The practice of acidification by the addition of different amounts of a strong acid to the juice modifies both the concentration and ionic strength of the juice, in addition to its pH. Modification of pH by the addition of 2 M citric acid required adding no more than 3% by volume of the concentrated acid, and thus, the dilution factor was insignificant. However, concentrations of citric acid.
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(a) D4,3 measured in the entire juice (black bar), in the sediment (gray bar), and in the supernatant (white bar) in commercial juice acidified with 2 M citric acid and allowed to separate for 3 days, (b) particle layers in a monodisperse suspension during settling, and (c) particle layers in a bidisperse suspension during settling. Figure 6. Optical micrographs with 20× magnification of carrot juice cloud particles at different pH values created by acidification of the juice. Journal of Agricultural and Food Chemistry Article 11532 dx.doi.org/10.1021/jf5042855 | J. Agric. Food Chem. 2014, 62, 11528−11535 the final juice ranged up to 0.062 M at the lowest pH, and this would increase the juice ionic strength by up to ∼0.04 M at pH values below 5.5.
These acidified juices can then be compared to those in which buffer salts were added directly to keep the ionic strength uniform between samples.
You should know all about the benefits of carrot juice
The Relative Turbidity Results,
in which citric acid/sodium citrate buffers were used to alter pH, are replotted and compared in Figure 7 to carrot juice to which 2 M citric acid was added to decrease pH. Juices acidified using citrate buffer salts (prepared at a constant ionic strength of 0.1 M) showed a smoother increase in turbidity over the entire pH range, while the turbidity data of the juice acidified with 2 M citric acid displayed a more sigmoidal shape. The latter juices were slightly less stable at high and low pH than the juices containing buffer salts. This result is somewhat surprising because the higher ionic strength in the buffer salt-containing mixtures might be expected to promote flocculation by decreasing the Debye length and lowering the energy barrier between particles.10,13 To probe this effect further, the extent of clarification of juices was compared when sodium chloride versus calcium chloride was added in specific amounts to unbuffered commercial carrot juice at pH 6.2 (Figure 8). The juices were stored at 4 °C for 6 days, undisturbed before relative sediment height was measured. The results in Figure 8 indicate that CaCl2 significantly induced clarification, in contrast to NaCl, which had little effect on the cloud stability. Results with the sodium salt are consistent with those in Figure 7 and indicate that any effect of Na+ on the particle diffuse double layer thicknesses does not seem to impact the particle stability. Benıtez et al. ́ 10 explored the effect of the ionic strength on apple juice cloud stability, by first separating apple juice cloud particles from the juice serum and reconstituting them in aqueous solutions containing potassium chloride concentrations between 10−4 and 10−1 M. Turbidity and, hence, cloud stability were greatest at the lowest ionic strength, as expected. A key difference between the results in Figure 8 and those by Benıtez et al. ́ 10 is that, in the latter study, the contributions of the native juice serum have been removed. To account for the surprisingly weak effect of ionic strength on our carrot juice cloud results, it is important to recognize that the native carrot juice serum will itself contain ions that contribute to the overall ionic strength. Data from the
United States Department Of Agriculture
(USDA) National Nutrient Database27, as well as measurements of electrolyte concentrations in other types of juices28−32, suggest that native ionic strength values of the order of 20−100 mM10 are likely. At such concentrations, the diffuse double layer is already less than 1−2 nm, and further thinning upon the addition of further ions may not have a large effect on modifying interparticle repulsion. In addition, with such small Debye lengths, electrostatic repulsion may occur over length scales comparable to short-range forces, such as steric interactions, giving the latter an important contributing role.33 Benıtez et al. ́ 26 found that, even at very low pH values, where the ζ potential of isolated cloudy apple juice particles was close to zero, particles remained significantly stable because of a short-range repulsion.
Ackerley and Wicker24 also concluded that, at high pH values (pH 7), steric stabilization as a result of a hydrated pectin layer accompanies repulsive forces in preventing aggregation and clarification of orange juice. Thus, despite the high pH and high negative charge in the native carrot juice, such short-range interactions may be playing a significant role in carrot juice cloud stability, relative to the longer-range double-layer interactions. Results in Figure 8 also indicate an important contribution because of specific ion effects.
Previous research has demonstrated that the presence of divalent cations increases fruit juice cloud instability, consistent with our results for carrot juice.11,34−36 Calcium bridging of pectin is an important mechanism in the clarification of orange juice.37 Such particle destabilization has been attributed to the formation of junction zones, in which divalent cations, such as calcium, induce pectin chain association.34 Consequently, one explanation for the greater stability of the juices modified with buffer salts (Figure 7), despite their higher ionic strength, is that the higher concentration of sodium ions in the system competes with native calcium for binding to pectin. This would make calcium-induced flocculation less likely in these samples compared to juices acidified with citric acid.