Whey is a by-product of cheese and casein manufacture. Two types of whey, i.e. sweet (using rennet) and acid (using acid coagulation) whey are produced. These whey sources are used to manufacture a wide range of commercial whey protein products including whey protein concentrates, isolates, fractions, hydrolysates, etc. Whey proteins have been used in food products for their excellent functional properties, i.e. hydration properties, interfacial properties and gelling properties, but also for their nutritional properties; they are the best nutritive proteins for human consumption (El- Salam, El-Shibiny, & Salem, 2009).
To date, the key functional property of whey proteins that has attracted much research and commercial attention is their ability to form strong elastic gels on heating. Gelling is a consequence of protein denaturation followed by aggregation involving the formation of covalent disulphide bonds and non-covalent associations. Most of the current whey protein products produced by the dairy industry are designed to perform well as gelling agents, as this suits functional applications in many food systems. However, gelling behaviour is a limiting property in many other food systems, especially beverages. In these applications, high levels of whey protein are required for nutritional reasons, but aggregation and gelation are not wanted; indeed, the whey proteins are required to be functionally “inert” with respect to gelation.
A significant market demand has been created for a whey protein product that is still nutritionally superior but lacks the ability to form heat-induced gels. The only known way of achieving this is to pre-denature the whey proteins by means of chemical or heat modifications (Morr & Josephson, 1968; Mulvihill & Donovan, 1987). Chemical modification involves major technical and nutritional challenges making it very difficult to implement. Therefore, heat modification is the only practicable option for modification of whey protein functionality. However, heat modification of whey proteins can still be a hurdle; when whey proteins are heated they will undergo denaturation, aggregation and gelation making them very difficult to handle in a commercial process. Therefore, one of the challenges for manufacturers is to produce nutritional but not functional (gelling) whey proteins.
Many processes have been proposed to produce heat-denatured whey protein products (information on this is reviewed in Section 2.8). They include producing micro- particulated whey proteins by combining heat and mechanical action, producing micro- particulated whey proteins from diluted whey protein solutions with little or no shear applied, using acid or enzyme hydrolysis, etc. While these processes provide technically feasible ways of producing heat-denatured whey protein products, they are not commercially feasible (cost-effective), hence the lack of denatured whey protein products on the market. These processes are not cost-effective for two main reasons: whey proteins are heated at low protein concentrations (e.g. 1 to 10% w/w) and high energy demand equipment is used (e.g. homogenizer). It is desirable to establish a cost- effective process for making denatured whey protein products that can be used in high protein food applications.
McSwiney et al. (1994) showed that when whey proteins were heated, protein molecules denatured almost completely before a gel network was formed (Figure 1.1).
Figure 1.1: Relationship between the loss of native proteins and the gel strength during
heating; loss of native proteins curve; gelation curve; weaker gelation curve (modified plot from McSwiney et al. (1994))
This figure highlights two possible technically feasible options for producing whey protein products that are functionally inert. The first is to quickly heat-denature whey proteins and then dry them before they start gelling as shown in option 1 (Figure 1.1).
The second is to manipulate process conditions so that the whey proteins form a weak gel that can easily be further processed as shown in option 2 (Figure 1.1), and this provides the impetus for the research presented in this thesis.
The effects of calcium on the heat-induced gelation of whey proteins have been extensively studied over the last two decades – but mainly in model systems (Hongsprabhas, Barbut, & Marangoni, 1999; Matsudomi, Rector, & Kinsella, 1991; Mercade-Prieto, Paterson, & Wilson, 2008; O'Kennedy & Mounsey, 2009; O'Kennedy, Mounsey, Murphy, Pesquera, & Mehra, 2006; Patocka & Jelen, 1991; Simons, Kosters, Visschers, & de Jongh, 2002; Xiong, Dawson, & Wan, 1993). Model systems are completely controlled in terms of whey protein composition, whey protein concentration, salt concentration, ionic strength, pH, etc. These studies were carried out with the purpose of understanding how calcium affects the functional properties of whey proteins. The work reported here focuses on understanding the effects of added calcium on the heat-induced behaviour of whey proteins as they are heated in a commercial processing environment. Can calcium be added into a feed stream of whey protein concentrate to modify the gelling behaviour of whey proteins so the process can be continued without blocking the commercial plant?
The objectives of the work were:
1) to determine the effects of added calcium on the denaturation and aggregation of
heated whey proteins in three different whey protein systems;
2) to determine the effects of added calcium on the gelation of heated whey
proteins from three different whey protein systems;
3) to determine the effect of the added calcium to protein concentration ratio on the
kinetics of heat-induced whey protein aggregation;
4) to make recommendations on: i) the suitability of different whey protein systems
for use as a feed material for making denatured whey protein products; ii) the usefulness of adding calcium as tool for making denatured whey protein products; iii) future study.