In this post, we’ll look at the optimal moisture content you need to achieve for long term storage of spent grains, summary of different drying techniques (from superheated steam to burning spent grain for heat)
According to Brewery Wastewater, the key is to low moisture spent grain is efficient lautering:
- Spent grain (BSG) usually has 80-85% moisture
- To store dry BSG for long time storage, moisture content needs to be less than 10%
- The value of spent grain is based largely on the percent moisture. The drier it is, the more valuable it is.
- 80% moisture is too high, it drips on the road, sloshes in the trailer, and makes a large slumpy pile at the farm.
- 70% moisture makes a nice tall pile similar to gravel, and it stores longer.
Another way of looking at it is:
- 50,000 lbs of 80% moisture grain deliver 10,000 lbs of dry product equivalent
- 50,000 lbs of 70% moisture grain deliver 15,000 lbs of dry product equivalent
From a research paper by Aliyu and Bala exploring different drying spent grains:
Many breweries have plants for BSG processing using two-step drying technique:
- Water content is first reduced to less than 60% by pressing
- Drying to ensure the moisture content is below 10%.
The traditional process for drying BSG is based on the use of direct rotary-drum dryers ( but is energy-intensive).
Bartolome´ et al. (2002) study is referenced and concluded:
- Freeze-drying – reduces the volume of the product and does not alter its composition – not feasible economically.
- Oven drying – reduces the volume of the product and does not alter its composition. There is a risk that the grain temperature near the dryer exit may rise leading to toasting or burning of the dried grains.
- Freezing methods – affects the composition of some sugars such as arabinose
- Thin-layer drying using superheated steam was proposed by Tang et al. (2005) as an alternative method. The circulation of superheated steam occurred in a closed-loop system; this reduces the energy wastage that occurs with hot-air drying. Also, the exhaust steam produced from the evaporation of moisture from the BSG can be used in other operations. Thus, superheated steam method has several advantages including the reduction in the environmental impact, an improvement in drying efficiency, the elimination of fire or explosion risk, and a recovery of valuable volatile organic compounds.
- Membrane filter press: BSG is mixed with water and filtered at a feed pressure of 3 to 5 bar, washed with hot water (65°C), membrane-filtered and vacuum-dried to reach moisture levels of between 20 and 30% (El-Shafey et al. 2004). Moreover, chemical preservatives such as lactic, formic, acetic, benzoic acid and potassium sorbate can effectively be used for preserving the quality and nutritional values.(Santos et al. ‘03).
Free Convection vs. Ventilation Drying
- Wet BSG drying with free convection goes very slowly. After 20h of drying at 60 ºC the upper and lower 2.5 cm layers lose 24.8 g water in average from 100 g spent grains, compared with 4.7 g in the middle.
- Increasing the temperature from 35 to 52 degrees average water removal by ventilation from the 2.5 cm thick layer changes from 55g to 70 g water per 100 g wet material during 30 minutes of drying. The 5 cm thick layer dried up in 2 hours at 35 degrees Celsius by ventilation.
- The experimental results show that active ventilation can be used for BSG drying in small beer brewing companies. Surplus heat received from beer production can be used for ventilated air heating. The drying efficiency can be increased if the brewer’s spent grains are previously exempt from free water. REF
The results show that the BSG drying with free convection permitted only a thin layer with a periodical or continuous regular mixing.
Conclusion – Ventilation is more efficient BSG drying method and can be applied in a thicker layer with lower temperatures. It is an effective way for BSG drying in small beer brewing companies, particularly if previously free water separated.
Adding heat to dry spent grains according to Chris, from an Alaska based Brewery:
Alaskan Brewery uses about 4300 BTU’s to dry one pound of spent wet grain at 75% moisture. “we use about half of the dried spent grains we produce to refuel our burner for some additional drying heat, but the burner still requires oil in addition to the dried grains to produce enough heat for the drying process and also to help the grains burn without smoke. The grain does burn very similar to wood chips and has about the same amount of waste ash as wood which is another waste stream we have to deal with.
The dried grains which are below 10% have a BTU value of about 10,000 to 12,500 BTU per pound when burned. The system is not cheap to buy or operate and if you can send your malt to someone without drying I would highly encourage it, it is an added cost that is not easy to justify. We only do it because we can not get rid of the grain locally and have to ship it out.
Grain drying equipment includes rotary dryers, steam tube dyers, flash-type dryers, steam disc dryers, and belt presses.