Fresh spent grain contains roughly three quarters water by weight and enough residual sugar and protein to support rapid microbial growth (Mussatto et al., 2006). It leaves the mash hot, which keeps the initial microbial load low, and then cools slowly through the temperature range in which bacteria multiply fastest. Robertson et al. (2010), who profiled spent grain at the site of production, described the microbial population at discharge and its development on storage, and concluded that the material must be stabilised promptly if it is to be used for anything more demanding than feed. Reviews of the microbiology reach the same conclusion (Bianco et al., 2020). The time between discharge and stabilisation is therefore the central control variable of the whole undertaking.

Spent grain is not a uniform commodity. Its composition depends on the barley variety and malting, on the adjuncts in the grist, and on the mashing regime, and Santos et al. (2003) showed that it varies between brews even within one brewery. Craft breweries, the natural suppliers of a regional processor, change recipes often and use wheat, oats, rye and dark malts freely. Each of those changes the fibre and protein content, the colour, the flavour and the allergen status of the residue.

The literature on bakery and snack applications is consistent on one point. Small additions of dried spent grain raise fibre content with little penalty, and larger additions darken the product, coarsen its texture and reduce expansion or volume (Stojceska et al., 2008; Ktenioudaki et al., 2012). Particle size is part of the explanation, since coarse husk fragments are perceived as gritty. A cold-formed or lightly baked bar, which is dense by nature and usually contains nuts, seeds and whole grains, is more tolerant of such an ingredient than an aerated product would be. That tolerance is the reason for choosing a bar.

The decisive change is one of status. Grain destined for cattle is handled as feed, and it becomes a food ingredient only if it is treated as one from the moment it leaves the lauter tun. The proposed agreement with each brewery therefore specifies dedicated, lidded, food-grade containers that are cleaned between uses; a discharge route that keeps the grain away from floor drains, cleaning chemicals and yeast or trub waste streams; a record of the brew, its grist bill and the time of discharge; and a maximum interval before collection. It also specifies which brews are excluded, for instance those containing ingredients the processor does not wish to declare. The brewery's own malt purchasing specification, including its supplier's assurances on mycotoxins, is requested as part of the agreement.

Stabilisation means removing water quickly. Mechanical pressing first lowers the moisture content cheaply, and thermal drying then brings it to a level at which the material is shelf-stable; around ten per cent moisture is the figure commonly cited. The literature also advises keeping the drying temperature moderate, below about 60 °C, because higher temperatures generate unpleasant toasted flavours (Mussatto et al., 2006). For a small processor a tray or belt dryer with good air circulation is the realistic option. Where same-day drying capacity is lacking, the framework allows chilling or freezing as a holding step with its own time limit. It does not allow wet grain to be held at ambient temperature.

Dried grain is milled and sieved to a particle size fine enough to avoid grittiness, and each lot is identified with the brews from which it came. A lot specification is proposed with limits for moisture and water activity, particle size, total dietary fibre and protein, and microbiological criteria including indicator organisms and Bacillus cereus, a spore-former associated with cereals that survives mashing temperatures. Dietary fibre is determined by an accepted enzymatic-gravimetric method (Lee et al., 1992). The method is fixed in the specification, because later integrated methods, which also capture resistant starch and small non-digestible carbohydrates, do not give identical results. Mycotoxin testing is scheduled on a risk basis against the legal maximum levels for cereals and cereal products (European Commission, 2023).

The commercial point of the bar is its fibre content, so the formulation is worked backwards from the claim. In the European Union a food may be called high in fibre if it contains at least 6 g of fibre per 100 g or at least 3 g per 100 kcal, and a source of fibre at half those amounts (European Union, 2006). From the fibre content of the milled grain and of the other ingredients, the processor calculates the minimum inclusion needed, adds a margin for lot variation, and then tests whether a bar at that inclusion is pleasant to eat. A simple mixture approach is proposed, varying the proportions of spent grain flour, a base of oats or other cereal, and binder, with nuts, seeds or dried fruit held constant. The binder, usually a syrup with or without fat, controls both cohesion and water activity.

A hazard analysis following the Codex General Principles of Food Hygiene is carried out for the whole chain, including the brewery end (Codex Alimentarius Commission, 2020). Practical guidance on applying the method in a small business is available in standard texts (Mortimore and Wallace, 2013). Table 1 lists the hazards and quality risks that the literature and the nature of the material suggest, with proposed controls. Two deserve emphasis. Water activity is the principal barrier in the finished bar: microbial growth is not expected below a water activity of about 0.6, and most bacteria require much higher values (Scott, 1957). And spent grain from barley contains gluten, so the bar must declare cereals containing gluten and cannot be positioned as gluten-free, whatever the other ingredients (European Union, 2011).

Bars made from at least three separate lots of grain are stored in the intended packaging and examined at intervals for water activity, texture, rancidity and microbiological status. Lipid oxidation merits attention, since spent grain retains the lipids of the barley and bars often contain nuts and seeds. Acceptance is assessed with a hedonic test among consumers conducted according to the relevant international standard (ISO, 2014), against a comparable bar without spent grain. The purpose is to learn whether the product is liked for what it is, not only whether it is tolerated for its origin.