High-gravity brewing is the practice of producing concentrated wort followed, at some stage, by dilution to produce finished beer of a desired alcohol content. In essence, therefore, it is a strategy that involves brewing using a concentrate, which thereby reduces the requirement for handling water, the major component of wort and beer. In terms of plant efficiency, it follows that it is prudent to delay the dilution step until the last possible process stage. Usually this will be in bright beer tanks, prior to packaging. However, plant permitting, it can be before or during fermentation, although the latter two options fail to provide many of the available benefits (Schaus, 1971; Pfisterer & Stewart, 1976).
High-gravity brewing was originally introduced to the United States in the 1950s. It has since gained widespread popularity throughout the brewing world and is now very common especially for the production of Pilsener-type lagers. In the vast majority of cases a process is operated in which the concentrated wort is fermented to produce a high-gravity beer which is diluted just before packaging. The principal advantage is that fermentation capacity is increased with no need for capital expenditure. Allowing for the fact that there may be some increase in fermentation cycle times, Hackstaff (1978) considered that by high-gravity brewing the capacity of existing plant was increased by 20-30%.
Several other advantages may be recognised. There are reductions in both energy usage and labour costs per unit volume of beer produced. The relatively high ethanol concentrations formed during fermentation promote increased precipitation of polyphenol protein material and, therefore, high-gravity beers have better colloidal stability than standard gravity fermented products (Whitear & Crabb, 1977). Relatively high ethanol yields impart greater microbiological stability to beers. Yeast growth extent increases with increase in wort concentration; however, the relation is not pro rata and high gravity fermentations are more efficient and produce a com-mensurately greater yield of ethanol. Nevertheless, it is necessary to increase the yeast pitching rate in proportion with wort gravity and crop viabilities can be reduced when very concentrated worts are used (Fernandez et al., 1985). After dilution the beers from high-gravity fermentations are generally considered to have a smoother palate than sales gravity beers, probably due to the loss of polyphenols (Hackstaff, 1978).
There are some constraints that limit the benefits of high-gravity brewing. Although yields from individual fermentations are improved, there will be a concomitant increase in the fermentation cycle time, assuming all other control parameters stay the same. In this case the total number of fermentations which can be performed per unit time per vessel will decrease. For this reason, some of the gains in productivity are lost. A major aim of high-gravity fermentation management has been to identify regimes that avoid the penalty of protracted cycle times, yet produce beer, which after dilution is indistinguishable from the same product fermented at sales gravity. This theme is explored in greater detail in various sections of Chapters 3, 5 and 6.
A fundamental requirement of high-gravity brewing is that the brewhouse must be capable of producing concentrated worts. In the case of new breweries designed for the application of this technique, the brewhouse may be sized appropriately to accommodate the wort batch size and concentration that is required. In many cases, however, the process has been introduced to an existing site where the brewhouse was designed to service fermenters requiring less concentrated worts. In this case it has been necessary to adapt the process of wort production to meet the needs for increased capacity. Obviously this may be partially achieved by using a lower grist to liquor ratio. However, this is of limited value since the efficiencies of plant such as lauter tuns will allow only a certain margin of turn-up in their rated capacities.
Schaus (1971) proposed the use of a recycling wort production method, in which the run-off from an initial lautering was directed towards a mash mixer, added to a further charge of grist and used to produce a second concentrated wort. However, this approach has not seen widespread adoption and it would be suspected that wort viscosity would be problematic. In the case of plant designed specifically for high-gravity brewing, the new generation of membrane mash presses seem ideally suited to producing concentrated worts (Hermia & Rahier, 1992; Biihler, 1995). Copper boil of high-gravity worts reduces the efficiency of extraction of hop bitterness and it is necessary to counteract this by increasing dosage rates (Hackstaff, 1978). This is now less of a problem since bitter hop extracts can be added at later stages in the process.
The most commonly used method of increasing wort concentration without modifying the brewhouse is the use of liquid syrup adjuncts which may be added directly to the copper and therefore circumvent the earlier steps of wort production. This approach is superficially very attractive; however, it should be treated with caution since the use of pure sugar syrups dilutes the available nitrogen concentration of the finished wort. Dramatic changes in wort carbon to nitrogen ratios result in large shifts in the concentrations of flavour-active metabolites produced during fermentation, in particular elevated levels of acetate esters (Whitworth, 1978) (Section 3.7.3). The result is that it is difficult to match the flavour of existing beers fermented at low gravity with a high-gravity fermented product.
In terms of wort concentration, high-gravity brewing is taken to mean fermentation of worts of approximately 16-18°Plato (1.064-1.072) which are subsequently diluted to sales gravity beers of 10-12°Plato (1.040-1.048). This upper limit is based upon constraints, which relate to issues both economic and of beer quality. Whitworth (1978) considered that a breakpoint of about 15°Plato (1.060) for UK brewed beers was optimal. More concentrated worts attracted unacceptable penalties in terms of brewhouse efficiency and yeast wetting losses in fermenter. However, of greater significance was the observation that fermentation of worts of greater than 15°Plato was associated with an exponential increase in beer ester levels. Furthermore, use of more concentrated worts may result in sluggish or sticking fermentations and formation of low-viability yeast crops. These effects have been assumed to be a consequence of sensitivity of yeast to ethanol toxicity and high osmotic pressure.
It is now suggested that the adverse effects on yeast and fermentation, which result from the use of very concentrated worts, reflect nutrient deficiencies. These deficiencies may be exacerbated by dilution of malt constituents with syrup adjuncts. Casey and Ingledew (1986) concluded that a normal 12°Plato wort required a minimum free amino nitrogen (FAN) concentration of 160 mgl 1. High-gravity worts (18°Plato) could be used to produce beers with normal levels of esters after dilution and fermentation performance was satisfactory providing at least 280 mgl 1 FAN was provided. Provision of semi-aerobic conditions also produced similar effects (Casey et al., 1984, 1985). The same authors and others, for example, McCaig et al., (1992) have shown that worts of 24-30°Plato may be fermented satisfactorily providing that yeast pitching rate, wort oxygen concentration and FAN levels were increased pro rata. In the case of very-high-gravity brewing (more than 24°Plato) it may be necessary to supplement worts with yeast foods, which contain assimilable nitrogen, metal ions (especially magnesium) and unsaturated fatty acids.
Provided these nutritional deficiencies are remedied it may be demonstrated that brewing yeast strains are not less osmotolerant or ethanol tolerant than strains, such as saké yeasts, which are generally considered to have enhanced abilities in these respects (Mogens & Piper, 1989). Nevertheless, even discounting considerations of brewery plant efficiency, beers diluted from very-high-gravity fermentations are different in character to those brewed at lower gravities. Thus, McCaig et al. (1992) reported that very-high-gravity beers, after dilution, were well received but considered to be lighter, smoother, less sweet and more winey and estery than their normal-gravity counterparts. In addition, it was noted that yeast became less flocculent, a change considered to be a response to the high osmotic pressure, and thus, less yeast was cropped such that the high suspended cell count in green beer would significantly increase solids loading on centrifuges. Younis and Stewart (1998, 1999) concluded that the carbohydrate spectrum of very-high-gravity worts exerted an influence on the extent of volatile synthesis during fermentation. It was demonstrated that volatile levels were reduced when maltose was the predominant carbohydrate, as compared with glucose or fructose. Thus, 20°Plato worts containing 30% high maltose adjunct produced lower levels of higher alcohols and esters compared with similar concentrated wort made from all malt.
The most significant on-cost of high-gravity brewing is the requirement for blending plant. If, as is usually the case, this is performed with bright, beer the process requires careful control. The blending must be accurate to avoid product waste and to meet the requirements of legislation. The cutting water must be of high standard with regard to microbiological purity and it must be free from flavour and colour taints. Most importantly, it must be deaerated and have a mineral composition and pH that will not perturb the colloidal stability of the beer after dilution. An essential part of the blending process is to adjust the level of carbonation.
Dilution water must be very pure and totally free of particulate matter. In modern plants fully demineralised or reverse osmosis purified water is used. Deaeration is usually achieved by gas stripping, via passage through a column against a counter-current flow of carbon dioxide. A typical specification for oxygen is 0.05 ppm maximum dissolved oxygen concentration. Frequently, the purification and deaeration treatment may be followed by pasteurisation or treatment with ultraviolet light to reduce microbiological loading.
The simplest blending system is that in which a calculated volume of deaerated water is added to a batch of high-gravity beer in a tank and the mixture stirred to ensure homogeneity. Much more elegant automatic systems are now in use. These may combine blending and carbonation in one operation. Several approaches are used which differ in sophistication. All demand plant of excellent hygienic design and an operation which prevents oxygen ingress. In the simpler systems, the high gravity feed beer is analysed and the required blend rate calculated. This is then achieved by microprocessor control of flow rates of the feeds of water and high gravity beer. Carbonation is then adjusted in a separate process.
More sophisticated blending systems use in-line monitors to measure alcohol and/ or specific gravity. Some of these sensors are described in Section 188.8.131.52. In these systems the desired control parameter is measured in the diluted product stream and output from this sensor regulates the flow rates of the two feed streams. Automatic carbonation can also be achieved using a similar control system. More detailed descriptions of some commercially available deaerating and high-gravity beer blending systems may be found in the following papers: Rubio et al. (1987), Andersson and Norman (1997), Koukol (1997), Anon. (1997).
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