Changes in color parameters and phenolic compounds during the sun-drying grape raisining of cv. Pedro Ximenez to obtain sweet wines are studied. Browning increases during the process as a result of the contribution to a greater extent of the low and medium molecular size polymers than the high molecular size polymers. Raisining decreases hue and lightness and increases chroma, all measured as CIELab parameters, indicating a color change to dark reddish hues that is also preferentially due to low and medium molecular size polymers. Most of the phenols studied increase in concentration during raisining, essentially through the concentration effect resulting from the loss of water in the grapes. The concentration changes, however, are comparatively small for hydroxycinnamic esters and flavan-3-ol derivatives, suggesting that these phenolic fractions undergo predominantly oxidative degradation reactions by enzymatic pathways, contributing strongly to the browning of grapes.

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In recent years many studies have been carried out on new pigments derived from anthocyanins that appear in wine during processing and aging. This paper aims to summarize the latest research on these compounds, focusing on the structure and the formation process. The main pyranoanthocyanins are formed from the reaction between the anthocyanins and some metabolites released during the yeast fermentation: carboxypyranoanthocyanins or type A vitisins, formed upon the reaction between the enol form of the pyruvic acid and the anthocyanins; type B vitisins, formed by the cycloaddition of an acetaldehyde molecule on an anthocyanin; methylpyranoanthocyanins, resulted from the reaction between acetone and anthocyanins; pinotins resulted from the covalent reaction between the hydroxycinnamic acids and anthocyanins; and finally flavanyl-pyranoanthocyanins. On the other hand, the second generation of compounds has also been reviewed, where the initial compound is a pyranoanthocyanin. This family includes oxovitisins, vinylpyranoanthocyanins, pyranoanthocyanins linked through a butadienylidene bridge, and pyranoanthocyanin dimers.

Sherry wines are consumed worldwide and are principally produced in the Jerez and Montilla-Moriles regions of southern Spain. Acetaldehyde is a relevant carbonyl compound in wines and one of the main responsible for the particular personality of Sherry wines with a ripe apple odor descriptor. Aldehyde dehydrogenase plays an important role in yeast acetaldehyde metabolism. Acetaldehyde contents in Sherry wines subjected to biological aging strongly depend on yeast populations, and the formation of velum depends on specific amino acids, oxygen availability, and the composition of the wine. Both biological and oxidative aging processes increase the acetaldehyde content in Sherry, although some of the acetaldehyde is oxidized to acetic acid and subsequently transformed into acetyl-CoA. In sensory terms, 1,1-diethoxyethane and other acetals, acetoin, and sotolon are the main compounds formed from acetaldehyde in Sherry wines. The chemical browning pathway of wine by the condensation between phenols and acetaldehyde is especially important in Sherry wines. Acetaldehyde can inhibit the velum formation at higher concentration than its threshold tolerance; also, it could be responsible for the high mitochondrial DNA polymorphism observed in flor yeasts. Usually, acetaldehyde is used to control the biological aging of Fino Sherry. A faster production of acetaldehyde could be considered to shorten the aging process of Sherry. In recent years, the acetaldehyde formed during ethanol metabolism in alcoholic beverages has been associated with carcinogenic processes; however, no systematic data are available about this statement.

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The drying of Pedro Ximenez grapes in chamber at a controlled temperature of 40 or 50 degrees C is studied. Compared to traditional sun-drying, the chamber-drying shortened the drying time by about 40% at 50 degrees C. In color terms, the musts obtained from grapes dried at 50 degrees C were closer in CIELab coordinates to those obtained by sun-dried grapes, with similar h(ab) values and slightly lower L* and C(ab)*. To shorten further the drying times at 50 degrees C, the grapes were dipped in olive oil or ethyl oleate emulsions containing potassium carbonate. The ethyl oleate pretreatment shortened additionally the drying time by about 25%, providing musts with chroma, lightness, and hue similar to those without grape pretreatment. In general, except for the phenolic compounds corresponding to the drying with ethyl oleate pretreatment, most of these compounds in the remainding conditions studied increased to a lesser extent than expected because of water losses of the grapes during drying, revealing degradation reactions.

In this work, the effects of chamber drying under controlled temperature and moisture conditions on three varieties of red grapes (Merlot, Tempranillo, and Syrah) cultivated in warm areas in southern Spain were studied. This drying was made with a view to their use in the production of sweet red wines. Analyses included color parameters, browning index, and anthocyanin concentrations measured by HPLC-DAD/MS. Based on the results, drying increases color and the concentration of these phenolic compounds by the effect of dehydration of the berries and diffusion of the colored compounds from the skin to the pulp due to the structural alterations in their skin. In addition, drying increased the browning index (OD 420), although less markedly than OD 520, as well as decreased the hue (OD 420/OD 520). The musts exhibited the typical color of red wines and a marked darkness by the effect of their low lightness (L* < 20 CIELAB units). Although the sugar content of the musts obtained at 24 h of drying was less, these musts were better to use in the vinification process, even without the maceration step as a result of their higher anthocyanin content, less browning, and darker color. To increase the content of the high-molecular weight compounds and anthocyanin derivatives, more raisining time could be required.