Mashing

The mashing process consists of cooking (gelatinization) of the grain in water to solubilize the starches from the kernels and converting (saccharification) of the starch to "grain sugar" (primarily glucose and maltose). In general, cooking can be carried out at or above atmospheric pressure in either a batch or continuous process. During mashing, trace VOC emissions may result from constituents in the grain.

Small quantities of malted barley are sometimes added prior to grain cooking. After partial cooling, conversion of the starch to sugar is accomplished by adding barley malt and/or enzymes (from other sources) to the cooked grain. The mash then passes through a noncontact cooler to a fermenter. Between the mashing and fermentation, the process generally is closed during cooling, with no emissions. Distillers may vary mashing procedures, but generally conform to basic principles, especially in the maintenance of sanitary conditions.

Fermentation

Fermentation, which usually lasts 3 to 5 days for whisky, involves the use of a yeast to convert the grain sugars into ethanol and carbon dioxide (CO2). The converted grain mash is cooled prior to entering the fermenter or tank and inoculated with yeast. It is common practice to dilute the hot grain mash to its final solids concentration by adding backset stillage and/or water. Backset is liquid stillage which is screened or centrifuged from the distillation "beer still bottoms." The use of backset provides water conservation, nutrient supplements, pH adjustment of the fermentation, and some flavor components (e.g., sour mash).

The fermentation process varies slightly for the production of other distilled spirits. For instance, rum fermentations takes 1 to 2 days. In rum production, black strap molasses is the source of fermentable sugars and is stored in tanks prior to fermentation. The black strap molasses also is not "mashed" (i.e., cooked) prior to being diluted with water to obtain the proper concentration of fermentable sugars.

Congeners are flavor compounds which are produced during fermentation, as well as during the aging process. These congeners include trace aldehydes, esters, and higher alcohols (i.e., fusel oils). Lactic acid bacteria (lactobacillus) may simultaneously ferment within the mash and contribute to the overall whisky flavor profile. On rare occasions lactobacillus may provide some pH control. On other occasions, the addition of sulfuric acid, though rarely used, may result in trace hydrogen sulfide emissions from the fermentation tank.

In whisky production, significant increases in the amount of yeast consumed occur during the first 30 hours of fermentation, when over 75 percent of the carbohydrate (sugar) is converted to ethanol and carbon dioxide. Many fermentation vessels are equipped with agitation and/or cooling means that facilitate temperature control. Fermentation vessels may be constructed of wood or metal and may be open or closed top.

The final fermented grain alcohol mixture, called "beer," is agitated to resuspend its solids and may be transferred to the "beer well" storage vessel for holding until it is pumped to the "beer still." Distillers use mechanical or air agitation during transfer and storage to prevent settling of solids. In the instance of air agitation, trace amounts of aldehydes may be produced. The beer passes from the beer well through a preheater where it is warmed by the alcohol vapors leaving the still and then enters the still for distillation.

The beer still vapors condensed in the preheater generally are returned to the beer still as reflux.

Distillation

The distillation process separates and concentrates the alcohol products from the fermented grain mash. In addition to the alcohol and congeners, the fermented mash contains solid grain particles, yeast cells, water-soluble proteins, mineral salts, lactic acid, fatty acids, and traces of glycerol and other trace congeners. Whisky stills are usually made of copper, especially in the rectifying section, although stainless steel may be used in some stills. In a general whisky distillation process using a beer still, the whisky separating column consists of a cylindrical shell having three sections: stripping, entrainment removal, and rectifying. The stripping section contains approximately 14 to 21 perforated plates, spaced 56 to 61 cm (22 to 24 inches) apart. The fermented mash is introduced at the top of the stripping section and descends from plate to plate until it reaches the base where the stillage is discharged. Steam is introduced at the base of the column, and the vapors from the bottom of the still pass up through the perforations in the plates. Whisky stills are usually fitted with entrainment removal sections that consist of a plate above the stripping plate to remove fermented grain particles entrained in the vapor. Distillation columns operate under reflux (sealed) conditions and most vapors are condensed and collected, although small amounts of noncondensable gases will be emitted to the atmosphere. The rectifying section contains several bubble cap or valve rectifying plates in the top section of the still that produce distillates (ethanol) up to 190E proof.

The diameter of the still, the number of stripping and rectifying plates, capacity of any doubler, and proof of distillation are factors that can contribute characteristics to a particular whisky. The doubler is a type of pot still that is used to redistill the distillate from the beer still to enhance and refine the flavors desired in a specific whisky. Following distillation, the whisky, at high proof, is pumped to stainless steel tanks and diluted with demineralized water to the desired alcohol concentration prior to filling into oak barrels.

Grain & Liquid Stillage

At most distilleries, after the removal of alcohol, still bottoms (known as whole stillage) are pumped from the distillation column to a dryer house. Whole stillage may be sold, land applied (with appropriate permitting), sold as liquid feed, or processed and dried to produce distillers dried grains (DDG). The DDG consists of proteins, fats, minerals, vitamins, and fibers which are concentrated threefold by the removal of the grain starch in the mashing and fermentation process. Distillers' secondary products are divided into four groups: DDG, distillers dried solubles (DDS), DDG with solubles (DDG/S), and condensed distillers solubles (CDS).

Solids in the whole stillage are separated using centrifuges or screens. The liquid portion "thin stillage" may be used as a backset or may be concentrated by vacuum evaporation. The resultant syrup may be recombined with the solid portion or dried separately. This remaining mixture is then dried using one of a variety of types of dryers (usually steam-heated or flash dryers). The majority of DDG are used in animal feed, although increasing quantities are being sold as food ingredients for human consumption due to its nutrient and fiber content.

Warehousing/Aging

In the aging process, both the charred oak barrel in which beverage alcohol is stored and the barrel environment are key to producing distilled spirits of desired quality and uniqueness. The aging process gives whisky its characteristic color and distinctive flavor and aroma. Variations in the aging process are integral to producing the characteristic taste of a particular brand of distilled spirits. Aging practices may differ from distillate to distiller, and even for different products of the same distiller.

Blending/Bottling

After the whisky has completed its desired aging period, it is dumped or pumped from barrels into stainless steel tanks and reduced in proof to the desired alcohol concentration by adding demineralized water. The diluted whisky is processed and filtered. Following a filtration process the whisky is pumped to a tank, proof adjusted, and bottled.

Loss Exposures

The major hazard in respect of the distillery stems from the production of flammable organic compounds like ethanol. Trace elements of ethyl acetate, isobutyl alcohol etc are also present. The hazards arise from leaks in the tanks themselves, casks and ancillary equipment such as transfer pumps, pipe work and flexible hoses, all of which can release significant quantities of liquid on failure. A vapour cloud explosion may be possible depending on the size of the release, the spillage surface, and the presence of confined volumes or adjacent structures that produce flame acceleration.

The grain processing section also has a considerable fire hazard due to the production of flammable grain dust other particles generated in the process.

Old distilleries have wooden constructions in respect of intermediate floors and racks which can cause combustion. Also wooden cask stores have considerable fire load and can supply enough fuel to sustain a fire.

Whisky maturation warehouse sites sometimes hold a variety of other hazardous materials, which may need to be considered in the report. These include a natural gas supply to a boiler, a LPG cylinder, dangerous powders and/or toxic substances in tanks or drums.

Ethanol is a highly flammable liquid with a relatively low flash point, -21 ºC – the flash point is dependent on the alcohol concentration. There is a concern that evaporating ethanol could pose an explosion hazard in bonded warehouses and in stills rooms. The vapour-air density of ethyl alcohol is 1.6 times that of air. Ethyl alcohol vapours are invisible, and the distance they will travel is not always predictable. Testing carried out by the Distilled Spirits Council of the United States indicates that beyond 0.5 metres from the source, vapours are generally less than 25% of the LEL and beyond 1.65 metres they are usually negligible.

However, from a study of the major losses involving distilleries, it has been assumed that the chances of fire and subsequent explosion originating in the storage areas are far too higher than the process areas. The natural ventilation is an integral part in the maturing process of the whisky, lending it a particular quality, which will be different from region to region. It has been assumed that a typical air change rate is of the order of ten air changes per hour. It would appear that there is a low probability of an explosion due to the ignition of an ethanol/air mixture. The evaporation rate of ethanol at ambient conditions of about 25 ºC is too low; the natural ventilation would almost certainly be able to dilute the gas cloud ethanol concentration down to well below its lower flammability limit. However if there is a possibility of recirculation zones or stagnant regions, where the gas cloud could, potentially, become enriched so as to fall between the lower and upper flammability limit. Explosions could be triggered in the event of an ignition due to any source of electricity like sparks, static electricity discharge etc.

Studies conducted in respect of distillery accidents have indicated that in most of the cases a fire would precede an explosion - radiation and convection enhancing the vaporisation of ethanol, destroying casks and/or structures leading to further spillage and subsequent ignition of the explosive mixture.

The following are five mechanisms for the production of a flammable mixture