Clases de la Interversión

During the same years of the 1990s when salt cold gelation was discovered, Kawamura et al. (1993) found that preheated WPI solutions could also form gels at room temperatures when the pH was lowered to less than 5.8. Low pH is desirable to the food industry as it increases shelf stability (Errington and Foegeding 1998) and requires less stringent sterilization processes (Potter and Hotchkiss 1995). An example of acid-induced cold gelation with whey proteins would be to increase the viscosity and WHC of yogurts, instead of using milk powder (Britten and Giroux 2001).

Acidification is usually performed using GDL. GDL is an internal ester that slowly hydrolyzes in the presence of water to gluconic acid, which is a weak acid (pKa 3.9) that further dissociates (de

Kruif 1997). These equilibrium reactions allow GDL to decrease the pH of the solution gradually, resulting in more regular gels. Inorganic or other organic acids, such as ascorbic or citric acids, cause an instantaneous decrease of the pH and therefore an irregular gelation.

Acid-induced whey protein gels using GDL have better mechanical properties, such as hard- ness, than salt-induced gels (Ju and Kilara 1998f), and the network formed is finer (Nakamura et al. 1995). Gel harness is maximum at pH 4.7, the adhesiveness at pH 4.4, while the cohesiveness does not change significantly between pH 5.3 and 3.5 (Ju and Kilara 1998f). The WHC is better at pH 5.2 and 3.9 than in between, while the elastic modulus and the stress at rupture tend to decrease at lower pH, and the strain at rupture remains unchanged (Cavallieri et al. 2007; Cavallieri and da Cunha 2008). The hardness of the gels increases with the final protein concentration in a power- law manner (Ju and Kilara 1998f). Higher incubation temperatures (20–50°C) result in gels with finer networks, and larger fracture stress and strain (Kawamura et al. 1993). Once a gel is formed, the modulus is minimum at ~20°C and it increases at lower and higher temperatures (Britten and Giroux 2001).

Dynamic studies of acid cold gelation have been performed by Cavallieri and da Cunha. (2008) by following the formation of the mechanical properties of the gels with time as the pH decreases. Previous studies only took into account the final pH of the system, not the acidification rate. A higher acidification rate, which leads to a lower final pH, reduces the time required for the system to gel and increases the rate of growth of the complex modulus after gelation, but the pH at the gelation point is very similar (5.8 and 5.6 at low and high acidification rates, respectively). Once a self-supported gel is formed, the stress at rupture and the elasticity value increase quickly in the first 30 h at a gelation temperature of 10°C (not at room temperature as in many other studies), increas- ing slowly thereafter until reaching an equilibrium value. These values are very similar at a final pH of 5.1, 4.9, and 4.7, about 20 kPa for the stress at rupture and 15 kPa for the elastic modulus. Much lower values are found at a pH of 4.2, ~15, and ~5 kPa, respectively. The time profiles at pH 4.2 are different from those at higher pH: the elastic modulus increases quickly in the early ~15 h but it decreases thereafter and the rupture stress does not increase further. This corresponds when the pH decreases below pH ~4.3. The strain at rupture of the gels increases quickly in the first 17 h, regardless of the acidification rate, followed by a slight decrease during a prolonged storage. If the evolution of the mechanical properties is followed with the pH, not with time, the data do not superimpose. This suggests that the acidification rate is relevant in the formation of the gel network, resulting in weaker gels at higher rates. Gels at equal pH, but at higher acidification rates, have lower rupture stress and elasticity modulus. The network deformability is also dependent on the rate, more than on the pH. Dynamic analysis of the cold gelation process also shows that the time required to reach the equilibrium pH is much shorter than to reach the equilibrium stress rupture. Increasing the acidification rate shortens the former but increases the latter. This results in a greater amount of molecular rearrangements occurring after the pH is set at higher acidification rates. This aging process is particularly important at high rates because a significant increase in the mechanical prop- erties occurs once the pH of the gel does not change further.

In addition to chemical acidification using GDL, acid cold gelation can also be achieved with acid-producing bacteria. Bacterial acidification is often preferred in the food industry. In addition, the acidification method can vary the acidification rate and the final pH by varying the inoculum size (concentration) and glucose concentration (feed stock). Alting et al. (2004a) used a lactid acid bacteria (Lactobacillus plantarum) to induce the acid cold gelation of WPI solutions below pH 5.9. Increasing the concentration of glucose decreases the final acidification pH. The gel hardness does not change significantly between 5.2 and 4.5, while it decreased at lower values, similar to that in chemical acidification (Ju and Kilara 1998f). Changing the amount of bacteria affects the acidifica- tion profile, while the final pH is the same if the glucose concentration is kept constant, e.g., the time to reach pH 5.5 changes form 220 min to 750 min from an inoculum size of 10–0.5%, respectively. A fast acidification rate resulted in harder gels, probably because these gels have been in the gel state for longer.

Gelation and Thickening with Globular Proteins at Low Temperatures 163

10.1.4 interactionsbetWeen protein aggregates

There are several intermolecular interactions that lead to the formation and stabilization of protein gels: van der Waals (0.1–1 kJ/mol), hydrophobic (5–10 kJ/mol), hydrogen bonds (10–40 kJ/mol), electrostatic interactions (25–80 kJ/mol), and covalent bonds (200–400 kJ/mol) (Dickinson 1997). In this section the key interactions involved in the different types of cold-set gels reviewed previ- ously will be discussed, particularly salt and acid cold-set gels.