Feedstocks, fermentation and distillation for production of heavy and light rums 243

Chapter 16

Feedstocks, fermentation and distillation forproduction of heavy and light rums

J.E. MurtaghMurtagh & Associates, Winchester, Virginia, USA

Introduction

The US Bureau of Alcohol, Tobacco and Firearmsregulations (1982) define rum as: �an alcoholicdistillate from the fermented juice of sugar cane,sugarcane syrup, sugarcane molasses or othersugarcane by-products, produced at less than190o proof (95o GL), in such a manner that thedistillate possesses the taste, aroma andcharacteristics generally attributed to rum, andbottled at not less than 80o proof (40o GL), andalso includes mixtures solely of such distillates�.

It should be noted that unlike the USregulations for whisky, the regulations for rumdo not include any requirement that it be agedin oak barrels for a minimum period of time. TheUS regulations also do not specify any geo-graphic region in which rum may be produced,however the British regulations define rum as:�a spirit distilled directly from sugarcane productsin sugarcane-growing countries�.

The regulations are rather vague of necessityowing to the very wide variety of rums rangingfrom the light, nearly neutral, continuous stillproducts to the heavily flavored, dark, navy-typepot still products. The diversity of rum types andtheir production methods makes comprehensivecoverage of the industry almost impossible for a

publication such as this. Thus, readers shouldconsult the references for additional information.What are now considered the classic studies onrum production were published by Rafael Arroyoof Puerto Rico in the 1940s (see reference list).That was at a time when the island�s rum industryincreased production very significantly in orderto meet the demand arising from a wartimeshortage of whisky and grain spirits. Other morerecent studies and helpful reviews of the rumindustry have been published by Clutton (1974),Ianson (1971), Kampen (1975), Lehtonen andSuomalainen, (1977) and Paturau (1969).

Feedstocks: fresh sugarcane juice ordiluted molasses

Cane juice

Using cane juice for rum production has bothadvantages and disadvantages. The mainadvantage is that no additional processing isrequired. The cane is simply crushed and thejuice fed directly into the fermenters. Anotheradvantage is that cane juice does not have a highcontent of dissolved salts and therefore does not

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cause as much scaling and blocking of distillationcolumns as may occur using molasses.

One of the main disadvantages of using canejuice is that it normally contains only about 12-16% w/w sugar, so that the alcohol content ofthe fermented material is limited to about 6 -8%v/v as compared to 10-13% v/v obtainable frommolasses. Another significant disadvantage is thatcane juice cannot be stored in bulk for anyconsiderable length of time. Heavy contam-ination with bacteria and yeasts causes it to startto ferment spontaneously and very rapidly. Thismeans that the distillery must be located in closeproximity to the cane mill. It also means that thecane juice is only available during the caneharvesting season, which generally does notexceed six months per year. Thus, some rumdistilleries may use cane juice during the harvestseason and molasses for the remainder of theyear. Cane juice may also have costdisadvantages in that it is a primary, or at least anintermediate product in the production of sugar.In contrast, molasses is generally a by-product,considered to be of lower value.

Cane juice normally becomes heavily infectedwith bacteria and yeasts in the crushing process.Pasteurization prior to fermentation is usuallyinfeasible due to the large amount of suspendedfiber in the juice, which would tend to block mostheat exchangers. Thus the contaminatingorganisms may significantly reduce alcohol yieldsfrom cane juice.

Molasses

The use of molasses as a feedstock for alcoholproduction has been covered in an earlierchapter in this volume; therefore only a fewcomments are necessary with specific referenceto rum.

The source of the molasses can have a stronginfluence on the aromatic quality of rum. Thishas been demonstrated in trials on making rumfrom beet molasses where it was found that itwas not possible to obtain the same characteristicaromas as obtained from cane molasses (Arroyo,1948b). Arroyo (1941, 1942) reported that fresh

blackstrap molasses with its low viscosity, hightotal sugars, nitrogen, phosphorus and a low ashand gum content was preferable in theproduction of rums with desirable odors andtastes.

Fermentation methods

Heavy rum

Just as there are many different types of rum,there are many different methods of ferment-ation. In the production of heavily flavored rums,cane juice or diluted molasses mash may beallowed to ferment spontaneously using yeastspresent naturally in the feedstock. This isrelatively inefficient in terms of alcoholproduction; and the results may vary. However,it is the high level of bacterial contamination inthese fermentations that produces many of thedesirable congeners such as acids and esters.In some processes, e.g. in Jamaica, thespontaneous fermentation may be assisted bythe addition of �dunder� at the start offermentation. Dunder is old stillage that has beenstored in open tanks to allow development of astrong bacterial flora.

To attempt a more controlled fermentationin production of heavy rums, a pure culture ofyeast may be used together with a pure bacterialculture. A specially selected strain of yeast,usually Saccharomyces cerevisiae, which is abudding yeast, or possibly other species such asthe fission yeast Schizosaccharomyces pombe,is propagated from an agar slant. This culture istransferred every 12-24 hrs through a series ofErlenmeyer flasks each holding increasingamounts of sterilized, diluted molasses mashsupplemented with malt syrup or other sourcesof nutrients. When about 20 liters of yeast culturehave been produced, it may be used to inoculateabout 200 liters of diluted molasses mash in asmall plant propagator or �prefermenter�. Thecontents of the vessel are aerated, and afterabout 8-12 hrs of propagation are used toinoculate a larger prefermenter containing about2,000 liters of mash. This vessel may also be

Feedstocks, fermentation and distillation for production of heavy and light rums 245

aerated and fed incrementally with more dilutedmolasses medium until it contains the equivalentof about 10% of the capacity of the plantfermenters.

It is generally considered desirable to have aminimum of about 50 million yeast cells/ml ofmedium when the fermentation has beencompleted. Thus, after allowing for yeastpropagation during the filling process, the 10%yeast inoculum should have a cell count of about200 million cells/ml to achieve this objective.

A pure culture of bacteria such as Clostridiumsaccharobutyricum may be added after 6-12 hrsof the yeast fermentation. Usually the bacterialinoculum amounts to about 2% of the fermentercapacity; and the pH of the fermenting mash isadjusted upwards to about pH 5.5 before additionto give more suitable conditions for the bacterialpropagation. The bacteria produce a mixture ofacids, predominantly butyric, together with otherssuch as acetic, propionic, and caproic acids.These acids in turn react with the alcohol toproduce desirable esters.

Fermentation for light rums

In light rum production, the emphasis is generallyon maintaining clean, rapid fermentations tominimize development of undesirable congenersand to maximize fermentation efficiency. Purecultures of yeast may be propagated as prev-iously described. These may be accompaniedby the use of antibiotics such as penicillin orbactericides such as chlorine dioxide, ammoniumbifluoride or quaternary ammonium compoundsto control bacterial contamination.

Some plants use �mother yeasting�. In thissystem a pure yeast culture is propagated in theprefermenter. The pH is lowered to about 3.7to reduce bacterial growth; and up to 90% ofthe yeast volume is used to inoculate a fermenter.The prefermenter is refilled with mash to repeatthe process several times.

In other plants, the yeast may be recycledfrom one fermenter to another. This is done bycentrifuging the yeast out of the fermented beerprior to distillation and then subjecting it to an

acid washing at pH 2.2-2.5 to kill most of thecontaminating bacteria. As an alternative to acidwashing, the yeast may be treated with about 15ppm chlorine dioxide at a pH of about 3.5. Theobjective of yeast recycling is to save on sugarthat would otherwise be needed for yeast growthand to ensure a very high cell count in thefermenter inoculum. This provides a rapidfermentation in which bacteria lack sufficientopportunity to get well established.

Commercial dry yeasts may also be used inlight rum fermentations. A large quantity of dryyeast may be added to the molasses mash at thestart of fermenter filling. More commonly, inorder to save cost a smaller quantity of the dryyeast may be rehydrated and propagated forseveral hours in a prefermenter before transferto the fermenter.

Distillation

The method of distillation used has a consid-erable effect on the nature of the rum product.Heavy rums are usually produced by batchdistillation, while light rums are normally prod-uced by continuous distillation.

Batch distillation: heavy rum

Batch distillation may be performed with varioustypes of equipment. The simplest form is a single,simple pot still. Here, the fermented beer istransferred into a copper tank or �pot�, as shownin Figure 1. The pot is heated either by an internalsteam coil (calandria) or by a fire of wood orbagasse underneath, usually in a brick enclosure.The pot is fitted with a vapor pipe which leads toa condenser coil immersed in a water tank. Asthe beer is heated, the alcohol and other volatilecongeners are distilled off, condensed and runinto a storage tank. Usually the first fractiondistilled contains much of the more volatile,pungent �heads� congeners and is discarded asthe �heads cut�. The process is continued untilmost of the alcohol has been distilled out of thebeer.

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Figure 1. Simple pot still for heavy rum production.

Feedstocks, fermentation and distillation for production of heavy and light rums 247

The beer residue, or �stillage�, is emptied out ofthe pot and the distillate is returned from thestorage tank to the pot to be redistilled toincrease the proof. A heads cut may again bediscarded together with a �tails cut� taken towardthe end of the distillation. The �center cut�, whichis the bulk of the distillate, has the more pleasantaromas of fruity esters characteristic of rum.Normally, there are only two pot distillations, butthere may be a third if desired.

Two or three pot stills may be interconnected,as shown in Figure 2, to eliminate the need totransfer the distillate back into the first pot forredistillation and to conserve energy. Theefficiency of the pot still in separating congenersmay be improved considerably by includingperhaps five or six bubble cap trays between thepot and the vapor pipe, and a simple condenserabove the trays to provide some reflux as shownin Figure 3. This is the design of pot still mostfrequently used by small, on-farm producers ofcommon tafia and aguardiente rums. It is a simpleprogression from this pot still to the batch kettleand column still used by larger scale rumproducers, as shown in Figure 4.

In some countries crude rum or aguardienteis produced on a small scale by numerousfarmers using pot stills. Their product may thenbe sold to companies that redistill it in a batchsystem to standardize quality for sale as heavyrum or for blending with neutral or nearly neutralspirits to make light rums.

Continuous distillation: light rum

Continuous distillation is usually confined to theproduction of light rums. A two column beerstill with a Barbet type prestripper, as shown inFigure 5, is normally preferred as it permitsremoval of heads, volatile sulfur compounds andsome other undesirable fractions before the bulkof the alcohol enters the concentrating column.(Operation of this type of beer still is discussedin the chapter on neutral spirit production).

When a lighter or less flavorful rum than canbe produced on a simple two column beer still

is required, an extractive distillation column anda rectifier as employed in neutral spirit productioncan be used (Figure 6).

Combinations of batch and continuousdistillation

There are many possible variations and comb-inations of distillation processes. For example,crude pot still distillates may be put through asimple continuous distillation unit to produce amore acceptable, standardized product. Arroyohas described two techniques for combinationsof continuous and batch distillations to produceboth heavy and light rums at the same time. Inone system Arroyo (1948c) suggested that thebeer first be distilled in a single continuouscolumn with only about five concentratingplates. Provision for fusel oil removal allowssome control over the fusel content in thedistillate, which has a proof of about 70o GL. Hethen recommended that the distillate be dilutedto about 40o GL and be submitted to a batchdistillation.

The distillate from the batch column shouldbe collected as five separate fractions based onmonitored odor variations and rising distillationtemperature. Arroyo found that the first fraction,which distilled between 69 and 72oC at a proofof about 91o GL, represented about 5% of thetotal distillate volume and consisted ofunpleasant aldehydes, organic acids and esters.He recommended this fraction either bediscarded or held for subsequent reprocessingto recover some of the ethanol. The secondfraction, distilling over a temperature range of72-77oC at a proof of 93-94o GL, representedabout 10% of the total distillate volume andcontained ethanol with appreciable amounts ofaldehydes and esters. The third fraction, whichdistilled at a fixed temperature of 78oC and aproof of about 95.5o GL, was the largest involume at 55-60% of the total and containedmostly ethanol with very small amounts ofcongeners such as aldehydes, esters and higheralcohols. It also contained the lowest

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Figure 2. Triple pot still system for heavy rum production.

Feedstocks, fermentation and distillation for production of heavy and light rums 249

Figure 3. Pot still with bubble cap tray section.

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Figure 4. Batch still for rum production.

Feedstocks, fermentation and distillation for production of heavy and light rums 251

Figure 5. Modified Barbet beerstill for light rum production.

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Figure 6. Four column distillation and rectification system for the production of light rum.

Feedstocks, fermentation and distillation for production of heavy and light rums 253

concentration of volatile organic acids. Thefourth fraction was collected at a distillingtemperature range of 78.5-85oC at a proof ofabout 90o GL. This fraction was characterizedby the presence of most of the higher alcoholsfrom the original continuous still distillate togetherwith more esters, aldehydes and acids than inthe previous fraction. The fifth and final fractionwas collected over a distilling temperature rangeof 85-90oC and had a much lower proof thanthe other fractions (25-30o GL). This fraction wasopalescent or turbid due to the presence of someof the highest boiling point esters and aldehydeswhich are more soluble in ethanol than in water.

Arroyo then recommended mixing the lastfour fractions in various proportions to makeboth light and heavy rums. He quoted anexample of using 25% of the second fraction,50% of the third fraction, 40% of the fourthfraction and 15% of the fifth fraction to make alight rum while using all the balances to make aheavy rum and avoid wastage.

In another combined process, Arroyo(1949b) suggested that the various congenersidestreams removed in the production of verylight rums or neutral spirit by continuousdistillation should be mixed, diluted with waterand subjected to a fractional distillation almostidentical to that used in the previous process.The distillate should be split into a similar set offive fractions with the first discarded, the thirdmixed with the light rum and the other threefractions used in various proportions forblending heavy rums. Arroyo claimed that theserums matured more rapidly than normallyproduced heavy rums when aged in oak barrels.As such there was a saving on warehouse spaceand other costs.

Rum maturation and blending

The normal and most reliable method of maturingrum to make it more suitable for consumption isby aging in oak barrels. Light rums with very lowlevels of congeners may require very little agingand may be acceptable after just a few weeks in

barrels. In fact, many of the currently popularlighter rums are not aged at all. The heavy rums,however, tend to require much more aging tobecome palatable and may be held in barrels forfive years or longer.

The barrels normally used for rum aging areobtained from the US, where governmentregulations specify that bourbon whiskey mustbe aged in new oak barrels thereby creating alarge supply of once-used barrels. The bourbonbarrels are charred on the inside and are usedfor aging rum either with the char intact or afterits removal, depending on the preference ofindividual distillery.

Oak wood has a strong influence on thematuration and ultimate flavor of the rum, largelydue to the extraction of compounds such astannins and calcium salts and colorings such asquercitine. The type and quantity of theextracted compounds depend largely on thelength of time and temperature of aging, theproof of the rum, the type and former usage ofthe barrel and the amount of contact surface itoffers. New oak barrels, or fresh, once-usedbourbon barrels naturally have more extractablecompounds than old barrels that have been usedmany times over several years. Thus, a light rumstored in new or once-used barrels for a year ortwo may emerge with much of the odor andtaste of a bourbon whiskey, as the maincharacteristics of bourbon are derived from theoak wood. This odor and taste may or may notbe desirable. A program should therefore beestablished to identify individual barrels and codethem as to history of usage in order to selectparticular types or ages of barrels for particularrum products. For example, in countries requiringthat rum be aged in wooden barrels for aminimum period of time (which may be 1-3years), light rums should be aged in the oldestavailable barrels as these will contribute the leastamount of flavor. Even then, it may be necessaryto treat the aged product with activated charcoalto reduce the color and flavor before bottling.

The main reactions that take place during agingto make rums more palatable are esterification,condensation and oxidation. The formation of

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esters involves a reaction between acids and theethanol or other alcohols in the rum, and cantake a considerable amount of time. Thepresence of esters gives the rum a generallydesirable fruity aroma. In the condensations,molecules such as aldehydes and alcohols maycombine to form acetals. In oxidations that occurwith air passing through the pores in the wood,ethanol may be oxidized to acetaldehyde, whichin turn may be oxidized to acetic acid, to thenundergo esterification to ethyl acetate.

The temperature at which rums are aged willgreatly affect the rate of maturation. It is reportedthat the rate of maturation at 25oC may doubleat 35oC (Kampen, 1975). This means that rumsaged in the tropics will tend to mature morerapidly than those aged in temperate climatesunless the warehouses are heated. The highertemperatures do, however, increase the annualrate of evaporation loss of rum through thewood.

The wood contact surface area may beincreased by adding some charred or toastedoak chips to the barrels. The chips are relativelyinexpensive as they can be made from scrapwood and can provide a means of standardizingthe rate of maturation if fresh supplies are usedeach time.

Some rum producers add fruit extracts, sugarand even artificial rum essences to their products.The use of artificial essences should, however,be avoided or kept to a minimum as the productwill usually have a noticeably artificial character.This is because an essence containing possiblyfour or five chemical compounds cannot givethe overall �roundness� of odor and taste thatcomes from the dozens or even hundreds ofdifferent congeners present naturally in a goodheavy rum.

Characteristics of the aged rums should becarefully checked prior to bottling. Someblending may be required to ensure consistencywith rum bottled previously under the samebrand labels. Caramel coloring may be addedto adjust the color of the rum to a standard inorder to compensate for color variationsacquired from the barrels used in aging.

Disposal of rum distillery wastes

The disposal of molasses stillage is a majorproblem in most rum producing areas as theliquid waste has a high biological oxygen demand(BOD) and will cause serious pollution ifdischarged into rivers or other watercourses.

There are some limited uses of the stillage. Itmay, for example, be sprayed on unpaved roadsin dry seasons to reduce dust. Similarly, ifevaporated to a syrup referred to as �condensedmolasses solubles�, it may be used as a dust sup-pressant and source of minerals in cattle feeds.In some countries, a dried molasses stillagepowder is sold for cattle feeding.

Evaporated stillage syrup may be burned inboilers as a partial replacement for bunker-C fueloil; but the energy yield barely covers the energyrequired for the evaporation. There have alsobeen reports of the use of molasses stillage as abinder in the production of concrete blocks.Generally, however, molasses stillage must besubjected to waste treatment processes. It wasreported by Szendrey (1983) that the BacardiCorporation in Puerto Rico uses an anaerobicdigestion system to treat stillage and to producemethane for use as a boiler fuel. Other stillagewaste treatment systems include anerobic andaerobic lagoons.

References

Arroyo, R. 1941. The manufacture of rum. Sugar36(12).

Arroyo, R. 1942. The manufacture of rum. Sugar37(1), (5), (7).

Arroyo, R. 1945. US Patent 2,386,924. Prod-uction of heavy rums.

Arroyo, R. 1945. The production of heavy bod-ied rum. International Sugar Journal40(11):34-39.

Arroyo, R. 1945. Studies on rum. Research Bul-letin No. 5 (University of Puerto Rico, Agri-cultural Experiment Station, Rio Piedras,Puerto Rico), December.

Feedstocks, fermentation and distillation for production of heavy and light rums 255

Arroyo, R. 1947. The economics of rum prod-uction. International Sugar Journal49:292-294.

Arroyo, R. 1947. The economics of rum prod-uction. International Sugar Journal49:325-327.

Arroyo, R. 1948a. The production of straight lightrums from blackstrap. International SugarJournal 50:150-152.

Arroyo, R. 1948b. The flavour of rum - recentchromatographic research. InternationalSugar Journal 50:210.

Arroyo, R. 1948c. Simultaneous production oflight and heavy rum. International Sugar Jour-nal 50:289-291.

Arroyo, R. 1949a. The Arroyo fermentation pro-cess for alcohol and light rum from molas-ses. Sugar Journal 11(8):5-12.

Arroyo, R. 1949b. A new rum distillation process.Sugar 44 (7):34-36.

Arroyo, R. 1949c. Rum distillery yields and effi-ciencies-factors affecting them. InternationalSugar Journal 51:163-169, 189-191.

Arroyo, R. 1950. Advanced features in rum fer-mentation. International Sugar Journal52:42-44.

Bureau of Alcohol Tobacco and Firearms. 1982.Regulations under the Federal Alcohol Ad-ministration Act. Washington, D.C.

Clutton, D.W. 1974. Rum. The Flavour Industry,November/December pages 286-288.

Ianson, P. 1971. Rum Manufacture. Process Bio-chemistry, July, pages 35-39.

Kampen, W.H. 1975. Technology of the rum in-dustry. Sugar & Azucar 70(8):36-43.

Lehtonen, M. and M. Suomalainen. 1977. Rum.In: Alcoholic beverages (A.M. Rose, ed).Academic Press, London. pages 595-635.

Paturau, J.M. 1969. By-products of the cane sugarindustry. Elsevier, Amsterdam.

Szendrey, L.M. 1983. The Bacardi Corporationdigestion process for stabilizing rum distill-ery wastes and producing methane. In: En-ergy from Biomass & Wastes VII Sympo-sium, Lake Buena Vista, Florida, Institute ofGas Technology, Chicago, Il. pages 767-794.

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