Wine Chemistry and Biochemistry

(Steven Felgate) #1

222 E.J. Waters and C.B. Colby


sodium carbonate and photo-oxidation with UV radiation was found to be effective


in removing residual protein from bentonite.


As discussed by Waters et al. (2005), in both these studies, protein was desorbed


from bentonite by increasing the pH, but only in a batch system. As adsorption


and desorption are equilibrium processes, it is hardly surprising that residual pro-


tein was retained on the bentonite surface. In contrast, in a continuous flow sys-


tem, fresh reactant could be continually fed to the adsorption system, while ion


exchange products could be continually transported away from the system, driving


the exchange process to completion. Continuous flow contactors, such as continuous


stirred tanks, packed bed columns and fluidised beds, are commonplace in many


chemical industries, as is continuous catalyst regeneration (Fogler 1992). There-


fore, it was postulated that effective regeneration of bentonite might be achieved by


treatment with a base such as sodium hydroxide in a simple continuous flow system


(Waters et al. 2005). However, the difficulty in regenerating bentonite by salts and


alkalis could also suggest that protein adsorption on bentonite may involve other


mechanisms than solely cation exchange; thus complete regeneration of bentonite


by such a process may not be feasible.


6C.6.1 The Sensory Impact of Bentonite Fining


One of the perceived disadvantages of bentonite fining is that it is thought to reduce


flavor and aroma of wines. This perception is not supported by strong evidence in


the normal operational range of bentoniteadditions by winemakers. Sensory evalu-
ation of wines treated with bentonite showed no significant differences between the


control and the fined samples (Leske et al. 1995). Similarly, using difference testing,


Pocock et al. (2003) reported that bentonite fining of a Chardonnay and Semillon


wine had no effect on wine aroma and palate. This contrasts with previous findings


(Miller et al. 1985) that demonstrated reduced concentration of aroma compounds


after bentonite addition to juice, mustor wine. More recently Pollnitz et al. (2003)


elegantly confirmed that aroma compounds can be absorbed by bentonite, as did


Cabaroglu et al. 2003, although the later study found no sensory effect of bentonite


fining ofV. viniferacv. Muscat Ottonel or Gewurztraminer wine. Rankine (1989)


stated that bentonite fining results in the loss of aroma and flavor and Mart ́ınez-


Rodriguez and Polo (2003) extended this conclusion to sparkling wines when ben-


tonite is added to the tirage solution. This has led to the widespread conclusion,


throughout both literature and industry, thatbentonite fining at typical addition rates


has a detrimental effect on wine aroma and flavor, despite the fact that the conclu-


sions of Miller et al. (1985) were influenced by many factors and “should not be


considered general” (Miller et al. 1985). Similarly, Leske et al. (1995) concluded


that “lack of significant differences observed in this trial suggests that the negative


effect of fining may not be evident to a consumer in a commercial situation” and


“the magnitude of any such [negative sensory] effect will presumably depend on the


individual wine, the bentonite chosen, and the rate of addition chosen”.

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