The purpose of this lab was to determine the pH and total acidity of grape juice and wine. A second purpose was to experimentally determine the pKa of a monoprotic weak acid, polyprotic weak acid, as well as, a free amino acid, alanine.
Acids and bases were defined by Bronsted-Lowry in 1923, in which acids were described as substances capable of donating a proton and bases are substances capable of accepting a proton (Thompson and Dinh 2009). In addition to defining acids and bases, Bronsted-Lowry further classified acids and bases as being either strong or weak. By definition, a strong acid or base is a substance that is completely dissociated in aqueous solutions and in contrast, weak acids only partially dissociate and reach their equilibrium with their conjugate base (Harris 2003).
Buffers are comprised of weak acids and their conjugate bases. In the food industry, buffers are commonly used to protect changes in pH of food stuffs. The two conjugate components of the buffer resist changes in pH by absorbing the addition of any hydrogen or hydroxyl ions (Christen and Smith 2000). When weak acids and their conjugate bases are at equilibrium, their concentrations can be expressed in terms of the dissociation constant, Ka. For a strong acid, the Ka value is greater than 1 x 10-2M and less than 1 x 10-2M for a weak acid (Thompson and Dinh 2009). For weak acids, the relationship of hydrogen ion concentration and pH can be defined by the Henderson-Hasselbach equation (Thompson and Dinh 2009).
Wine quality greatly depends on the quality of the grapes and the microorganisms involved in the fermentation process (Husnik and others 2006). When compared to lactic acid bacteria, yeasts are better suited for growth in the low pH (3.0-3.3), high sugar environment of grape musts (Lonvaud-Funel 1999). As outlined by Lonvaud-Funel (1999), alcoholic fermentation starts very quickly and is completed by wine yeast strains of Saccharomyces sp. Once all reducing sugars are fermented to ethanol, yeast levels begin to decline and lactic acid bacteria are able to grow.
Alcoholic fermentation is followed by malolactic fermentation (MLF), which is a current method wine makers utilize to reduce wine acidity and to obtain microbial stability in wine (Husnik and others 2006; Ugliano and Moio 2005; Lonvaud-Funel 1999). L-malic and L-tartaric are the two main organic acids found in grape musts (Husnik and others 2006). The purpose of MLF is to transform L-malic acid into L-lactic acid, with the release of carbon dioxide. The decarboxylation of L-malic acid to L-lactic acid is usually conducted by lactic acid bacteria genera, such as Oenococcus, Pediococcus, or Lactobacillus (Ugliano and Moio 2005). Once all malic acid is degraded, lactic acid bacteria must be eliminated by sulfiting (Lonvaud-Funel 1999).
Sugar and alcohol content are two basic, yet important quality factors of wines (Son and others 2009). Sugar and alcohol concentrations are...