Roger B. Boulton

Physiological factors contributing to the variability of sensory assessments: relationship between salivary flow rate and temporal perception of gustatory stimuli

Abstract Stimulation by oral manipulation or ingestion of stimuli causes the salivary flow rate to increase. Not only do gustatory stimuli affect salivary response, but saliva in turn can affect perception of taste by titration, dilution, or precipitation of stimuli. Average bitterness and astringency time-intensity (TI) curves and individual TI parameters generated in response to wines varying in ethanol, pH and phenolic composition revealed differences in temporal perception among the salivary flow groups. For both attributes, low-flow (LF) subjects took a longer time to reach maximum intensity and had a longer duration than high-flow (HF) subjects. For wines with tannic acid at pH 3·0 and pH 3·6, LF subjects recorded a higher intensity at maximum of bitterness and astringency than HF subjects. This perceptual difference was more pronounced at pH 3·6, where a greater difference in salivary flow rate was also elicited between flow groups. The log intensity decay curves for bitterness and astringency were linear, suggesting that a first-order decay process governed the perception function. Desorption is thus considered to be the primary phenomenon, rather than diffusion through a boundary layer. For all flow groups, the decay constants for bitterness were higher than those for astringency. However, the decay constants within one modality were similar for all flow groups, indicating similar desorption rates. Reciprocal plots of salivary flow rate were linear, suggesting that a second-order process governed salivary flow.

Enhancement of red wine colour by pre-fermentation addition of copigments

Abstract The effects of pre-fermentation additions of catechin and caffeic acid to musts were investigated. The cultivar Listan negro, a major red grape grown in the Canary Islands, was used for these studies. The addition of catechin at 120 mg/l resulted in only 10% enhancement in wine colour (AU at 520 nm) after fermentation, while caffeic acid, at the same level, enhanced the wine colour by 60%. These results appear to be the first report of such a significant increase in wine colour achieved by pre-fermentation treatments with natural grape constituents, supporting the view that red wine colour is generally limited by the levels of cofactors for copigmentation rather than by the level of anthocyanins alone. This demonstrates the importance of non-pigment composition in establishing red wine colour and the influence of initial must composition.

Effect of Caffeic Acid on the Color of Red Wine

The copigmentation effect of prefermentation additions of different doses of caffeic acid was investigated during the 1997 harvest. Microfermentation with the major red grape cultivars Listán negro and Negramoll, grown in the Canary Islands, was carried out with the same protocol. Visible and UV spectra were registered periodically. HPLC chromatograms were carried out. The color enhancement of cv. Negramoll wine varied between 13 and 75% (AU at 520 nm), and that of cv. Listán negro wine between 25 and 45% at the end of fermentation. During aging these values were enhanced to reach even >100% in some cases. An initial complex of the 1:1 type, where one molecule of caffeic acid associates with one molecule of anthocyanin, has been identified using the mathematical procedure of Brouillard et al. (J. Am. Chem. Soc. 1989, 111, 2604-2610). Caffeic acid seems to contribute to color stability and protection against oxidation. The importance of nonpigment composition in pigment extraction and color retention during and after fermentation is demonstrated.

Automated HPLC analysis of glutathione and thiol-containing compounds in grape juice and wine using pre-column derivatization with fluorescence detection

Abstract An easy and sensitive method for the analysis of glutathione (GSH) and other thiol-containing compounds in grape juice and wine has been developed and optimized. Following a pre-column derivatization of thiols with o-phthalaldehyde (OPA) and 2 aminoethanol, isoindole derivatives are separated on reversed-phase HPLC column and quantified by a fluorescence detector. The minimum detection limits for thiols are: GSH, 3.3 nmol/l (1 μg/l); cysteine, 22 μmol/l (2.7 mg/L); methanethiol, 0.27 μmol/l (12.8 μg/l); ethanethiol, 0.65 μmol/l (11 μg/l). The method yields linear responses up to 40 and 21 mg/l for GSH and cysteine, respectively. GSH levels in two varietal grape juices during fermentation varied from 0 (starting juice) to 2.1 mg/l (wine) in Sauvignon blanc, while the GSH in a Palomino sample with 1.28 mg/l in the juice increased to 5.1 mg/l in the wine. This automated pre-column derivatization of thiols followed by an automatic injection procedure is sensitive, reproducible and rapid, with a run time of 35 min.

Regional sensory and chemical characteristics of Malbec wines from Mendoza and California

Malbec grapes are widely grown and studied in Argentina, whereas the smaller production in California is less well known. This study sought to define and compare Malbec wine compositions from various regions in Mendoza, Argentina and California, USA. The Malbec wines were clearly separated, based on their chemical and sensory profiles, by wine region and country. Descriptors of Malbec wines were aromas of cooked vegetal, earthy, soy and volatile acidity, as well as acidic taste and astringent mouthfeel, regardless of the region of origin. Malbec wines from Mendoza generally had more ripe fruit, sweetness, and higher alcohol levels, while the Californian Malbec wines had more artificial fruit and citrus aromas, and bitter taste. Compositional differences between the two countries were related more to altitude than precipitation and growing degree days. To our knowledge, this is the first time that an extensive regionality study has been attempted for Malbec wines.

Modeling the sweetness response using time–intensity data

Abstract A general mathematical model for the temporal intensity response to a transient sweet stimulus was developed. The model is a set of nonlinear ordinary differential equations, expressing the rates of diffusion across a saliva boundary layer and adsorption and desorption to receptor site proteins on the taste bud cells. In this model, the perceived intensity is considered to be the product of a scaling factor and the amount of stimulus bound to receptor sites at any given time. A parameter estimation routine was used to fit the differential equations to the time–intensity (TI) curves for five equisweet stimuli (sucrose, aspartame, thaumatin, monellin, and brazzein). Model parameters were computed for each of 19 trained judges. The predicted TI curves were close to the observed time-intensity responses for all judges and sweeteners. TI parameters varied significantly across both judges and compounds, while model predicted parameters, such as adsorption and desorption coefficients, varied significantly only across compounds. Except for one model parameter (predicted plateau time), there were no significant differences across judges for the model-predicted parameters, indicating that all judges responded similarly to each compound.

Influence of intrinsic factors on conventional wine protein stability tests

Abstract The influence of intrinsic factors on the results of ethanol, tannin and heat tests, routinely used to assess wine protein stability, was studied. Experiments were performed on 23 Portuguese and Austrian wines. The factors considered were total protein content, pH, ethanol content and the amount of several relevant cations (calcium, iron, copper, sodium and potassium). The protein profiles were analysed by HPLC fractionation. The heat test was a good indicator of total protein content while the ethanol and the tannin tests showed significant interference by the other factors. A factorial design at two levels in selected samples was also performed to assess the influence of pH, storage temperature, tannin concentration and ethanol concentration on the development of turbidity. Results indicated that ethanol content had no significant influence, and that pH and storage temperature had a significant influence, though only when tannin was added.

Barrel maturation, oak alternatives and micro-oxygenation: Influence on red wine aging and quality

The impact of micro-oxygenation (MOX) in conjunction with a variety of oak alternatives on phenolic composition and red wine aging was investigated and compared with traditional barrel aging. Although several studies concluded that MOX give similar results to barrel aging, few have compared them directly and none directly compared MOX with and without wood alternatives and barrel aging. Results confirmed that MOX had a positive effect on colour density, even after 5 months of bottle aging. This is supported by an increase in polymeric phenol and pigment content not only with aging but in the MOX compared to barrel matured wine treatments. Descriptive analysis showed that MOX in combination with wood alternatives such as oak chips and staves could mimic short term (six months) barrel aging in new American and French oak barrels in regards to sensory characteristics.

Kinetics of the adsorption of bovine serum albumin contained in a model wine solution by non-swelling ion-exchange resins

Abstract Adsorption of wine proteins is an essential step in the production of white and rose wines. In order to develop environmentally friendly adsorption processes, non-swelling adsorbents are required. The performance of selected non-swelling ion-exchange resins (Macro-Prep™ 50S and Streamline® SP) was studied by describing the process kinetics of the adsorption of BSA in a model wine solution. The process was assumed to be diffusion controlled and a shrinking core model was applied. Experiments were performed in the 5–35°C temperature range and with different equilibrium partition coefficients. The results obtained with the shrinking core model were theoretically consistent and the apparent diffusivity values correlated very well with theoretically estimated effective diffusivities combined with a linear dependence of porosity with temperature. Separating the temperature effect on porosity, the apparent diffusivity followed an Arrhenius type dependency with temperature with 16.9 kJ/mole activation energy.

Use of Near-Infrared Spectroscopy and Chemometrics for the Nondestructive Identification of Concealed Damage in Raw Almonds (Prunus dulcis).

Concealed damage (CD) is defined as a brown discoloration of the kernel interior (nutmeat) that appears only after moderate to high heat treatment (e.g., blanching, drying, roasting, etc.). Raw almonds with CD have no visible defects before heat treatment. Currently, there are no screening methods available for detecting CD in raw almonds. Herein, the feasibility of using near-infrared (NIR) spectroscopy between 1125 and 2153 nm for the detection of CD in almonds is demonstrated. Almond kernels with CD have less NIR absorbance in the region related with oil, protein, and carbohydrates. With the use of partial least squares discriminant analysis (PLS-DA) and selection of specific wavelengths, three classification models were developed. The calibration models have false-positive and false-negative error rates ranging between 12.4 and 16.1% and between 10.6 and 17.2%, respectively. The percent error rates ranged between 8.2 and 9.2%. Second-derivative preprocessing of the selected wavelength resulted in the most robust predictive model.