Elsevier

Journal of Food Engineering

Volume 124, March 2014, Pages 11-18
Journal of Food Engineering

Effect of power ultrasound pre-treatment on the physical and functional properties of reconstituted milk protein concentrate

https://doi.org/10.1016/j.jfoodeng.2013.09.013Get rights and content

Highlights

  • The ultrafiltered milk retentates were treated with PUS prior to spray drying.

  • We examine physical and functional properties changes of MPC for PUS pre-treatment.

  • The pH and conductivity of MPC reconstitution did not change significantly.

  • The functionality of MPC such as solubility increased for PUS pre-treatment.

Abstract

This work investigated the impact of power ultrasound (PUS) pretreatment on the physical and functional properties of reconstituted milk protein concentrate (MPC) 80. Ultrafiltered/diafiltered (UF/DF) milk protein retentates were treated with PUS (12.50 ± 0.31 W and 50% amplitude) for 0.5, 1, 2, and 5 min prior to spray drying.

Results revealed that the particle size (D50) reduced from 28.45 μm to 0.13 μm after 0.5 min of sonication. Solubility increased significantly from 35.78% to 88.30% after 5 min of PUS pre-treatment. Moreover, the emulsifying activity index (EAI) of MPC samples increased significantly as the time of ultrasonic treatment was prolonged. Additionally, the emulsion stability index (ESI) initially increased after ultrasound treatment for 1 min. Surface hydrophobicity was greatly increased with more hydrophobic groups exposed to the environment. PUS pre-treatment also promoted an increase in the storage modulus (G′) of the MPC solutions. Viscosity significantly decreased after PUS pre-treatment. This result was confirmed by the microstructure of the powder, with small particles formed and trapped in dents of large particles. However, sodium-dodecyl sulfate–polyacrylamide gel electrophoresis showed no significant change in protein molecular weight.

Introduction

Milk protein products are widely used valuable ingredients in the food and dairy industries; these products made a major contribution in the development of new food products. Traditional milk protein products are produced by acid or rennet precipitation, which involves the application of high heat or pH adjustments that denature the whey protein. Milk protein concentrate (MPC) powders are manufactured through ultrafiltration (UF), diafiltration (DF), and optional evaporation of the retentate prior to spray drying. These processes are relatively moderate without heat treatment and pH adjustment; hence, MPC contains undenatured casein and whey proteins. Compared with skim milk powders, MPCs have less lactose and higher amounts of protein. The casein/whey protein ratio of MPC is similar to that of skim milk powder. MPCs are crucial in the production of cheese, confectionary, yoghurt, and other food products. Generally, the protein powder needs to be dispersed and fully dissolved before it can be utilized. Therefore, prior dissolution of MPC in water must be as rapid as possible at room temperature with moderate agitation to minimize operating costs. However, MPC powders are poorly soluble because of their high protein content (40–90%), which restricts their applications. Several researchers have focused on different ways to improve the solubility of MPC powders in cold water. Examples include the addition of monovalent ions prior to spray drying (Carr et al., 2002), removal of calcium ions using a cation exchanger (Bhaskar et al., 2003, Dybing et al., 2003), acidification to lower pH followed by UF/DF or addition of a calcium chelating agent (Schuck et al., 2002, Bhaskar et al., 2003), and high shear treatment of the milk concentrates prior to spray drying (Augustin et al., 2012).

In contrast to low-intensity ultrasound (typically less than 1 W cm−2, with a frequency range of 5–10 MHz), high-intensity ultrasound (HIUS), which uses much higher power levels (typically in the range of 10–1000 W cm2, with a frequency range of 20–1000 kHz), causes physical disruption of the material and promotes certain chemical reactions (Mason, 1998). HIUS has been applied in many processes, including homogenization, cutting, drying, and extraction, inactivation of microbes and enzymes, degassing of liquid foods, nucleation, and even preparation of submicron emulsions (Bhaskaracharya et al., 2009, Camino et al., 2009, Knorr et al., 2004).

Several studies have focused on the effect of HIUS treatment on whey proteins (Wang et al., 2008, Jambrak et al., 2008, Guzey et al., 2006) and soy proteins (Jambrak et al., 2009). Augustin et al. (2012) examined the effect of various shear treatments (homogenization, microfluidization, or ultrasonication) of UF/DF concentrates on the solubility of MPC. Their work demonstrated that the solubility of MPC powder can be improved by ultrasonication.

To date, systematic studies on the effects of ultrasonication treatment on the functionality of MPCs are lacking. In the present work, skim milk retentate was treated with power ultrasound (PUS) at different durations to evaluate the influence of PUS pre-treatment on the solubility, emulsification, and other functional characteristics of MPC powder.

Section snippets

Materials

Raw cow’s milk was obtained from the dairy farm of the China Agriculture University. The raw milk was composed of 3.4% (w/w) protein, 4.5% (w/w) lactose, and 4.0% (w/w) fat (data collected from Beijing Dairy Cattle Center). The raw milk was kept at 4 °C until use.

Sample preparation

The fat content of the raw milk was removed by a disc bowl centrifuge (FT15, Armfield Company, UK). MPC was manufactured through UF of skim milk with DF. Pasteurized skim milk (60 kg) was heated to 50 °C and ultrafiltered to approximately

pH and conductivity

The pH and conductivity of the samples did not change significantly (p > 0.05) upon PUS pre-treatment with probe (Table 1).

Jambrak et al. (2008) observed no significant changes (p > 0.05) between the pH of the control and sonicated samples at 20 kHz. However, some researchers found that milk shows lower values of pH after ultrasound treatment (Bermudez et al., 2008). The decrease in pH in milk after sonication can be attributed to several reasons. Walstra et al. (2006) mentioned that a decrease in

Conclusions

The present study showed that PUS pre-treatment (20 kHz, 12.50 ± 0.31 W and 50% amplitude) can significantly improve the functional properties of MPC, such as solubility, emulsification, and gelation. The pH and conductivity of all samples did not change significantly (p > 0.05). The retentates presented shear-thinning behavior when the shear rate was in the range of 1–100 s−1, which assisted in the optimization of spray drying. These changes were closely related to the increase in surface

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