The functionality of micellar casein produced from retentate caprine milk treated by HP

Highlights

• HP-treatment of MFRs decreased turbidity and size while increased pH, soluble Ca and P.

• The G'max and yield stress values of the gels made from MFR treated by HP decreased.

• HP have improved the properties of MCC powders, as solubility, foamability and emulsification.

• HP a good alternative technology to expand the application of food protein industry.

Abstract

The effect of high pressure (HP) treatment on the physicochemical, structural, and rheological characteristics of microfiltration retentate (MFR) caprine milk prior to spray drying was investigated. In addition, the functional and microstructure properties of micellar casein concentrate (MCC) were determined. Microfiltered/diafiltered milk protein retentates was treated at (25 ± 2) °C for 15 min with a different levels of pressure (100–500 MPa), and then spray dried. The results showed that the turbidity and casein size decreased significantly while pH, soluble calcium and phosphorus significantly increased with an increase in pressure level. Further, the structural characteristics of MFR for all samples were studied. There was an increase in hydrophobicity of HP-treated samples compared with the untreated. In contrast, the G'max and yield stress values of the HP-treated MFR gels decreased. The MFR sample treated with HP at 100 MPa had the highest average of the G'max (99.75 Pa), and the lowest value of rennet coagulation time (25.9 min). Moreover, the solubility, emulsifying and foamability of MCC samples increased significantly with HP-treated samples compared to control. Maximum percentages of solubility (80.3), foam capacity (47) and foam stability (79.8) were observed at MCC-400 MPa. However, MCC-400 MPa sample showed the lowest average turbiscan stability index at 1 and 24 h (4.9 and 7.3, respectively). Thus, HP pre-treatment could be used as a good alternative technology to promote MCC products' application in the food industry.

Introduction

In general, the structure of a protein is categorized as primary, secondary, tertiary, and quaternary structure depending on the arrangement of the polypeptide chains. Caseins, the major protein fraction in milk, consists of four gene products, α_s1-casein (α_s1-CN), α_s2-casein (α_s2-CN), β-casein (β-CN) and kappa-casein (ĸ-CN). Caseins in milk are associated with large colloidal aggregates, known as the casein micelles. The casein micelles are self-assembled caseins and the micelles are formed through the binding of calcium and phosphate in the Golgi vesicles (De Kruif and Holt, 2003). The size of casein micelles ranges from 80 nm to 500 nm in diameter and with an average size of around 200 nm in diameter (De Kruif, 1998).

Micellar casein concentrate (MCC) produced from microfiltration, which is a concentrated liquid colloidal suspension (microfiltration retentate) consists mainly of casein in micellar form, minerals, lactose and a minor amount of serum proteins. The percentage of serum protein removal ranges from 60 to 95% (w/w) based on the total serum proteins in the original skim milk (Beckman et al., 2010).

Information about the effect of processing (e.g., high pressure [HP]) on the casein micelles structure is of importance since the casein micelles are the main ingredients in many dairy products such as cheese and yogurt. It also has potential applications where concentrated milk proteins occur as ingredients such as in nutritional meal-replacement products, whipped topping, and coffee whiteners (Zhang et al., 2018b). Several authors reported that the micellar structure of caseins is known to be pressure sensitive which leads to dissociation and re-aggregation of these micelles (Huppertz et al., 2006a, 2006b; Knudsen and Skibsted, 2010; Orlien et al., 2006). It is well-established that the major effects of HP on casein micelles of milk are changes in the mineral balance (colloidal calcium phosphate is solubilized) (Huppertz and de Kruif, 2007; López-Fandiño, 2006; Nassar et al., 2019; Orlien et al., 2010) and reduction in the size of casein micelles (hydrophobic and electrostatic interaction between proteins are disrupted under pressure) (Gaucheron et al., 1997).

It is also established that HP affects the quaternary structure (e.g., through hydrophobic interactions), the tertiary structure (hydrophobic and hydrogen bonding) and the secondary structure (H- and electrostatic interactions), while the primary structure of proteins remains intact during HP treatment (Goyal et al., 2018). These effects strongly depend on the applied pressure, the pressurization time and temperature as well as the composition of samples.

To this day, compared with the numerous studies of structural changes induced by HP in bovine MCC (Baier et al., 2015a; Baier et al., 2015b; Cadesky et al., 2017; Udabage et al., 2012), there are little publications available on changes in caprine MCC structure induced by HP (Law et al., 1998). Therefore, the first aim of this work was to point out the physico-chemical, rheological, and structural properties changes in microfiltration retentate (MFR) caprine milk samples induced by HP-treatments.

On another hand, during dehydration, storage and reconstitution, the functionality of the proteins is retained. An issue related with MCC powders is their poor reconstitution properties, which several technological approaches such as chemical and physical methods have been used in an attempt to improve the solubility of MCC powders (Anema et al., 2006; Meena et al., 2017). Furthermore, the structural changes of proteins due to HP indicate an increase in surface hydrophobicity, which may alter the functional properties of the system such as the solubility, foaming, emulsifying and water binding capacity of the proteins (Altuner et al., 2006). Therefore, the second aim of this study was to investigate the solubility, emulsification, and other functional properties of MCC powders produced from MFR pre-treated by HP.