly important are: (i) the changes to the integrity of the casein micelles and the extent to which they are reversible on cooling of the heated milk, (ii) the changes to the speciation of the components of the serum as they re-equilibrate in response to the changed environment during heating and on cooling, (iii) the heat-induced denaturation of the whey proteins, (iv) the interaction between the components of the micelles and those in the milk serum, particularly those interactions that lead to aggregation or other changes that affect the functional properties of the milk on heating. This project includes thermal treatment (90°C/10 min) of control skim milk solutions (9% Milk Solids Non Fat) with or without addition of calcium chelating agents (orthophosphate (Pin) & Ethylenediaminetetraacetic acid (EDTA)) and concentrated skim milk solutions (up to 21% MSNF). The pH range chosen was 6.2 to 7.2. Almost all of the studies on heat stability to date have been carried out by heating the milk and determining the changes that have occurred after the milk is cooled. This project is focussed on the direct measurements in real time of the changes that occur at the exact temperature. The experimental techniques included pH, calcium activity and 31P NMR measurements at high temperatures to investigate the consequences to the change in mineral speciation, Size Exclusion Chromatography in combination with SDS-PAGE analysis for protein speciation during heating and Diffusing Wave Spectroscopy and viscosity measurements to determine the heat stability of milk systems. pH and calcium activity decreased with increase in temperature for all the milk systems studied. These changes were largely reversible as enough time was given for equilibration. pH and calcium activity changes during heating are a function of milk composition. The quantity, size and the composition of the protein aggregates present in the serum phase after mild centrifugation (~33,000g) of heated (90°C/10min) milk solutions were found to be a function of pH and milk composition (including the consequent differences in speciation of the components of milk). DWS and the viscosity measurements showed that pH at the temperature of heating is one of the primary determinants in influencing the aggregation of the proteins, which led to thermal stability of milk systems. Hence, changing the milk composition resulted in differences in pH at the temperature of heating, which led to different behaviours of heat stability of milk systems. Careful control of the composition of milk and thereby the pH at the temperature of heating allows a greater control of thermal stability of milk systems

Lactose crystallization as affected by presence of lactic acid and calcium in model lactose systems

Acid whey is considered a hard to process stream mainly due to failure of lactose to crystallize, apparently attributed to the presence of lactic acid (LA) and calcium (Ca). Understanding the physical state and the thermal behaviour of lactose during crystallization in the presence of LA and Ca is important in order to develop adequate strategies to improve processability of acid whey and thus was the objective of this study. Presence of high concentration of LA (1% w/w) resulted in a decline in lactose crystal yields (∼59%). This was alleviated by lowering the LA content (0.2% w/w) which improved the yield (∼77%). At high LA content, small amounts of Ca (0.072% w/w) appeared to diminish the negative impact of LA presence by increasing the yield. Pure L solution yielded lactose particles of ∼84 μm. Crystal size slightly increased to ∼91 μm in the system containing high concentrations of LA along with low Ca concentration and although the presence of low concentrations of both LA (0.2% w/w) and Ca (0.072% w/w) resulted ∼ 163 μm sized crystals. Water molecules in the hydration layer around lactose molecules appeared to play a significant role governing the crystallisation behaviour and characteristics of lactose crystals in the presence of LA and Ca and/or in combination. Hence, the removal of some Ca to concentrations ≤0.072% w/w from acid whey may improve the crystallisation of lactose and thereby improve the processability of acid whey

Effect of concentration, pH and added chelating agents on the colloidal properties of heated reconstituted skim milk

The thermal processing of milk changes the composition and surface properties of the colloidal particles present and alters the physical properties of the milk. Whilst some changes such as those used to improve the texture of products such as yoghurt and are desirable, others such as gel formation during the manufacture of Ultra-High Temperature milk are highly undesirable. This work aims to characterize the effects of milk composition and pH on the chemical and physical changes that occur when milk is heated in order to understand and control the effect of thermal processing on the functional properties of the milk. Particularly important are: (i)the changes to the integrity of the casein micelles and the extent to which they are reversible on cooling of the heated milk, (ii)the changes to the speciation of the components of the serum as they re-equilibrate in response to the changed environment during heating and on cooling, (iii)the heat-induced denaturation of the whey proteins, (iv)the interaction between the components of the micelles and those in the milk serum, particularly those interactions that lead to aggregation or other changes that affect the functional properties of the milk on heating. This project includes thermal treatment (90°C/10 min) of control skim milk solutions (9% Milk Solids Non Fat) with or without addition of calcium chelating agents (orthophosphate (Pin) & Ethylenediaminetetraacetic acid (EDTA)) and concentrated skim milk solutions (up to 21% MSNF). The pH range chosen was 6.2 to 7.2. Almost all of the studies on heat stability to date have been carried out by heating the milk and determining the changes that have occurred after the milk is cooled. This project is focussed on the direct measurements in real time of the changes that occur at the exact temperature. The experimental techniques included pH, calcium activity and 31P NMR measurements at high temperatures to investigate the consequences to the change in mineral speciation, Size Exclusion Chromatography in combination with SDS-PAGE analysis for protein speciation during heating and Diffusing Wave Spectroscopy and viscosity measurements to determine the heat stability of milk systems. pH and calcium activity decreased with increase in temperature for all the milk systems studied. These changes were largely reversible as enough time was given for equilibration. pH and calcium activity changes during heating are a function of milk composition. The quantity, size and the composition of the protein aggregates present in the serum phase after mild centrifugation (~33,000g) of heated (90°C/10min) milk solutions were found to be a function of pH and milk composition (including the consequent differences in speciation of the components of milk). DWS and the viscosity measurements showed that pH at the temperature of heating is one of the primary determinants in influencing the aggregation of the proteins, which led to thermal stability of milk systems. Hence, changing the milk composition resulted in differences in pH at the temperature of heating, which led to different behaviours of heat stability of milk systems. Careful control of the composition of milk and thereby the pH at the temperature of heating allows a greater control of thermal stability of milk systems

Water-lactose behavior as a function of concentration and presence of lactic acid in lactose model systems

The presence of high amounts of lactic acid in acid whey restricts its ability to be further processed because lactose appears to remain in its amorphous form. A systematic study is lacking in this regard especially during the concentration step. Hence, the main aim of the study was to establish the structure and behavior of water molecules surrounding lactose in the presence of 1% (wt/wt) lactic acid at a concentration up to 50% (wt/wt). Furthermore, the crystallization nature of freeze-dried lactose with or without lactic acid was established using differential scanning calorimetry and Fourier transform infrared spectroscopy. Two mechanisms were proposed to describe the behavior of water molecules around lactose molecules during the concentration of pure lactose and lactose solutions with lactic acid. Pure lactose solution exhibited a water evaporation enthalpy of ~679 J·g-1, whereas lactose + lactic acid solution resulted in ~965 J·g-1 at a 50% (wt/wt) concentration. This indicates a greater energy requirement for water removal around lactose in the presence of lactic acid. Higher crystallization temperatures were observed with the presence of lactic acid, indicating a delay in crystallization. Furthermore, less crystalline lactose (~12%) was obtained in the presence of lactic acid, indicating high amorphous nature compared with pure lactose where ~50% crystallinity was obtained. The Fourier transform infrared spectra revealed that the strong hydration layer consisting lactic acid and H3O+ ions surrounded lactose molecules via strong H bonds, which restricted water mobility, induced a change in structure of lactose, or both, creating unfavorable conditions for lactose crystallization. Thus, partial or complete removal of lactic acid from acid whey may be the first step toward improving the ability of acid whey to be processed

Heat stability and acid gelation properties of calcium-enriched reconstituted skim milk affected by ultrasonication

The aggregation of proteins after heating of calcium-fortified milks has been an ongoing problem in the dairy industry. This undesirable effect restricts the manufacture of calcium rich dairy products. To overcome this problem, a completely new approach in controlling the heat stability of dairy protein solutions, developed in our lab, has been employed. In this approach, high intensity, low frequency ultrasound is applied for a very short duration after a pre-heating step at ©1/470 °C. The ultrasound breaks apart whey/whey and whey/casein aggregates through the process of acoustic cavitation. Protein aggregates do not reform on subsequent post-heating, thereby making the systems heat stable. In this paper, the acid gelation properties of ultrasonicated calcium-enriched skim milks have also been investigated. It is shown that ultrasonication alone does not change the gelation properties significantly whereas a sequence of preheating (72 °C/1 min) followed by ultrasonication leads to decreased gelation times, decreased gel syneresis and increased skim milk viscosity in comparison to heating alone. Overall, ultrasonication has the potential to provide calcium-fortified dairy products with increased heat stability. However, enhanced gelation properties can only be achieved when ultrasonication is completed in conjunction with heating

Sonication of milk protein solutions prior to spray drying and the subsequent effects on powders during storage

Solutions of 10 wt% solids, reconstituted from three types of dairy powders (milk protein concentrate, calcium caseinate and whey protein concentrate) were sonicated prior to lab scale spray drying at inlet and outlet temperatures of 180 °C and 80 °C respectively. The aim was to investigate the effects of sonication on the surface composition and morphology of the resulting powders. The effects of storage on the functional behaviour of these powders were also observed over 60 days at 25 °C, under two different humidity conditions (23.1% and 75.3%). No significant changes to the surface composition (fat, protein and lactose) of the sonicated and non-sonicated powder samples occurred during storage. The microstructure of non-sonicated powder samples showed the appearance of particle agglomerates upon storage, whereas this was not observed with sonicated samples. Importantly, it is shown that sonication prior to drying can increase powder stability during storage and delay the loss of powder solubility. When powders are reconstituted, the increase in solution viscosity that is normally associated with long term storage is also slowed. This is beneficial to industry where powder stability is important for reconstitution and use in the manufacture of secondary dairy products