Higher heating temperature and longer holding times increase the viscosity of the preheated solu- tion (Bryant and McClements 2000d). The apparent viscosity plateaus with time once the aggrega- tion process is finished. A lower solution pH, in the range of 6–8, results in more viscous solutions (Mleko and Foegeding 2000). Shear thinning is observed in the most viscous solutions, those heated at higher temperature and longer times. The flow behavior is usually described by a power- law model,
η=kγn−1, (10.1)
where η is the apparent viscosity, γ is the shear rate, n is the flow index, and k is the consistency. Increasing the protein concentration in the heating process results in more viscous solutions and higher consistencies, but up to 10 wt% WPI they are still very Newtonian (n~1) (Vardhanabhuti and Foegeding 1999). Shear thinning behavior is clearly observed at 11 wt% WPI, with n~0.45–0.75. The overall rheology is similar to that observed for other hydrocolloids at lower concentrations (Mleko and Foegeding 1999; Vardhanabhuti and Foegeding 1999). The large aggregates formed when heating at high protein concentrations result in more viscous solutions after being diluted than heated solutions at the diluted concentration (Mleko 1999; Mleko and Foegeding 1999).
10.1.2 tHickening
There are many hydrocolloids that are used as instant thickeners for food applications, such as pregelatinized starch, but they lack the nutritional benefits of whey proteins (Regester et al. 1996). Whey proteins can also be used as food thickeners at room temperature as opposed to high tem- peratures (Kinsella and Whitehead 1989), resulting in weak cold-set gels capable of holding water (Hudson et al. 2000, 2001). Protein-based thickening agents could succeed in applications where it is desirable to reduce the consumption of carbohydrates while increasing the consumption of proteins. Such an example was presented by Hudson, Daubert, and Foegeding (2001) for dysphagia patients (difficulty swallowing). For thickening applications, there is no need for a secondary gela- tion step, only an aggregation step. This aggregation step however can be more severe than that dis- cussed previously. For example, aggregates can be produced from the destruction of gelled systems. For thickening applications, these aggregates should be in a powder form.
A few patents have been granted for modified whey protein powders for food applications (Tamaki, Nishiya, and Tatsumi 1991; Holst et al. 1993; Kawachi, Takeachi, and Nishiya 1993). Studies have been performed to study their improved functionality as emulsifiers (Firebaugh and Daubert 2005) or as thickeners that improve the WHC in yogurts (Li and Guo 2006). The elemen- tary steps to form a derivatized whey protein thickening powder are as follows: protein hydration, (thermal) gelation, drying, and milling. In principle, any initial conditions capable of resulting in a heat-set gel (e.g., pH, salts) could be used during heat gelation. However, these powders will be mixed with other foods, thus they must be highly soluble in order to have any application. Soluble powers (>60%) are only formed at acid conditions, using, for example, HCl, at low enough pH to obtain fine-stranded gels (pH<4)(Hudson et al. 2000). Particulate gels formed close to the pI or fine-stranded gels formed above the pI yield powders with very low solubility values and therefore of little use. Only low pH conditions, particularly pH 3.35, have been used in subsequent studies (Resch, Daubert, and Foegeding 2005a, 2005b). Therefore, heating conditions for thickening appli- cations are very different than for cold gelation applications.
The derivatized proteins also present good stability once they are reconstituted in water. Like many food thickeners, reconstituted powders show shear thinning behavior. Using the right gelation conditions (e.g., pH 3.35, no salt, 80°C for 3 h), the rheological characteristics of the solutions can be fairly independent of the temperature and of the solution pH (4 or 8) (Hudson et al. 2000). Increasing the protein concentration of the reconstituted solution obviously increases the viscosity greatly.
The consistency of a 5.6 wt% solution is one order of magnitude higher than of a 3 wt% solution, and that of a 7 wt% is two orders of magnitude higher than of a 5.6 wt% solution. Increasing the pro- tein solution also accentuates the sear thinning behavior; the flow index in Equation 10.1 decreases from 0.86 at 3 wt% to 0.75 at 5.6 wt% and to 0.24 at 7 wt% (Clare et al. 2007). The WHC of the reconstituted solutions is around 8–10 g water/g dry protein (Resch and Daubert 2002; Resch et al. 2005b; Clare et al. 2007).
The addition of NaCl prior to heating under acidic conditions has a detrimental effect on the derivatized whey proteins; it decreases the solubility of the powders and reduces the viscosity of the reconstituted powder solutions. A prolonged heating time during thermal gelation results in an improvement in the solubility and thickening capability. It has been suggested that the beneficial effect of a prolonged heating and of a low salt concentration is caused by the higher degree of acid hydrolysis under those conditions. Good derivitized powders showed significant amounts of low- molecular-weight peptides (<10 kDa) (Hudson et al. 2000).
At constant pH conditions (3.35), the type of acid used to acidify the protein solution has a marked effect on the initial gel formed, and in the final derived powders. Gels formed with lactic acid were stronger and the reconstituted solutions more viscous than using the usual HCl (Resch et al. 2005b). Very weak heat-set gels are achieved using phosphoric acid and this result in powders with little thickening capability and worse WHC. The addition of citric acid speeds up the heat gela- tion kinetics compared to lactic acid, although the final complex modulus is very similar. However, the powders made from citric acid are very difficult to solubilize, resulting in a solution with very low viscosity and no thickening ability. Resch et al. (2005b) argued that the differences among the different acid anions follow the Hofmeister series, which already occurs in the thermal stability of βLg (Damodaran 1989). Citric acid, and the reciprocal citrate anion, destabilize whey proteins—a strong salting out effect—causing rapid aggregation, resulting in particulate-like gels. As discussed previously at different pH, particulate gels are bad candidates to form thickening powders. On the other hand, phosphoric acid has a stabilizing effect on whey proteins, increasing the denaturation temperature, reported as the peak transition temperature with DSC, from 85 to 89°C. This explains why good gels are not formed with phosphoric acid when heating at 80°C.
Increasing the gelation temperature using HCl and lactic acid improved the thickening func- tionality up to 85°C; higher temperatures did not show further advantages. Phosphoric acid, which yields bad gels and powders at low heating temperatures, can equal the HCl and lactic acid powders by increasing the gelation temperature to 90°C, because the denaturation temperature is higher in the presence of phosphoric acid. The rheological differences observed using different acids are nev- ertheless too subtle to be observed under scanning electron microscope (SEM) (Resch et al. 2005a). In summary, the thickening and water-holding functionality of reconstituted derivatized powders parallels the rheological properties of the gels from which the powders were formed. Both param- eters are affected by the type of acid used to decrease the pH before the heating process.
While lower heating rates allow gelation to be observed at lower gelation temperatures, in nonisothermal experiments, the final rheological properties, once the modulus plateaus, appear very similar between 0.2 and 2°C/min. Only at very low values (e.g., 0.1°C/min) is an increase in the gel strength observed. However, this small benefit will be offset by higher processing costs, making the heating rate an impractical parameter to manipulate for improving gelation and derivatized powder functionality in a commercial setting (Resch et al. 2005a).
The final production of the whey powder is commonly carried out in the lab by freezing 12 wt% heat-set gels (80°C for 3 h) at –5°C for 16–18 h, followed by freeze-drying up to a moisture content <5%, and finally milling. This procedure is economically unfeasible on an industrial scale. Resch, Daubert, and Foegeding (2004) developed a friendlier protocol for the food industry, which is as fol- lows: an 8 wt% WPC solution is heated under constant agitation at 80°C for 1 h, resulting in a semi- solid gel which is transferred to a laboratory spray-dryer, using an inlet temperature of 190°C and an outlet temperature of 90°C. The derivatized powders using both methods presented comparable thickening capabilities, despite the different heating protocols, which can lead to the development of
Gelation and Thickening with Globular Proteins at Low Temperatures 157
a (semi-)continuous manufacturing process. The functionality of the derivatized powders depends on the initial composition of the whey protein powder used. In addition to pure βLg and WPI, com- mercial WPC powders are also capable to from good thickening agents (Resch and Daubert 2002; Firebaugh and Daubert 2005).
Whey-protein-based thickening agents at room temperature show great promise in food appli- cations, but following the previous procedures a higher concentration—more than double—is so far required compared to traditional hydrocolloids (e.g., carrageenan, xanthan gum, or starch) to achieve the same viscosity (Resch et al. 2004). Nevertheless, the thickening ability of derivatized whey powders can be greatly improved in the presence of calcium during hydration (after heating) (Clare et al. 2007). Increasing the calcium concentration to 75 mM in a 5.6 wt% solution greatly increases the consistency (two orders of magnitude), while the flow index decreases (from 0.75 to 0.17). This leads to a significant increase of the apparent viscosity, particularly at low shear rates, comparable with a reconstituted solution of 10 wt% without calcium (Hudson, Daubert, and Foegeding 2000). Calcium also induces significant gel strengthening during cold storage in a time frame of hours (Clare et al. 2007).
10.1.3 gelation step
Once soluble denatured proteins and aggregates are obtained, the final gelation step can be achieved by many means. For example, the final gelation can be achieved after another heat treatment, when the pH (Mleko 1999) or the presence of salts (McClements and Keogh 1995) can be adjusted before the second heat treatment. However, these examples do no constitute a cold gelation protocol, although they fall within the generalized two-step gelation discussed previously (Figure 10.1). In this section the cold gelation after the addition of salts and acids will be considered in detail.