Effects of conventional processing methods on whey proteins in production of native whey powder

Abstract

Native whey, produced by microfiltration of skim milk, has great potential in infant nutrition and as a functional food ingredient; however, processes to produce whey powder using high temperatures can adversely affect protein quality. The individual effects of three critical processing steps were investigated: standard thermal pasteurisation, membrane concentration and spray drying on protein quality when performed under the mildest conditions considered feasible for industrial operation. HPLC-analysis was used to measure the degree of denaturation of the six most abundant whey proteins (β-lactoglobulin, α-lactalbumin, bovine serum albumin, immunoglobulins, lactoferrin and lactoperoxidase) before and after each processing step. Overall, denaturation was minimal throughout our tests, although the tendency to denature was unequal between the individual proteins and also varied between tested processing types and medium compositions. In the production of native whey with microfiltration, retentions varied greatly between the proteins but were not affected by pasteurisation of milk.

Introduction

The biological functions of milk during the neonate's growth are numerous and mostly mediated by proteins, or peptides released upon their digestion (Fox, 2008; O'Mahony & Fox, 2013) with established roles in being the source of amino acids, transferring nutritionally important ingredients, protecting against microorganisms, modulating the immune system and acting as releasing signal agents that affect many physiological systems (Korhonen, 2009; Pellegrino, Masotti, Cattaneo, Hogenboom, & de Noni, 2013; Sanghoon & Hae-Soo, 2009). More recently they have also been linked with growing number of health benefits (Korhonen, Pihlanto-Leppälä, Rantamäki, & Tupasela, 1998; Madureira, Pereira, Gomes, Pintado, & Malcata, 2007), especially for infants whose protein digestion is less efficient than adults due to higher gastric pH and lower protease activity (Perdijk, van Splunter, Savelkoul, Brugman, & van Neerven, 2018). Native whey proteins have also been proposed as a possible reason why consumption of raw milk correlates with lower risk of developing allergies, asthma and respiratory infections in children (Abbring, Hols, Garssen, & van Esch, 2019; Perdijk et al., 2018). Although health benefits may be less accessible for the adult digestive system, undenaturated whey proteins have proven their value in improving functional properties of many dairy products (Mulvihill & Donovan, 1987).

To perform their biological functions, proteins fold into their specific, three-dimensional form, which can be easily disturbed by changes in their environment leading to their denaturation and loss of the original biological activity (Denature, 2020; Haque & Adhikari, 2015). For the functionalities to reach a consumer, the proteins have to tolerate certain amount of processing, such as agitation and elevated temperatures; this poses a challenge for industrial applicability as several of these proteins are prone to heat denaturation at temperatures typically used in pasteurisation, evaporation and drying of dairy products. As an example, in their recent study, Brick et al. (2017) calculated that a typical pasteurisation of at 72 °C for 20 s was enough to reduce the number of identifiable proteins by approximately 20%.

As there are obvious health risks related to consumption of unpasteurised milk, an alternative approach on how to process and deliver bioactive proteins to consumers would be favourable. Milk microfiltrate, or native whey (also referred to as milk serum, serum protein and ideal whey), is a promising ingredient as it contains great number of native whey proteins with small quantity of caseins and has some unique properties compared to typical cheese whey in both nutritional and technological functionality (Gésan-Guiziou, 2015; Korhonen, 2009). In the production of native whey, the denaturation of the native proteins can be avoided using low temperatures, i.e., below 20 °C, which has the additional benefit of increased release of β-casein from the casein micelle, allowing it to permeate the microfiltration membrane together with whey proteins (Hekken & Holsinger, 2000). Therefore, native whey could be used to simultaneously supplement a dairy product with bioactive proteins and to adjust its casein composition. A prime example would be an infant formula with desired bioactivities from whey proteins and more human-like protein composition from increased β-casein content and near absence of bovine milks α_S2-caseins (Woychik, 1991), which has also been linked to better digestibility in infant nutrition (Nakai & Li-Chan, 1987). Whether or not the demands for digestibility and product safety could be met with such infant formula is open for debate, but perhaps nutritional benefits could be applied in products aimed at slightly older children.

The denaturation rate of whey proteins is known to be influenced by their surrounding media, especially pH, ionic strength and mineral composition, dry matter content and casein content (Anema, 2006; Kessler & Beyer, 1991). Under the processing conditions typical for milk or whey, the corresponding proteins easily denature and aggregate (Anema, 2009; Kessler & Beyer, 1991). Denaturation of whey proteins during heat treatment has been researched on many occasions, using different analytical methods and varying feeds, most often milk or sweet whey (Mulvihill & Donovan, 1987; Wijayanti, Bansal, & Deeth, 2014), but far more rarely with native whey. The aim of this work was to investigate whether it would be possible to produce a whey protein product in powder form with a truly native protein content using only processing steps commonly used within the dairy industry and to determine the optimal process.