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Jouret C., Pace E., Parfait A. 1990a. L’acide formique composant de l’acidité volatile des rhums. Industries alimentaires et Agricoles 107, 1239 – 1241.
The formic acid component of the volatile acidity of rums
By JOURET C. *, PACE E. *, PARFAITA.**
*INRA – Station expérimentale d’Œnologie de Pech Rouge, Gruissan
**INRA – Antilles Guyanne – Pointe à Pitre Cedex
The dosage of formic acid was carried out on rums of diverse types (agricultural, molasses, grand arôme), aged or not in oak barrels using a specific enzymatic method.
The rates recorded vary according to the origin of the samples and show the influence of several factors: the role of microorganisms and that of the wood of the preservation containers appear to be of importance.
Formic acid is the first acid in the series of fatty acids that make up the Volatile acidity of rums. In the legislative concept of volatile acidity are all the organic acids, in the free state or in the salified state, drivable by water vapor [volatile by steam distillation]; lactic, succinic and sorbic acids, as well as carbon dioxide and sulfur dioxide, are not taken into account in this measure.
Various authors (MAUREL A. et al., 1965; PARFAIT A. and SABINE, 1975) have studied the global volatile acidity of rums, FAHRASMANE L. et al. (1983) and NYKAINEN L and SUOMALAINEN H. (1983) presented quantitative data for various Volatile acids found in sugarcane alcohols. In addition, LEHTONEN H.J. and SUOMALAINEN H. (1977) have demonstrated the presence of a particular volatile acid in rums: 2 ethyl 3 methyl butyric acid.
However, published results on the levels of formic acid in these alcoholic beverages are rare (KERVEGANT D. 1946, TER HEIDE R. 1986) and it therefore seemed interesting to measure this volatile acid in the different types of rums existing on the market to have a better knowledge of the chemical composition of these eaux-de-vie.
We have chosen for its specificity and simplicity, the enzymatic assay technique proposed by the company Boehringer-Mannheim.
The principle of the assay is based on the oxidation of formic acid quantitatively to carbon dioxide, in the presence of formate dehydrogenase (FHD) by nicotinamide adenine dinucleotide (NAD).
The amount of NADH formed is stoichiometric with respect to the oxidation of formic acid. NADH is measured by increasing the absorbance of the medium at 365 nm.
Formic acid level of rums (in mg/l of rum at 50° GL)
We followed the dosing method indicated by the supplier of the Analytical Certificate with some slight modifications to adapt it to the problems of the brandies: on the one hand, by evacuating under a vacuum to half to reduce the volume of the sample; which makes it possible to get rid of volatile substances interfering with the assay (excessive ethanal, ethyl formate, formaldehyde, etc.); on the other hand, rums aged in barrels and thus colored were diluted to half or quarter of the intensity of their coloring before the vacuum evaporation operation. This is followed by a charcoal treatment (50 mg for 5 ml) and a fine membrane filtration.
Reproductivity is 5% in the worst cases.
Results and discussion
The assays were carried out on the different types of rums from the French West Indies of well-known origin: rums made from cane juice, rums from molasses, white and aged in oak barrels, grand arôme rums (obtained from a particular fermentation medium), as well as samples from an experiment on rums wooded and stored for 3 years in spent oak barrels.
The results expressed as mg of formic acid per liter of rum at 50° GL were collated in the following table.
It can be noted, very generally, that white agricole rums are poor in formic acid: 1.3 mg/l on average; the white rums of molasses are already richer with amounts of the order of 3.5 mg/l and this increase in the formic acid content is important for rums aged in oak barrels, as well as for white grand arôme rums. There are, however, some exceptions to this finding.
These quantitative differences can be explained by the multiple pathways of the biological formation of formic acid and, of course, by the technological processes used in the elaboration of different rums.
We can, therefore, question, first of all, the richness in formic acid of the various raw materials used in the manufacture: juice of sugar cane, molasses, vinasse [dunder, stillage].
Then, the influence of the microorganisms involved in the transformation; if the yeasts produce practically no formic acid from the sugars during a normal alcoholic fermentation, on the other hand, different microorganisms can degrade the sugars or other substrates present in the fermentative medium (AHRENS I., DIZER H. 1978) to give formic acid.
During the elaboration of rums, the health status of sugar canes (effect of pre-harvest burning, the time elapsed between cutting and implementation, etc.) as well as the conditions for obtaining and preserving molasses and vinasses; the state of maintenance of the premises and equipment; the microbiological quality of the water used for the extraction of sugar or the dilution of molasses; fermentation techniques (seeding, temperature control, …) lead to considerable variations in the composition of the fermentation flora and hence to the quality of the rums (GANOU – PARFAIT B. 1984, GANOU – PARFAIT B., FAHRASMANE L. et al 1989).
Also involved are the physicochemical phenomena that occur during the more or less advanced heating of carbohydrate substances in obtaining molasses or during distillation (SUGISAWA H. 1966, COTTIER L. et al., 1989). Vinasse, used only in certain rum fabrications or for grand arôme rums, is a bottoms product and, consequently, is richer in polar compounds, especially organic acids.
Finally, during the storage in barrels, several physicochemical phenomena known for the aging of wood spirits can be taken into account; the concentration of polar components resulting from the evaporative loss of volatile compounds as well as the oxidation of alcohols to acids. These two elements must, of course, play a reduced role, given the average volume losses of 5% per year and the low level of methanol existing in the rums. The role of wood appears, a priori, more important. In fact, the barrels undergo, during their manufacture, a heating for the bending of the staves and often a charring of the inner wall. The resulting degradation of carbohydrate elements gives furfurolic components and formic acid. This path is more or less active depending on the degree of charring and the stage of use of the barrel.
Depending on the situation, several of the elements thus rapidly defined may be added to explain the general observation and the special cases.
Thus, white agricole rums have reduced levels of formic acid because cane juice contains little. For No. 4 and No. 14 samples and to a lesser extent No. 13 and No. 10, the significant enrichment in formic acid was certainly bacterial in origin because the water required for their preparation was derived from a water table that became brackish and loaded with various microorganisms after a very dry period.
White rums from molasses contain more formic acid because the raw material used provides more for various reasons (concentration, heating, preservation). No. 4, particularly rich, is a rum having had a manufacturing accident due to bacteria present in the water. No. 14 and 15 are rums that have been rectified during distillation to reduce non-alcohol components. The degree of distilling between 92° GL and 93° GL makes it possible to eliminate, in particular, polar elements such as acids.
Apart from the grand arôme white rums whose rich formic acid is explained by their conditions of preparation (molasses with the addition of vinasse, yeast and bacterial flora complex), the rums aged in barrels are loaded with formic acid : No. 9 and No. 10 agricole rums and No. 4 molasses rum have relatively low levels as they have been kept for only 6 months in barrels and have been classified here as “Old” to facilitate the presentation of the results. The other samples have at least 3 years of wood storage, which is legally the minimum period to qualify as Old rum.
We have, moreover, a more precise idea of the increase of formic acid in the rum due to the preservation in barrel with the experimentation concerning the addition of boise [wood extract]. The example, white rum, is a mixture of agricultural rum, and two rums derived from molasses, one with a low non-alcoholic coefficient, less than 100 g/hl / AP [pure alcohol], the other with a high non-alcoholic coefficient, more than 225 g/hl / AP. It contains 1.80 mg/l of formic acid. It reaches, after 3 years, a rate of 11.50 mg/l. Addition of boise at the start causes an increase in the rate to 4.40 mg/l, the two batches of boise rums reach 12.60 mg/l after 3 years of storage. The barrels used in this test had already been used for several years to try to avoid an excessive interaction of the polyphenolic compounds of the barrel compared to those of the boise.
In a very general way, we can say that the rate of formic acid in rums remains in the range of figures found for other eaux-de-vie, white or aged: the composition of the raw material, thermal degradation of substances carbohydrates, as well as the concentration of acids due to the evaporation of less polar volatile substances, the equilibrium formic acid / ethyl formate, the oxidation of methanol present in the initial brandy, the role of wood during barrel preservation are biochemical and physicochemical factors that play a significant role in the formic acid composition of beverage alcohol.
However, the intervention of various microorganisms, other than yeasts, can lead to a significant increase in the formic acid content of white rums. If a specific microbial activity is desired for obtaining the very strong rums, that are the rums grand arôme, other uncontrolled bacterial interventions give alcohols with defects more or less serious. The determination of the level of formic acid can then be presented as a complementary element of appreciation of the conditions for the production of a rum and the evaluation of its quality.
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