|This is the final article in a series of three which covers the fungal and bacterial origins of wine aromas. These articles detail esters, aldehydes, volatile fatty acids, volatile phenols, sulphurous compounds and higher alcohols. The old adage “one man’s trash is another man’s treasure”, holds true with most of these compounds. The article herein will deal primarily with volatile acidity and hydrogen sulphide, which are nearly unanimously classified as wine faults by vintners.Some sulphurous compounds can be pleasant, as is the case with grape derived thiols that are intrinsic to the “passion fruit”, “box wood”, and “grapefruit” aromas of Sauvignon blanc. However, the sulphurous compounds which are purely derived from yeast are not considered valuable to the vintner.
The volatile fatty acids are likely only considered to be contributors of positive aromas by those who produce vinegar. In wine, the volatile fatty acids are responsible for a major fault when they accumulate beyond their sensory threshold.
Volatile fatty acids
The volatile fatty acids found in wine consist primarily of short-chain fatty acids (tails of less than 6 carbons) and medium-chain fatty acids (tails with 6 – 12 carbons). The short and medium-chain fatty acids are the most studied fatty acids in wine and are responsible for what is known as volatile acidity (VA).
VA is a measure of all the steam distillable volatile acids present in wine. These can include acetic, lactic, formic, butyric and propionic acid (Zoecklein et al., 1999). Other organic acids, excepting acetic acid, are of little consequence to wine flavour and aroma. Around 90% of all the volatile acidity in wine comes from acetic acid, which, in conjunction with ethyl acetate, possesses a vinegar-like aroma (Pretorius & Lambrechts, 2000). Yeast produce acetic acid during fermentation within the range of 100 mg/ℓ – 200 mg/ℓ, depending on the yeast strain and vigour of fermentation (e.g. temperature and juice nutrient status) (Boulton et al., 1996). This usually occurs during the beginning lag phase of fermentation (Whiting, 1976). Excessive acetic acid production is usually an indicator of microbial spoilage by Acetobacter and Gluconobacter (Boulton et al., 1996).
Acetic acid from microbial sources is derived through various pathways. One mode is through the degradation of sugars by lactic acid bacteria via the phosphoketolase process (the way in which bacteria can break down residual sugar). Alternatively, acetic acid can simply be produced as part of the citric acid cycle. Acetobacter and Gluconobacter can oxidise ethanol to acetic acid enzymatically with alcohol dehydrogenase (first oxidised to acetaldehyde then to acetate with aldehyde dehydrogenase) (Swiegers et al., 2005).
Acetic acid (e.g. high VA) is a common issue when creating ice wines. Under ice wine conditions, the yeast are under high osmotic stress conditions. In order to adapt to this condition, yeast cells will exude glycerol, which prevents the movement of water from the yeast cell into the must. Glycerol is formed through a NADH dependent enzymatic reaction. The subsequent conversion of NADH to NAD+ changes the redox balance, which is corrected for by the production of acetic acid, shifting the redox potential back to equilibrium (Erasmus et al., 2004). Thus, this results in excessive levels of acetic acid.
Giudici and Zambonelli (1992) suggest that perhaps the reason for acetic acid production by yeast in normal table wines is due to acetic acid’s role as an intermediate in the formation of acetyl Coenzyme A (CoA) from acetaldehyde. However, Boulton et al. (1996) state that a consensus has not been reached concerning the mechanism of the enzymatic formation of acetic acid.
Pretorius and Lambrechts (2000) suggest that a typical VA of an unspoiled wine is around 200 – 400 ppm. There is no given “threshold of detection” for VA. The perception of these compounds can differ between wines, since high levels of sugar and ethanol mask them (Corison et al., 1979; Zoecklein et al., 1999). Further, winemakers expect an increase in VA of about 60 – 120 mg/ℓ in barrel-aged wine after one year. This is not necessarily due to microbial spoilage, but rather the degradation of the hemicellulose of the oak barrel itself. Also, phenolic compounds can oxidise over time to form peroxide, which oxidises to acetaldehyde and, after, to acetic acid (Zoecklein et al., 1999).
Sulphur compounds can be pleasant or disagreeable and generally have a low threshold of detection. Volatile sulphur compounds, such as thiols, are responsible for the ripe/fruity aromas of Sauvignon blanc and are formed during fermentation (Tominaga et al., 1998). They can also contribute a “box-tree like aroma” in the same variety (Tominaga et al., 1996). Further, sulphurous compounds at low concentrations may cause a perceived “minerality” in some wines (Goode, 2005). Additionally, Lactobacillus can metabolise methionine, which forms volatile sulphur compounds, such as methanethiol, dimethyl disulphide and propionic acid. Oenococcus oeni also metabolises methionine; current research suggests that the most significant by-product is propionic acid, which contributes a chocolate aroma and may be partially responsible for the pleasing and complex aroma profile of malolactic fermentation (Pripis-Nicolau et al., 2004).
The molecule H2S is the most studied sulphur compound. Normally considered aversive, H2S can have a pleasing aromatic impact by providing a “yeasty” flavour to wine at low levels. Higher concentrations of H2S have a “rotten egg” aroma and a very low sensory threshold of 10 – 100 µg/ℓ Pretorius & Lambrechts, 2000). In order to synthesise sulphur-containing amino acids, yeast can reduce sulphite to sulphide, which is then enzymatically combined with a nitrogenous compound to form cysteine or methionine. If those nitrogenous compounds are not present, the result is the release of hydrogen sulphide, which freely bypasses the cell wall (Kaiser, 2010). Deficiencies in vitamin B5 have also been found to be limiting in juices that produce H2S. This vitamin is important for the formation of Coenzyme A, which is necessary for the formation of methionine and cysteine. Without this enzyme, these amino acids cannot form, and the sulphur produces hydrogen sulphide (Wang et al., 2003). However, a vitamin deficiency is extremely rare and difficult to test for in a lab (Boulton et al., 1996).
H2S can also result from:
- Reduction of elemental sulphur from spray residues (relatively uncommon).
- Presence of other sulphur containing compounds (glutathione).
- High or very low juice turbidity (recommended ~0.5% turbid).
- Low redox potential of must (e.g. reductively held or tall/skinny tanks).
- Release of bound sulphurous compounds in yeast lees during lees ageing.
The impact of lees on H2S production isn’t always negative though. The mannoproteins of yeast can form disulphide bridges with sulphur compounds and lessen their aromatic impact.
Excessive SO2 use leads to the formation of H2S by inhibiting acetaldehyde reduction to ethanol. If a deficiency is also present in O-acetylserine and O-acetylhomoserine (precursory compounds necessary for the formation of sulphur containing amino acids), H2S is produced from the enzymatic reduction of sulphite (from SO2). This gives the yeast a sulphur source to produce these amino acids (Margalit, 2004). A high metal ion (e.g. residual copper from Bordeaux mixture) concentration within the must suppresses cellular respiration, which lowers redox potential and ultimately, elevates H2S levels (Boulton et al., 1996). If the redox potential of a must is not increased during H2S formation (e.g. aerated must), the H2S can react with other compounds such as ethanol and sulphur containing amino acids to form mercaptans (most notably, methyl mercaptan). The details of the mechanism of the formation of mercaptans are currently unknown. These molecules create a pungent, rotten cabbage aroma. Mercaptans can be easily oxidised to form a less aromatic disulphide. This misleads winemakers into believing the problem has dissipated, although mercaptans can re-form under reductive bottle conditions. Yeast strains differ widely in their propensity to form H2S and are chosen based on this characteristic.
Wine is commonly referred to as a “complex matrix”. By breaking wine down into its fundamental components, we can begin to understand how to better manage our vineyards and wineries to attain the wine styles that our markets desire. Volatile fatty acids and sulphurous compounds which are purely microbially derived should be avoided. It is crucial to understand how these compounds arise and how winemakers and viticulturists can manage them effectively and efficiently.
Boulton, R., Singleton, V., Bisson, L. & Kunkee, R., 1996. Principles and Practices of Winemaking. New York City: Springer Science and Business Media Inc.
Corison, C., Ough, C., Berg, H. & Nelson, K., 1979. Must acetic acid and ethyl acetate as mold and rot indicators in grapes. American Journal of Enology and Viticulture 30(2), 130 – 134.
Erasmus, D., Cliff, M. & Van Vuuren, H., 2004. Impact of yeast strain on the production of acetic acid, glycerol and the sensory attributes of icewine. American Journal of Enology and Viticulture 55, 371 – 378.
Giduci, P. & Zaomonelli, C., 1993. Increased production of n-propanol in wine by yeast strains having an impaired ability to form hydrogen sulphide. American Journal of Enology and Viticulture 44(1), 123 – 127.
Goode, J., 2005. The Science of Wine: From Vine to Glass. Los Angeles: University of California Press.
Kaiser, K., 2010. Controlling Reductive Wine Aromas. Retrieved August 26, 2012, from Brock University: http://brocku.ca/ccovi/files/uploads/Karl_Kaiser_-_Controlling_reductive_wine_aromas.pdf.
Margalit, Y., 2004. Concepts in Wine Chemistry (2nd ed.). (J. Crum, Ed.) San Francisco: The Wine Appreciation Guild.
Pretorius, I., & Lambrechts, M., 2000. Yeast and its importance to wine aroma: a review. South African Journal of Enology and Viticulture 21 (special issue), 97 – 129.
Pripis-Nicolau, L., De Revel, G., Bertrand, A. & Lovaud-Funel, A., 2004. Methionine catabolism and production of volatile sulphur compounds by Oenococcus oeni. Journal of Applied Microbiology 96(5), 1176 – 1184.
Swiegers, J., Bartowsky, E., Henschke, P. & Pretorius, I., 2005. Yeast and bacterial modulation of wine aroma and flavour. Australian Journal of Grape and Wine Research 11, 139 – 173.
Tominaga, T., Darriet, P. & Dubourdieu, D., 1996. Identification of 3-mercaptohexyl acetate in Sauvignon wine, a powerful aromatic compound exhibiting box-tree odor. Vitis 35(4), 207 – 210.
Tominaga, T., Furrer, A., Henry, R. & Dubourdieu, D., 1998. Identification of new volatile thiols in the aroma of Vitis vinifera L. var. Sauvignon blanc wines. Flavour and Fragrance Journal 13(3), 159 – 162.
Wang, X., Bohlscheid, J. & Edwards, C., 2003. Fermentative activity and production of volatile compounds by Saccharomyces grown in synthetic grape juice media deficient in assimilable nitrogen and/or pantothenic acid. American Journal of Enology and Viticulture 94(3), 349 – 359.
Whiting, G., 1976. Organic acid metabolism of yeast during fermentation of alcoholic beverages: a review. Journal of the Institute of Brewing 82, 84 – 92.
Zoecklein, B., Fugelsang, K., Gump, B. & Nury, F., 1999. Wine Analysis and Production. New York City: Kluwer Academic/Plenum Publishers.
Gut Microbial Metabolism of Polyphenols from Black Tea and Red Wine/Grape Juice Is Source-Specific and Colon-Region Dependent
- F. A. van Dorsten
- S. Peters
- G. Gross
- V. Gomez-Roldan
- M. Klinkenberg
- R.C. de Vos
- E.E. Vaughan
- J. P. van Duynhoven
- S. Possemiers
- T. van de Wiele
- D. M. Jacobs
The colonic microbial degradation of a polyphenol-rich black tea extract (BTE) and red wine/grape juice extract (RWGE) was compared in a five-stage in vitro gastrointestinal model (TWINSHIME). Microbial metabolism of BTE and RWGE polyphenols in the TWINSHIME was studied subsequently in single- and continuous-dose experiments. A combination of liquid or gas chromatography with mass spectrometry (LC-MS or GC-MS) and NMR-based metabolic profiling was used to measure selected parent polyphenols, their microbial degradation into phenolic acids, and the production of short-chain fatty acids (SCFAs) in different colon compartments. Acetate production was increased by continuous feeding of BTE but not RWGE. During RWGE feeding, gallic acid and 4-hydroxyphenylpropionic acid remained elevated throughout the colon, while during BTE feeding, they were consumed in the distal colon, while 3-phenylpropionic acid was strongly produced. Gut microbial production of phenolics and SCFAs is dependent on colon location and polyphenol source, which may influence potential health benefits.
How Short-Chain Fatty Acids Affect Health and Weight
Short-chain fatty acids are produced by the friendly bacteria in your gut.
In fact, they are the main source of nutrition for the cells in your colon.
Short-chain fatty acids may also play an important role in health and disease.
They may reduce the risk of inflammatory diseases, type 2 diabetes, obesity, heart disease and other conditions (1Trusted Source).
This article explores how short-chain fatty acids affect health.
Short-chain fatty acids are fatty acids with fewer than 6 carbon (C) atoms (2Trusted Source).
They are produced when the friendly gut bacteria ferment fiber in your colon, and are the main source of energy for the cells lining your colon.
For this reason, they play an important role in colon health (1Trusted Source).
Excess short-chain fatty acids are used for other functions in the body. For example, they may provide roughly 10% of your daily calorie needs (2Trusted Source).
About 95% of the short-chain fatty acids in your body are:
- Acetate (C2).
- Propionate (C3).
- Butyrate (C4).
Propionate is mainly involved in producing glucose in the liver, while acetate and butyrate are incorporated into other fatty acids and cholesterol (4Trusted Source).
Many factors affect the amount of short-chain fatty acids in your colon, including how many microorganisms are present, the food source and the time it takes food to travel through your digestive system (5Trusted Source).
BOTTOM LINE:Short-chain fatty acids are produced when fiber is fermented in the colon. They act as a source of energy for the cells lining the colon.
One study of 153 individuals found positive associations between a higher intake of plant foods and increased levels of short-chain fatty acids in stools (7).
However, the amount and type of fiber you eat affects the composition of bacteria in your gut, which affects what short-chain fatty acids are produced (8Trusted Source).
For example, studies have shown that eating more fiber increases butyrate production, while decreasing your fiber intake reduces production (9Trusted Source).
- Inulin: You can get inulin from artichokes, garlic, leeks, onions, wheat, rye and asparagus.
- Fructooligosaccharides (FOS): FOS are found in various fruits and vegetables, including bananas, onions, garlic and asparagus.
- Resistant starch: You can get resistant starch from grains, barley, rice, beans, green bananas, legumes and potatoes that have been cooked and then cooled.
- Pectin: Good sources of pectin include apples, apricots, carrots, oranges and others.
- Arabinoxylan: Arabinoxylan is found in cereal grains. For example, it is the most common fiber in wheat bran, making up about 70% of the total fiber content.
- Guar gum: Guar gum can be extracted from guar beans, which are legumes.
BOTTOM LINE:High-fiber foods, such as fruits, veggies, legumes and whole grains, encourage the production of short-chain fatty acids.
Short-chain fatty acids may be beneficial against some digestive disorders.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are the two main types of inflammatory bowel disease (IBD). Both are characterized by chronic bowel inflammation.
Because of its anti-inflammatory properties, butyrate has been used to treat both of these conditions.
Studies in mice have shown that butyrate supplements reduce bowel inflammation, and acetate supplements had similar benefits. Additionally, lower levels of short-chain fatty acids were linked to worsened ulcerative colitis (15Trusted Source, 16Trusted Source).
Human studies also suggest that short-chain fatty acids, especially butyrate, can improve symptoms of ulcerative colitis and Crohn’s disease (17Trusted Source, 18Trusted Source, 19Trusted Source, 20Trusted Source).
Another small study found that butyrate supplements resulted in clinical improvements and remission in 53% of Crohn’s disease patients (18Trusted Source).
For ulcerative colitis patients, an enema of short-chain fatty acids, twice per day for 6 weeks, helped reduce symptoms by 13% (21Trusted Source).
BOTTOM LINE:Short-chain fatty acids may reduce diarrhea and help treat inflammatory bowel diseases.
Lab studies show that butyrate helps keep colon cells healthy, prevents the growth of tumor cells and encourages cancer cell destruction in the colon (24Trusted Source, 25Trusted Source, 26Trusted Source, 27Trusted Source).
Several observational studies suggest a link between high-fiber diets and a reduced risk of colon cancer. Many experts suggest the production of short-chain fatty acids may be partly responsible for this (28Trusted Source, 30Trusted Source).
In one study, mice on a high-fiber diet, whose guts contained butyrate-producing bacteria, got 75% fewer tumors than the mice who did not have the bacteria (33Trusted Source).
Interestingly, the high-fiber diet alone — without the bacteria to make butyrate — did not have protective effects against colon cancer. A low-fiber diet — even with the butyrate-producing bacteria — was also ineffective (33Trusted Source).
This suggests that the anti-cancer benefits only exist when a high-fiber diet is combined with the correct bacteria in the gut.
However, human studies provide mixed results. Some indicate a connection between high-fiber diets and reduced cancer risk, while others find no link (34Trusted Source, 35Trusted Source, 36Trusted Source, 37Trusted Source).
Yet these studies did not look into the gut bacteria, and individual differences in gut bacteria may play a role.
BOTTOM LINE:Short-chain fatty acids have been shown to protect against colon cancer in animal and lab studies. However, more research is required.
A review of the evidence reported that butyrate can have positive effects in both animals and humans with type 2 diabetes (38Trusted Source).
Yet there are fewer studies involving people, and the results are mixed.
One study found that propionate supplements reduced blood sugar levels, but another study found that short-chain fatty acid supplements did not significantly affect blood sugar control in healthy people (46Trusted Source, 47Trusted Source).
BOTTOM LINE:Short-chain fatty acids seem to help regulate blood sugar levels, especially for people who are diabetic or insulin resistant.
Studies have shown that short-chain fatty acids also regulate fat metabolism by increasing fat burning and decreasing fat storage (8Trusted Source).
Several animal studies have examined this effect. After a 5-week treatment with butyrate, obese mice lost 10.2% of their original body weight, and body fat was reduced by 10%. In rats, acetate supplements reduced fat storage (40Trusted Source, 56Trusted Source).
However, the evidence linking short-chain fatty acids to weight loss is based mainly on animal and test-tube studies.
BOTTOM LINE:Animal and test-tube studies indicate that short-chain fatty acids may help prevent and treat obesity. However, human studies are needed.
Many observational studies have linked high-fiber diets to a reduced risk of heart disease.
However, the strength of this association often depends on the fiber type and source (57Trusted Source).
Butyrate is thought to interact with key genes that make cholesterol, possibly reducing cholesterol production (66Trusted Source).
For example, cholesterol production decreased in the livers of rats given propionate supplements. Acetic acid also reduced cholesterol levels in rats (62Trusted Source, 67Trusted Source, 68Trusted Source).
BOTTOM LINE:Short-chain fatty acids may decrease the risk of heart disease by reducing inflammation and blocking cholesterol production.
Short-chain fatty acid supplements are most commonly found as butyric acid salts.
However, supplements may not be the best way to increase your levels of short-chain fatty acids. Butyrate supplements are absorbed before they reach the colon, usually in the small intestine, which means all of the benefits for colon cells will be lost.
Additionally, there is very little scientific evidence about the effectiveness of short-chain fatty acid supplements.
Butyrate reaches the colon best when it’s fermented from fiber. Therefore, increasing the amount of high-fiber foods in your diet is probably a much better way to improve your short-chain fatty acid levels.
BOTTOM LINE:Eating high-fiber foods is the best way to increase short-chain fatty acid levels, as supplements are absorbed before reaching the colon.
Due to their anti-inflammatory and anti-cancer properties, it is likely that short-chain fatty acids have a wide range of beneficial effects on your body.
One thing is for certain: looking after your friendly gut bacteria can lead to a whole host of health benefits.
The best way to feed the good bacteria in your gut is to eat plenty of foods high in fermentable fiber.
There’s a take-off on the industry slogan, “Beef: It’s What’s For Dinner – Beef: It’s What’s Rotting in Your Colon.” I saw this on a shirt once with some friends, and I was such the party pooper—no pun intended—explaining to everyone how meat is fully digested in the small intestine, and never makes it down into the colon. It’s no fun hanging out with biology geeks—but, I was wrong!
It’s been estimated that with a typical Western diet, up to 12 grams of protein per day can escape digestion, and when it reaches the colon, it can be turned into toxic substances like ammonia. This degradation of undigested protein in the colon is called putrefaction, so a little meat can actually end up putrefying in our colon. The problem is that some of the by-products of this putrefaction can be toxic.
It’s generally accepted that carbohydrate fermentation—the fiber and resistant starches that reach our colon—results in beneficial effects for the host because of the generation of short-chain fatty acids like butyrate, whereas protein fermentation is considered detrimental for us. Protein fermentation mainly occurs in the lower end of the colon, when carbohydrates get depleted, and results in the production of potentially toxic metabolites. Perhaps that’s why we see more colorectal cancer and ulcerative colitis lower down, because that’s where the protein is putrefying. The simplest strategy to reduce the degree of potentially harmful compounds by protein fermentation is probably a reduction in dietary protein intake.
But, the accumulation of these harmful byproducts of protein metabolism may be attenuated by the fermentation of undigested plant matter. This study showed that if you give people foods containing resistant starch—starch resistant to small intestine digestion so it can feed our good bacteria down in our colon–foods such as cooked beans, peas, lentils, raw oatmeal, and cold pasta, you can block the accumulation of potentially harmful byproducts of protein metabolism. The more starch ended up in the stool, the less ammonia, for example.
But there’s protein in plants too. The difference is that animal proteins tend to have more sulfur-containing amino acids like methionine, which can be turned into hydrogen sulfide in our colon–the rotten egg gas that may play a role in the development of inflammatory bowel diseases like ulcerative colitis, as I’ve covered previously.
The toxic effects of hydrogen sulfide appear to be mediated through blocking the ability of our colon cells from utilizing butyrate, which is what our good bacteria make from the fiber we eat. So it’s like this constant battle in our colon between the bad metabolites of protein, hydrogen sulfide, and the good metabolites of carbohydrates, butyrate. Using human colon samples, they were able to show that the adverse effects of sulfide could be reversed by butyrate. So we can either cut down on meat, eat more plants, or both.
But there’s two ways hydrogen sulfide can be produced. Though it’s mainly present in our large intestine as a result of the breakdown of sulfur-containing proteins, rotten egg gas can also be generated from inorganic sulfur preservatives like sulfites and sulfur dioxide.
Sulfur dioxide is used as a preservative in dried fruit, and sulfites are added to wines. We can avoid sulfur additives by reading labels or by just choosing organic, since by law they’re forbidden from organic fruits and beverages. Cabbage family vegetables naturally have some sulfur compounds, but thankfully, after following more than 100,000 women for over 25 years, cruciferous vegetables were not associated with elevated colitis risk.
But because of the animal protein and preservative-laden processed foods, the standard American diet may have five or six times more sulfur than a diet centered around unprocessed plant foods, which may help explain the rarity of inflammatory bowel disease among those eating traditional whole food plant-based diets.
More than 35 years ago, studies started implicating sulfur dioxide preservatives in the exacerbation of asthma. This so-called “sulfite-sensitivity” seems to affect only about 1 in 2,000 people, so I recommended those with asthma avoid it, but otherwise I considered the preservative harmless. I am now not so sure, and advise people to avoid it when possible. How could companies just add things to foods without adequate safety testing? See Who Determines if Food Additives are Safe? For other additives that may be a problem, see Titanium Dioxide & Inflammatory Bowel Disease and Is Carrageenan Safe?
For more on the relationship between hydrogen sulfide and inflammatory bowel disease, see my video Preventing Ulcerative Colitis with Diet. More on this epic fermentation battle in our gut in Stool pH and Colon Cancer.
Does the sulfur-containing amino acid methionine sound familiar? You may remember it from such hits as Starving Cancer with Methionine Restriction and Methionine Restriction as a Life Extension Strategy.
These short-chain fatty acids released by our good bacteria when we eat fiber and resistant starches is what may be behind the second meal effect: Beans and the Second Meal Effect.
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