chemical entities such as proteins. For the remainder of this article we will consider two industries where the manufacture of the final products requires substantial removal of an aqueous by-product: distilled spirits (specifically whiskeys) and cheese. Cheese and whiskey may seem like strange bedfellows (although a peated Scotch complements a strong blue cheese admirably), but a brief comparison of the two processes (Fig. 1) clearly shows that not only is there a requirement for aqueous by-products to be removed, but also implies that these streams are by no means “pure” water and usually require further processing prior to disposal. In the case of Scotch malt whisky production, the aim is to recover the “essence” of a beer feed as an alcohol concentrate. By way of example, if a beer feed of 9% ABV is distilled to a 71% ABV still-strength spirit then for every 100 gallons of feed (ie nine gallons of alcohol), only around 12 gallons of spirit are recovered if we assume alcohol recoveries in excess of 95%. This means that some 88 gallons of aqueous by-product remain. Similarly the production of cheese involves the coagulation of proteins and subsequent separation of whey. This whey, rich in calcium and proteins represents around 90% of the initial milk feed volume. So, in both cases, the largest products by volume, if not in value, are their respective by-products. As we implied earlier, the term “aqueous by-product” is rather disingenuous. Aqueous they may be, but compositionally they are challenging. If we first consider Scotch whisky production, the first point to bear in mind is that the feed for the still is not dissimilar from unhopped beer, known in Scotland as wash. Typically wash is at around 7 – 9% ABV and, as new make Scotch malt whisky spirit is made in simple pot stills, spirit strengths required for satisfactory maturation (> 63% ABV) are generally not achievable in one distillation step, so most often two stills are used: a wash still and a spirit still. It is the by-product from the wash still, called pot ale, which we will focus on here. This is typically 4.0 – 4.5% (w/v) solids content, made up of yeast, yeast residues, soluble protein and carbohydrates, with an appreciable level of copper. Its high
Biological Oxygen Demand (BOD) adds significant cost in many effluent treatment situations, so it is most commonly evaporated to form pot ale syrup (PAS) and sold as is or the PAS is co-pelletized with residual grains from upstream processes to form distillers’ dark grains (DDGS) that can be sold as cattle or pig feed. More recently there have been great strides made in the conversion of by-products such as pot ale into energy sources, and as an example, more than 500 MW of electrical equivalent capacity are produced across the UK, as electricity or biogas from processes labeled anaerobic digestion (AD). Essentially cultures of microorganisms are used in static or continuous tanks to biodegrade byproducts. Advantages include little logistics costs for AD plants co-located at the distillery, and a potentially substantial reduction in energy costs. This conversion though is not without difficulties, with protein degradation often generating nitrogen as ammonia and sulfur as hydrogen sulfide, both of which are toxic to many micro-organisms. There is therefore a desire to remove this protein prior to AD. The separation and analysis of pot ale proteins reveals an amino acid profile that has potential as a protein ingredient for feeds. One example, which has been developed by Horizon Proteins, using their own patented technology to isolate purified proteins from pot ale at low cost, with the intention of using it as a supplement for soybean and fish meal proteins in aquaculture. If the feeding trials go well, the cost of the processing will be more than offset by the market value of the protein, with the additional benefit of a residual, de-proteinated feedstock that is more amenable to AD. That’s not quite the whole story though. The growth of aquaculture is limited largely by the availability of suitable proteins, with soy having anti-nutritional properties and fish meal protein, derived largely from anchovies, susceptible to oceanic phenomena off the Pacific coast of South America. So the introduction of another protein ingredient should help to stimulate further growth in aquaculture. The protein source does not necessarily need to be derived from malted barley, the raw material of Scotch malt whiskies. Other grain-derived processes (including both potable
FIGURE 1 Comparison of cheese- and whiskey-making processes. The black dotted box indicates the operations that generate up to 90% of the aqueous by-products from both processes.
RAW MATERIAL PREPARATION
CRUDE PRODUCT FORMATION
REMOVAL OF WATER-RICH BY-PRODUCTS
ELABORATION TO FINAL PRODUCT
MILLING; STARCH AND PROTEIN HYDROLYSIS
FERMENTATION TO CREATE ETHANOL
DISTILLATION TO CONCENTRATE ETHANOL
DILUTION, MATURATION, FILTRATION
PASTEURIZATION, FERMENTATION, RENNET ADDITION
COAGULATION OF PROTEINS
SALT, PRESS, RIPEN Fig. 1. Hughes et al.
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