Module 6 - Substances that compliment the activity of sulfur dioxide Flashcards
What is the main inhibitory activity of sorbic acid?
The main inhibitory activity of sorbic acid and its salts is against yeasts and
moulds.
What yeasts has sorbic acid been found to be an effective inhibitor against?
What about bacteria?
It has been found to be an effective inhibitor against most yeast genera such as Saccharomyces,
Brettanomyces, Candida and Pichia.
Bacteria are much less affected by sorbates although Acetobacter has been shown to be inhibited to
some extent.
How does sorbic acid inhibit yeast?
As with sulfur dioxide, this is believed to be related to the greater ease of penetration of the undissociated form through the cell membrane. Sorbic acid then is able to prevent unwanted yeast growth by inhibition of the yeast’s dehydrogenase system.
How does pH influence the activity of sorbic acid?
As also with sulfur dioxide, the proportion of acid that is dissociated is pH dependent. Lower pH values increase the proportion of the acid in the undissociated form.
However unlike sulfur dioxide, the pK of sorbic acid is sufficiently high (4.76) that in the usual wine pH range (3.0 to 4.0) more than 90% is in the active undissociated form.
Low pH conditions are also beneficial in that they intrinsically inhibit the rate of microorganism growth, thereby allowing the same degree of antimicrobial activity to be gained with smaller amounts of sorbic acid.
What is the legal limit of sorbic acid in wine?
The Wine production requirement section (4.5.1) of Australian and New Zealand Food Standards Code states that wine must contain less than 200 mgL-1 of sorbic acid.
Why do further precautions (avoidance of microbial contamination and sterile filtratrion) still need to be maintained when using sorbic acid?
Although sorbic acid is usually effective against yeast growth in wine at levels of less than the legal limit of 200 mg L, it must be remembered that a diversity of yeasts can exist.
For instance Zygosaccharomyces spp., a sulfur dioxide tolerant yeast that is typical of refermentations in wine, has a resistance to sulfur dioxide that is matched by its resistance to sorbic acid. Sorbic acid levels of up to 300‑700 mg L (well above the Australian legal limit)
may be required in wine to prevent refermentation by Zygosaccharomyces spp.
For this reason, use of sorbic acid
must be in balance with other precautions, such as avoidance of microbial contamination and sterile filtration wherever appropriate.
At what level is sorbic acid detectable to sensitive and typical tasters?
How does it effect the palate?
Sensitive tasters to sorbic acid have been shown to have a detection threshold of approximately 130 mg L
However most people are not usually able to detect the presence of sorbic acid until levels of 300‑400 mg L are reached.
An accentuation of astringency and bitterness is commonly associated with sorbic acid at these levels.
How is ethyl sorbate formed? How is the aroma of ethyl sorbate ester described?
Sorbate has been shown to undergo direct esterification with the wine ethanol to form ethyl sorbate. The off–odour of the ethyl sorbate ester has been described as pineapple-celery, and has a threshold of 0.3mg L-1.
How can the presence of sorbic acid cause the geranium odour in wine?
With growth of lactic acid bacteria in wine containing sorbic acid, there is development of an unpleasant,
abnormal and persistent geranium‑like aroma. This is due to a sequence of chemical changes brought about by the bacteria, in which sorbic acid is converted to aroma intense alcohols or their ethyl
ethers.
Why should sorbic acid never be used when a malolactic fermentation might occur?
These very low detection thresholds for sorbinol and the ethoxyhexadiene indicate the inherent danger in the use of sorbic acid.
Even slight bacterial activity can produce the geranium‑like off‑odour. When using sorbic acid, growth of lactic acid bacteria must be avoided. This requires that sorbic acid never be used when a malolactic fermentation might occur.
In turn this requires that it only be used in combination with sulfur dioxide, an effective bactericide, and only when continued presence of molecular sulfur dioxide can be guaranteed.
Why is the use of sorbic acid problematic in red wines?
Anthocyanin pigments effectively bind sulfur dioxide and reduce free sulfur dioxide, and consequently ‘true’ molecular sulfur dioxide, to levels which cannot guarantee bacterial stability.
Why is potassium sorbate used instead of sorbic acid?
The relatively non‑polar carbon chain attached to the carboxylic acid group of sorbic acid reduces the water
solubility of the molecule, so that the free acid is only slightly soluble.
Due to the low solubility of sorbic acid in water, it is usually added in the form of potassium sorbate.
The solubility of potassium sorbate in water at 20º C is 58 %(w/v) compared with that of sorbic acid of 0.16 %
(w/v).
Why is it important to use freshly prepared solutions of sorbic acid?
In solution the acid gradually turns yellow and loses activity through oxidation.
Why is it important to add sorbic acid solutions slowly and while thoroughly mixing?
When sorbic acid is added to wine, the acidity of the wine immediately returns the acid salt to the poorly soluble undissociated acid.
Unless the addition is slow, with thorough mixing, there is a risk of its concentration at the point of addition being too great for it to remain in solution.
Once precipitated, the acid will redissolve only very slowly.
What is ascorbic acid?
Ascorbic acid, commonly known as vitamin C, is an organic substance that is very readily oxidised (i.e.
it very readily reduces the oxidation state of other substances).
Why does the presence of ascorbic acid in a wine suggest it has been added?
It occurs naturally in the grape berry in the juice at concentrations of between 5-150 ppm.
This level decreases during fermentation, and losses also occur through air contact in processing; consequently it is usually absent in wine.
Why can’t ascorbic acid replace sulfur dioxide?
- The reaction of ascorbic acid with oxygen produces the vigorous oxidising agent hydrogen peroxide
Free sulfur dioxide must be present to destroy the hydrogen peroxide; - Its oxidation produces dehydroascorbic acid. This can readily degrade to products including
xylosone, 2-3-diketo-L-gulonic acid and furfural. These compounds are able to participate in reactions that
result in undesirable yellow coloured compounds being produced. Free sulfur dioxide is necessary to prevent
this colour development by binding to the carbonyl functionality of dehyroascorbic acid and its degradation product; - It lacks the ability of sulfur dioxide to inhibit oxidative enzyme activity, to bind acetaldehyde, and to inhibit
yeast and bacterial growth.
How is ascorbic acid used as an oxygen scavenging agent?
To a greater extent than other substrates in juice or wine, ascorbic acid reacts readily with dissolved oxygen gas and can remove oxygen before it is able to oxidise other substrates.
A detrimental aspect is that the process is similar to the oxidation of phenols whereby hydrogen peroxide is
generated. The latter is a very vigorous oxidising agent capable of oxidising ethanol, phenolic compounds and, probably, some flavour compounds. Adequate levels of free sulfur dioxide (20‑35 ppm) are necessary to efficiently destroy it.
How is ascorbic acid used as a reducing agent?
Ascorbic acid helps to maintain juice or wine in a reducing condition by direct reaction with some components. An example is the reduction of quinones (oxidised phenolic compounds) back to the original phenolic form.
What are some other benefits of using ascorbic acid?
In wine, ascorbic acid is generally reported to maintain a freshness and fruitiness of flavour in dry or sparkling white wines.
The success of ascorbic acid is dependent upon the variety of grape and the style and type of wine being
considered.
What wines is it most suited too?
It is most suited to those varieties with delicate fruit flavour and to wines that will be drunk young and fresh. It is least suited to varieties that are rich in fruit character and to wines that will develop well in wood or in bottle. In these instances ascorbic acid can sometimes retard the wine’s development.
Why should you not use ascorbic acid in red wines?
The requirement that it be used in the presence of adequate free sulfur dioxide makes it unsuitable to red wines where very low levels of free sulfur dioxide are present due to pigment binding.
How can the use of ascorbic acid help juice?
What concentration is normally recommended?
- limit the effects of enzymatic oxidation. - It does not inhibit the oxidase enzymes,
but it may help to reduce the dissolved oxygen content that is available to the enzymes. - It can also offset some effects of oxidation, for instance by reducing quinones back to phenolic compounds,
Concentrations of 100 ppm or more are usually recommended for white juice provided that there is always adequate free sulfur dioxide.
