So you want to join our community!

If you already have an account, all you have to do is

Use and continue

New World Wine Maker Blog - Technical Articles

Understanding and measuring tannins in red wines

by Jose Luis Aleixandre-Tudo, Mihaela Minhea & Wessel du Toit 

The importance of phenolic compounds on wine quality require methods that accurately measure the tannin content.

Tannins are phenolic compounds that play an important role in the astringency perception of red wines. Tannins also are very complex molecules with different sizes and conformations. The decrease in the astringency intensity during ageing is due to different phenomena, including cleavage reactions (appearance of smaller less astringent molecules), precipitation from solution, due to insolubility situations, conformational arrangements (shape of the tannins molecule) and interactions with other components (such as anthocyanins). Precipitation based methods (BSA and MCP tannins assays) are highly suitable for routine tannin analysis. Both methods positively correlate with each other and also with the astringency intensities measured by a sensory panel.

Introduction

Tannins are phenolic compounds that are involved in red wine mouthfeel attributes. They are also thought to play a very important role in the astringency perception. When drinking red wine, the tannin compounds interact with the salivary proteins, creating a macromolecular complex that precipitates from solution and causes a drying and puckering mouthfeel sensation, also known as astringency.1 The intensity of this feeling depends on many factors. The size and conformation of the molecules, the combination with other wine components and the levels found in wine define the astringency intensity.

It is also known that the wine astringency decreases over the ageing process.2,3 This behaviour that was initially attributed to a decrease in the total tannin content present in the wine, is nowadays ascribed to different phenomena. Starting with the assumptions that tannins polymerise during ageing and that the ability of the tannins to elicit astringency increases with tannin size (i.e. the bigger the molecule, and the higher the number of sites available to interact with the salivary proteins, the higher the ability to combine and precipitate proteins)2 other phenomena that explains the decrease in the astringency intensity needs also to be playing role. First of all, cleavage reactions, which means large tannin molecules break down giving rise to smaller, less astringent tannins, have been proposed by some researchers.4 Moreover, molecular conformational arrangements have also been identified as a possible reason.5 Bigger and larger tannin molecules can also be too bulky (which means that due to the molecular conformation, the active binding sites are not available to interact with the salivary proteins). In this specific scenario larger tannin won’t give rise to an increased astringency perception. It is also well accepted that anthocyanins play an indirect role in wine astringency. The anthocyanin-tannin molecules cause a reduction on the ability of the newly formed polymeric pigment to interact with salivary protein thus reducing astringency (phenomenon that could also be related to the abovementioned conformation rearrangement scenario).6 The later phenomena together with the precipitation of tannins from solution, due to insolubility conditions, may explain why the wine astringency softens during ageing.

Tannin measurement by acid hydrolysis

The quantification of tannins has been challenging researchers over the past years as these compounds are of a very diverse nature, a fact that makes it difficult to estimate their concentrations. However, a number of methodologies are currently available for the measurement of the wine tannin levels. A method that has been commonly used for a long time exploits the ability of the tannin molecules to break down in a heated acid environment (acid hydrolysis method).7 The individual molecules show a red coloration after the heating process and can then be measured by quantifying the intensity of the red tonality using a conventional spectrophotometer. This method that is used worldwide presents a number of limitations. It does not take into account the structure of the tannin pool and it also does not consider other components (anthocyanins) that can interfere in the reaction and measurement. Due to this, the tannin concentration in wine is often overestimated and it is common to observe an increase in the wine total tannin content during ageing. Nevertheless, the method also has some advantages as the ease of implementation and reliability.

Tannin measurement by precipitation

Once understood that the astringency perception is caused by the precipitation of the salivary proteins after the interaction with the tannin molecules, the following question comes into our minds: Why not using the same principle that occurs naturally in our mouth to measure tannins? Based on this reasoning two new methods for tannin analysis were recently developed. The first one relies on the interaction of tannins with an animal protein. This method uses bovine protein and is known as the bovine serum albumin protein or BSA method.8,9 The first step of the method consists of the precipitation of the tannin compounds after interaction with the BSA protein. However, a further step is required as the protein shows similar spectral properties than the tannin compounds and cannot therefore be quantified at the maximum absorption band of the tannin molecules. The reaction of the tannin complex with ferric chloride, that gives rise to blue coloured compounds, is thus measured …

>> CLICK HERE TO READ FULL ARTICLE

Read article

The Wonderful World of Lees

Have you ever wondered if there is any value in the lees that you accumulate during the early portions of harvest and wine processing? Have you questioned the various uses associated with wine lees?

Then you’re in luck! Consulting subscribers now have access to my recent article, Wine Lees: A Powerful Tool for Winemakers in the Learning Center that discusses the advantages and disadvantages associated with the use of wine lees during wine production. If you’re looking for a way to make your wine lees work for you, then this Article may help.

If you’re missing out on Denise Gardner Winemaking’s Learning Center, the information at the bottom of this blog post can help you get started!

Lees, or the settled portion of yeast after primary fermentation, offers several opportunities for its continued use through wine production. During good vintage years, clean lees can provide molecular components, such as nitrogen-containing compounds or polysaccharides that naturally alter the wine’s mouthfeel over time. Depending on lees contact time, the mechanism of contact, and the degree of yeast autolysis, lees can also contribute aromas or flavors to the wine that may offer changes in complexity.

Many winemakers opt to keep lees in barrels with the wines intended for longer-term aging, and if they are needed, remove the lees from those wines. A more traditional practice, bâtonnage, is a form of lees contact in which white [Chardonnay] Burgundy wines stored in barrel include routine stirring of the lees. Another great stylistic benchmark region associated with bâtonnage and lees contact is Muscadet produced from the Loire Valley (France). On Muscadet wine bottles, an additional term “sur lie” can be found to indicate this process …

>> CLICK HERE TO READ THE FULL ARTICLE

Read article

Using MLF to create a buttery wine or not

by Natasha Pretorius, Lynn Engelbrecht & Maret du Toit – Wineland Media

Various factors influence the amount of diacetyl, acetoin and 2,3-butanediol produced during fermentation impacting the buttery aroma.

Malolactic fermentation (MLF) is a secondary fermentation carried out by lactic acid bacteria (LAB)This process can occur spontaneously or can be induced by using MLF starter cultures. Currently, the commercially available MLF starter cultures belong to the species Oenococcus oeni and Lactobacillus plantarum. The use of starter cultures to induce MLF is preferred to avoid the risks associated with spontaneous MLF. The starter cultures can be inoculated simultaneously with the yeast, known as co-inoculation, or after the completion of alcoholic fermentation, known as sequential inoculation. MLF is a desirable process as the decarboxylation of l-malic acid to l-lactic acid and carbon dioxide decreases the acidity and increases the microbial stability of wine. This process also influences the organoleptic properties of wine.

In addition to malic acid, some MLF starter cultures can also degrade citric acid usually present in grape must at concentrations of 0.031 g/ℓ to 0.42 g/ℓ. The metabolism of citric acid leads to the production of acetate, d-lactate, diacetyl, acetoin and 2,3-butanediol (Figure 1). The production of acetate is one of the reasons for the 0.1 g/ℓ to 0.3 g/ℓ volatile acidity increase during MLF as citric acid metabolism is linked to malic acid degradation.

Figure 1.

 

When present at low concentrations, diacetyl can contribute to the complexity of wine. Diacetyl has a buttery aroma which contributes to wine complexity when present at concentrations above its sensory threshold value of 0.2 mg/ℓ to 2.8 mg/ℓ. However, high diacetyl concentrations above 5 mg/ℓ can give rise to an overwhelming buttery aroma that masks the fruity and/or vegetative aromas in wines. Diacetyl can be reduced to the less sensory active acetoin and 2,3-butanediol (Figure 1) with much higher sensory thresholds of 150 mg/ℓ and 600 mg/ℓ, respectively. The reduction of diacetyl to these compounds is therefore encouraged during winemaking if a buttery style wine is not wanted. Several factors influence this reduction, as well as the sensory perception of diacetyl in wines.

A few of these factors include:

Composition of grape must

The grape must composition influences the concentrations, as well as the sensory perception of diacetyl. There are three main components of grape must that can influence the diacetyl concentrations during fermentation. These components are:

  • pH

Diacetyl is more rapidly reduced to acetoin during the fermentation of grapes from warm climate regions that have a higher pH. Wines from these regions might therefore have less diacetyl than wines from cool climate regions that are usually associated with a low pH.

  • Citric acid concentration

Grape must with a higher citric acid concentration leads to increasing concentrations of acetate, d-lactate, diacetyl, acetoin and 2,3-butanediol. Excess acetate and d-lactate causes over acidification and inhibits bacterial growth thus prolonging MLF. A longer MLF duration can result in more diacetyl being produced during the fermentation.

  • Phenolic compounds

Several studies have previously indicated that diacetyl in white wines was less stable and more likely to be reduced to acetoin and 2,3-butanediol than in red wines. However, the buttery aroma of diacetyl is more likely to occur in white wines than in red wines, due to the presence of phenolic compounds such as p-coumaric, caffeic, ferulic, gallic and protocatechuic acid. These phenolic compounds lower the buttery aroma in red wines by binding to diacetyl.

>> CLICK HERE TO READ THE FULL ARTICLE

Read article

Microbial strategies for wine ethanol reduction

by Debra Rossouw & Florian Bauer 

Introduction

A prominent goal in global wine research is the reduction of ethanol levels in wine, without the use of physical alcohol removal methods such as spinning cone columns and reverse osmosis (Howley & Young, 1992; Pickering, 2000). This research is motivated by a confluence of social, economic and environmental factors: On the one hand, many styles of wine that are popular in the global market require grapes to be fully ripe at harvest. Desirable levels of ripeness are frequently linked to high sugar concentrations in the grapes, which invariably, all other factors being equal, lead to higher final ethanol concentration in the wine. On the other hand, while achieving desired stylistic outcome, such high alcohol levels may negatively affect the taste and balance of the wine (Guth & Sies, 2002). From a health perspective, high ethanol levels add to concerns related to alcohol addiction and illnesses. Finally, many countries tax wine based on the percentage of alcohol creating a strong commercial incentive to decrease ethanol levels (Heux et al., 2006).

While pre- and post-fermentation processes have been considered in low ethanol research, a growing body of international research is targeting microbial technology in an attempt to alter fermentative ethanol production. Microbial strategies present an attractive opportunity to decrease ethanol levels while preserving the quality and aromatic integrity of the wine. One aspect of this research relates to yeast strain development through breeding or genetic engineering: The principle behind these approaches is the engineering of yeast strains through altered gene expression to modify carbon fluxes in the cell. One of the key target carbon sinks in these approaches has been glycerol, as several research groups have attempted to re-direct carbon towards glycerol in order to decrease the flow of carbon to ethanol. (Remize et al., 1999; Lopes et al., 2000). These approaches have seen some success in terms of decreasing the ethanol concentration in wine, but off-flavours, such as acetic acid and butanediol, are often produced. Rossouw et al. (2013) demonstrated that an alternative metabolite in central carbon metabolism, trehalose (a disaccharide made from two molecules of glucose), can be targeted as a carbon sink without resulting in the accumulation of undesirable redox-linked metabolites (Rossouw et al., 2013).

A second microbial strategy that has seen growing interest in the last decade involves the use of non-Saccharomyces yeasts in co-fermentation with conventional S. cerevisiae wine yeast strains. S. cerevisiae wine yeast strains have been selected for fermentation efficacy, and as such their primary fermentation kinetics (in particular ethanol yields) fall within a very narrow range. While finding S. cerevisiae strains showing lower than usual ethanol yields is thus not likely, this avenue can be pursued for non-conventional wine-associated yeasts where ethanol yields have been found to vary widely (Contreras et al., 2014; Rossouw et al., 2015). Several studies have reported lower ethanol yields when using non-Saccharomyces yeasts, however, the decreased ethanol production was often linked to high residual sugar levels in these wines (Ciani et al., 2006; Magyar & Toth, 2011). Recent studies have highlighted the diversity of the South African vineyard microbial landscape (Setati et al., 2012), providing the opportunity to access novel species and strains in the context of ethanol research.

Impact of yeast strain and environmental parameters on ethanol yields

Varying key environmental and fermentation parameters, such as must nitrogen content, pH and fermentation temperature, has been shown to significantly impact the production of key yeast metabolites such as glycerol (Rankine & Bridson, 1971; Torija et al., 2003). If glycerol production can be manipulated through environmental factors, and given that glycerol and ethanol production by yeast are often inversely correlated, ethanol production could likewise be manipulated. We therefore undertook to determine which fermentation factors (alone or in combination) could influence the production of ethanol. This systematic approach comprehensively investigated the effects of various combinations of pH, temperature and nitrogen settings on the ethanol yields produced by 15 commercial wine yeast strains. These experimental factors were selected as they can in principle be controlled by the winemaker, thus making implementation practical, cost-effective and user-friendly if successful. However, the data indicate that there are no statistically significant differences in ethanol yields between strains, or between different conditions …

>> CLICK HERE TO READ THE FULL ARTICLE

Read article

How to reduce the pinking potential of white wines

By Anton Nel of Wineland Magazine

When white wines with the potential to pink are exposed to oxygen, the phenomenon known as pinking occurs. It is common in Sauvignon blanc, but also occurs in Chardonnay, Colombar, Chenin blanc and Viognier.

Literature exists about the possible compounds and components that promote pinking, but not all findings concur and there is still much uncertainty about the cause of the phenomenon. Research into pinking is therefore ongoing. The following is a discussion of basic guidelines to reduce the pinking potential of white wines, but research is currently in progress and more comprehensive feedback will be given to the industry at the end of the study.

 

Reductive vinification

During reductive vinification the winemaker tries to eliminate oxygen (O2) as much as possible. The most common ways of doing so entail working with dry ice, inert gas or ascorbic acid. When ascorbic acid (vitamin C) is used during the winemaking process, the ascorbic acid is oxidised into dehydroxy ascorbic acid and hydrogen ions. In the presence of oxygen in the juice/wine the oxidation of ascorbic acid also results in the formation of hydrogen peroxide (H2O2). This is a strong oxidative agent and SO2 is required to bind to it and neutralise it.

 

Keep the following guidelines in mind:

  • It is important always to keep the free SO2 concentration of the wine as close as possible to 35 – 45 mg/ℓ when working with ascorbic acid.
  • As soon as dry ice and inert gas are involved, it is important to keep the SO2 of the wine as close as possible to the above-mentioned values.
  • Ascorbic acid reacts with SO2 at a ratio of 1:1.7 and not 1:1 as generally accepted. It is therefore important first to determine the existing concentration of ascorbic acid in the wine before making any ascorbic acid additions or adjustments.

 

Metals in wine

Much research has been done on the effect of heavy metals (Cu, Fe) in wine. It is always good to analyse the wine for heavy metals and if the metal content is high, the free SO2 levels should be kept at a minimum of 45 mg/ℓ.

 

>> CLICK HERE TO READ THE FULL ARTICLE

Read article

Skin-fermented white wine – all talk and no action?

When I started writing this blog, an old adage came to mind, “there is more than one way to skin a cat”. If you believe thatthere are many different ways to achieve an objective and that the reason for drinking wine is enjoyment, then skin-fermented wines could be the means to an end.

I tasted skin-fermented white wine (henceforth referred to as orange wine) for the first time in Croatia last year and during that tasting surmised that this type of wine is not for everybody. My second experience with orange wine was last week at a wine tasting chaired by James Pietersen of Wine Cellar (Observatory, Cape Town). During this tasting I was bombarded with so many new (and sometimes weird) aromas, that I soon realised that I was out of my depth. Take a look at Table 1 at the end of this blog for more details.

After the tasting, Edo Heyns (also present at the tasting) and I discussed the wines and he reckoned that this was probably the most difficult tasting that I could have kicked off with (I started at WineLand on 1 February 2017). Edo supplied this quote, “This burgeoning niche category offers some truly delicious wines. Chenin Blanc has particularly risen to the occasion, but there are also impressive blends and Sauvignon Blancs made in this style. The tasting yielded an intriguing list of descriptors and discussions, which is part of the category’s appeal. I specifically enjoyed wines that had a neat balance of texture and acidity. Judging this style definitely tests your tolerance for funkiness. While this is part of the excitement, it could also be its Achilles heel. Skin-fermented wines should first and foremost be good wines. To me, that was not the case for some of the wines in the line-up.”

And judging by how well some of these wines are selling locally, regardless of the relatively higher price, orange wine is doing a lot of well-placed scratching. Speaking of price, these five wines ranged from R135 to R275. While a lot is said about orange wines and their sometimes obvious faults, consider that one of the browner wines at this tasting scored quite well, because of the nose and palate. As it turns out, one cannot solely judge a book by its cover (I’m on a roll with proverbs today …

>> CLICK HERE TO READ FULL ARTICLE

Read article