fermentation is the biotransformation of organic substances via enzymes by microorganisms such as yeast and bacteria. It is widely recognized that the practice of fermentation is as old as civilization itself, but it is also divided into the epochs of pre- and post-Pasteur, after the French microbiologist, Louis Pasteur (1822–1895), who, assisted by a host of other names in the mid-nineteenth century, started elucidating the science of fermentation, which had been previously approached only empirically. Originally, Pasteur narrowly defined fermentation as an anaerobic process, but since then broader definitions have been accepted that also account for aerobic processes and are more helpful to understanding the breadth of spirits production. Arguably, the most important fermentation process in this context is of course the transformation of sugar to ethanol, but myriad other processes are observed that have significant bearing on both spirit quality and economy.
In 1883, Emile Christian Hansen was the first to isolate a pure yeast cell, Saccharomyces carlsbergensis, famously used in lager beers. Rapidly following that achievement, yeasts and other microorganisms were collected and cataloged from around the world.
Not all raw materials used in spirits production have readily fermentable sugars, and in the case of grains the process of saccharification must take place in which starch is broken down into fermentable sugars. See saccharification. In the Western tradition, this is facilitated by enzymes in malt, while the Eastern tradition has for centuries also employed the amylo process of starch hydrolysis performed as a type of fermentation itself, using the enzymatic action of molds like koji, also contained in qu. See qu. The amylo process has seen some limited use in American whisky production, starting as early as 1914. Widespread adoption has been hindered by the tendency such ferments have to emit aromas not congruent with whisky and a general lack of modern exploration outside of purified enzymes.
Alcoholic fermentation sees the yeast cells eating any available sugar and excreting primarily liquid ethanol and CO2 gas, but there are many additional congeners that are byproducts of yeast metabolism. Yeast is capable of producing aldehydes, various acids, esters, and higher alcohols, among others, with each species having its own propensity. See aldehydes. Under narrow conditions, select yeast strains are capable of releasing extremely high-value aroma derived from carotene but bound as glycosides, such as the rose ketone β-Damascenone. Understanding this aroma, which is only present in the minutest quantities but extremely odor active, is at the forefront of the art.
Except under the strictest of pure culture conditions, bacteria are also present alongside yeast, and their own fermentation may have significant influence on flavor. Bacteria are capable of being either aroma-beneficial or detrimental spoilage organisms and may produce substantial congeners like nonvolatile lactic acid at the expense of potential alcohol. Other types may produce volatile propionic or butyric acid from a variety of inputs, which have the potential to become pleasant-smelling esters under narrow conditions but are off-putting in their free state. If vinegar fermentation by aerobic bacteria is allowed to take hold, ethanol is broken down to acetic acid, decreasing yield. Byproducts of bacterial fermentation may help select for a yeast type in open-culture conditions or even halt alcoholic fermentation altogether, as many budding yeasts are sensitive to volatile acids.
Fermentation may be initiated via inoculation with pure cultures of yeast and bacteria under strict sanitary conditions or by varying degrees of open-culture technique, which is generally associated with finer spirits and traditional processes. Pure culture technique often requires antibiotics, which come with the risk of bacterial adaptation rendering them no longer effective, while fully open-culture techniques termed spontaneous often rely on either a high natural yeast load such as with fruit or high acid conditions created by recycling stillage where bacteria are mostly inhibited to a degree they cannot suppress alcoholic fermentation. See dunder. Muck may also be added because it contributes acids inhibitory to spoilage organisms but not to the desired yeast. Baijiu production often features a solid-state fermentation phase before the water content is increased to harness particular bacteria and yeast that may only be active at a particular level of free water. See rum, Jamaica; and; baijiu.
Open-culture ferments embrace gradual change and adaptation, while pure cultures are often strictly serialized so there is less opportunity for genetic mutations to cause a drift in the flavor. Eastern fermentation techniques revolving around starting a fermentation with qu are holistic and open culture, while the overwhelming impact of Pasteur was for Western technique during the twentieth century to gradually be dominated by single organisms in overly sanitary conditions. Currently, the spirits industry is seeing a return to open-culture techniques guided by better understanding of the complex mechanisms as well as the breeding of designer As a rule of thumb, a longer fermentation duration produces more aroma and is typically distilled in a way to take advantage of it. This duration before a spirit is distilled mostly encompasses alcoholic fermentation, but it may also come after a production stage where bacteria were used for biotransformation such as the souring of grains in the sour mash whisky process, or amylo process saccharification. As alcoholic fermentation drags on, risk escalates, bacterial fermentation processes gain in activity, and either aroma improves to offset the change in economy or off aromas develop. Bourbon ferments may last anywhere between 72 and 120 hours, while cognac may have a duration between two and three weeks. Rum and baijiu see the most varied duration. Light rums can ferment as quickly as twenty-four hours, while the heaviest rum ferments last multiple weeks and justify slow-batch distillation by pot still. Baijiu has durations as short as five days, while some are reported to last multiple months. The chief method of manipulating alcoholic fermentation duration is altering the quantity of yeast present followed by the amount of sugar to be fermented and then other potential stressors. Ferments for neutral spirits are inoculated with specially selected pure cultures and conducted in closed vessels to limit foreign yeasts and bacteria that may harm economy. Bourbon employs a pure yeast, while its lactic acid bacteria culture is either pure or sometimes open culture. Bourbon is typically fermented in open wooden vats that allow very slight opportunity for adventitious microorganisms despite the swift pace of fermentation. Fruits, employed in brandy making, arrive covered in native yeasts and bacteria, which can quickly start vigorous spontaneous fermentation; however, depending on the demands of economy and risk, pure-culture yeasts may be added to reduce the fermentation duration. Many rum ferments are conducted similarly to neutral spirits, while the heaviest rums are known to encourage any yeasts or bacteria that can tolerate the inhibitory conditions that develop in the various pre-fermented segments that compose the main ferment. Heavy rums typically encourage open culture, but rarely do the raw materials have a yeast load comparative to fruit to truly start spontaneously, and instead each subsequent ferment is seeded with a portion of the last. The diversity of practices is vast, and many stretch the bounds of what many would think possible. Citrus rinds have been used to create starters at the beginning of the cachaça season, possibly accelerating yeast growth, while the early bourbon donas were reported to use ample amounts of hops as an antibacterial. Some Jamaican distillers deliberately add cane-juice vinegar to heavy-rum ferments to help maintain the dominance of fission yeasts, which can metabolize a portion of the acetic acid. Sea water is added to create a particular rum mark in Barbados, which distinctly influences the congener profile. Some of the most modernist ferments immobilize yeast in alginate beads to protect them from stress and facilitate continuous fermentation. Fermentation organisms may be just as much an avenue for terroir as the raw materials themselves. Production decisions may help promote local voices that otherwise would never get to express themselves because they have a low frequency of occurrence in nature. Narrow fermentation conditions of which only certain yeasts are capable may also show the uniqueness of raw materials where much aroma needs to be unlocked by a biotransformation, and this is most apparent with carotene-derived aroma. Due to the nature of volatility, the fate of extraordinary terroir-related aroma is tied to ordinary congeners like fusel oil and cannot be captured in the final distillate if excess fusel oil is not kept to the minimum. Fermentation has a profound impact on aroma and spirit economy, which will influence subsequent production decisions. Not all ferments are fit for batch distillation, which is far less efficient than continuous distillation; the character of the ferment must justify it. A spirit worthy of barrel maturation is a product just as much of fermentation and which microorganisms were allowed to develop as of how it was distilled. Amerine, M. A. The Technology of Wine Making. 4th ed. Westport, CT: AVI, 1980. Arroyo, Rafael Studies on Rum. Agriculture Experiment Station Puerto-Rico, Research Bull. no. 5, 1945. Greg, Percival H. The Jamaica Yeasts. Bulletin Botanical Department, Jamaica (2) II, 157–160, 1895. Kervegant, D. Rhums et eaux-de-vie de canne. Vannes: Ed. du Golf, 1946. de Kruyff, E. De fabricatie van Batavia-arak. Archief voor de suikerindustrie 151, 1909. Willkie, H, and J. Prochaska. Fundamentals of Distillery Practice. Division of Education, Joseph E. Seagram & Sons, 1943. By: Stephen Shellenberger