Interaction between added whey protein ingredients and native milk components in non-fat acidified model systems

Abstract

Non-fat acidified milk model systems were constructed from frozen casein and whey protein concentrates produced from skim milk using membrane filtration, and combined with commercial whey protein ingredients, i.e. nano-particulated whey protein (NWP), micro-particulated whey protein (MWP), and whey protein concentrate (WPC). Model systems were characterised in terms of particle size distribution and fractionation, surface hydrophobicity and accessible thiol groups, rheological behaviour, water holding capacity and graininess. Samples containing NWP exhibited higher surface hydrophobicity and increase in accessible thiol groups, shorter gelation time, higher gelation pH and G′, but increasing particle size and number of grains, when compared with addition of MWP and WPC. Addition of MWP resulted in weak gels with a less connected protein network and decreased number of grains. Mixtures of NWP and MWP (1:1) had rheological properties closer to those seen for MWP. Systems with WPC differentiated themselves with a large quantity of small aggregates.

Introduction

With the growing interest in high-protein, low-fat, low-sugar and clean-label dairy products, the application of milk protein ingredients has become a common practice, due to both favourable nutritional and functional properties (de Wit, 1998; Hau & Bovetto, 2001; Liu, Buldo, et al., 2016; Liu, Jæger, et al., 2016; Torres, Mutaf, Larsen, & Ipsen, 2016). Such ingredients can aid in improving taste and texture, and reduce production costs. However, when added into yoghurt, ingredients such as skim milk powder (SMP), micellar casein isolate (MCI), whey protein isolate (WPI) and whey protein concentrate (WPC) can result in products with powdery, excessive firmness, higher whey expulsion and a grainy texture (Famelart, Tomazewski, Piot, & Pezennec, 2004; Guzma'n-González, Morais, & Amigo, 2000). In this connection, industrially aggregated ingredients such as micro-particulated whey protein (MWP) or nano-particulated whey protein (NWP) have been shown to be beneficial in low-fat yoghurt by increasing creaminess and viscosity (Janhoj, Petersen, Frost, & Ipsen, 2006; Liu, Jæger, et al., 2016b, Liu, Buldo, et al., 2016a; Torres, Janhøj, Mikkelsen, & Ipsen, 2011). It should be noted, however, that the applied processing conditions of whey protein ingredients such as MWP markedly affect performance (Torres et al., 2011).

The ingredient NWP is normally produced by adjusting pH followed by heat treatment of whey protein (Bovetto, Schmitt, Beaulieu, Carlier, & Unterhaslberger, 2007; Liu et al., 2016a; Nicolai, Britten, & Schmitt, 2011) and exhibits an average particle size ≤1 μm. The particles of NWP have been shown to efficiently interact with caseins to increase particle size upon heating and acidification, through disulphide bonding and non-covalent interactions (e.g., hydrophobic), consequently increasing G′ values and whey retention (Andoyo, Guyomarc'h, Cauty, & Famelart, 2014; Liu et al., 2016a). Liu, Jæger, Nielsen, Ray, and Ipsen (2017) investigated the role of NWP in acidified milk model systems and found it had earlier self-association (at pH ≥ 5.5) with decreased electrostatic repulsion and enhanced hydrophobic interaction. Furthermore, Liu, Jæger, Nielsen, Ray, and Ipsen (2018) found that NWP was able to form similar gels, regardless of changes in the ionic environment (deionised water, whey protein permeate or dialysed against skimmed milk).

MWP is produced similarly to NWP, but the particle sizes range mainly from 1 to 10 μm. The particle size and properties of MWP largely depend on the different processing conditions applied (methods and materials/ingredients used), pH and calcium concentration (Ipsen, 2017). MWP has been used as a commercial texture enhancer in various dairy products (yoghurt, ice cream, sauces, dressing, desserts, etc.). When applied in low-fat stirred and set-style yoghurt, MWP can contribute to sensory creaminess (Janhoj et al., 2006) and improve the texture (Lobato-Callerosa, Martı́nez-Torrijosa, Sandoval-Castillaa, Perez-Orozco, & Vernon-Carter, 2004). MWP used as a fat substitute in low-fat yoghurt can provide a softer texture (Tamime, Kalab, Muir, & Barrantes, 1995), while addition of WPC could lead to textural characteristics similar to full-fat yoghurt (Sandoval-Castilla, Lobato-Calleros, Aguirre-Mandujano, & Vernon-Carter, 2004). Torres et al. (2018) found that MWP with the highest ratios of native β-lactoglobulin and α-lactalbumin among the different types of MWP powders investigated increased the texture of stirred low-fat yoghurt the most.

WPC is produced from retentate of ultrafiltrated whey, which is subsequently concentrated and usually spray-dried (Abd El-Salam, El-Shibiny, & Salem, 2009). The protein concentration of WPC varies from 35 to 85% (Westergaard, 2004). Enriching milk with WPC to improve the textural properties of yoghurt has been reported in numerous studies (Aziznia, Khosrowshahi, Madadlou, Rahimi, & Abbasi, 2009; Lucey, Munro, & Singh, 1999; Mahomud, Katsuno, & Nishizu, 2017; Sodini, Montella, & Tong, 2005; Zhao, Wang, Tian, & Mao, 2016).

The production of acidified milk products includes the processing steps homogenisation, pasteurisation, incubation/acidification, cooling, and storage, all of which will affect the quality of the products (Lee & Lucey, 2010). The influence of commercial whey protein ingredients (WPC, MWP, NWP, etc.) in acidified milk products has been elucidated to some degree. However, a comprehensive understanding of the interactions of these ingredients with the native proteins present in milk as a consequence of processing is still needed. Furthermore, knowledge on how this in turn will affect the texture and other quality parameters of acidified milk is required. Such in-depth fundamental understanding will enable better control of the chemistry governing the interactions between milk constituents and added milk protein ingredients and ensuring fermented dairy products with improved quality and stability. Previous work (i.e., Andoyo et al., 2014; Liu et al., 2016b, 2017) in this area has been done on model milk systems made from various types of powders. This, however, is far from optimal since there are known issues with rehydration of, e.g., micellar casein concentrates due to process-induced changes (da Silva, Ahrné, Ipsen, & Hougaard, 2018).

In the present study, we have therefore mimicked the processing of fermented dairy products in model systems produced from frozen casein and whey protein concentrates, and investigated the influence of addition of commercial milk derived protein ingredients, i.e., MWP, NWP, and WPC. Particle size distribution, hydrophobicity, thiol groups, rheological behaviour, water holding capacity, and graininess were analysed at different processing steps to clarify the types of bonding and molecular interactions between milk constituents and the added ingredients.