By-product Utilisation Questions and Answers

Answer: A

Correct answer: Gossypol in untreated cake is toxic

Cottonseed carries gossypol, a phenolic compound toxic to monogastric animals. Poultry and pigs lack the rumen microorganisms that bind it, so growth falls away and at higher intakes organ damage follows.

The cheapness of the cake is the clue: properly processed cake has been heat treated to bind the free gossypol to protein, or solvent extracted to remove it, and that processing is what the price reflects. Excess protein and high energy are not toxic, and over-processing would reduce protein availability rather than cause the symptoms described.

Answer: B

Correct answer: Its sugars need no pretreatment

Molasses arrives with its sugars already in fermentable form, so the process needs only dilution and nutrient adjustment before the yeast goes to work. Rice straw is lignocellulose: the cellulose must first be freed from the lignin by a severe pretreatment and then hydrolysed enzymatically before any fermentation can begin.

Those two extra stages carry most of the cost of the lignocellulosic route. Straw is in fact available across a far wider area than molasses, and calorific value and silica are irrelevant to a fermentation.

Answer: C

Correct answer: Stabilise the bran by heat at once

The premium is for bran whose oil is still fit for edible extraction, and that depends entirely on speed. Milling ruptures the bran cells and brings lipase into contact with the oil, which is then hydrolysed to free fatty acids at several per cent a day.

Once the free fatty acid content passes a few per cent the oil can only be sold for industrial use at a fraction of the price. Heat stabilisation - extrusion or steam treatment - denatures the lipase and must follow milling within hours. Storing for a week guarantees the loss.

Answer: D

Correct answer: briquetting

Briquetting compacts a loose residue under pressure into dense blocks, raising the bulk density several fold - for rice husk, from around 100 to 650 kg/m\(^3\).

The purpose is transport and handling rather than energy: the calorific value per kilogram is essentially unchanged, but the energy per cubic metre rises with the density, so a lorry can carry several times the fuel. Pelleting is the same principle at a smaller size, gasification and digestion are conversion processes that change the material chemically.

Answer: A

Correct answer: producer

Producer gas is made by gasification - partial oxidation of biomass in a restricted air supply, so that the carbon is only partly burnt and the products are combustible: carbon monoxide, hydrogen and some methane, heavily diluted with nitrogen from the air.

Its calorific value is low, typically 4 to 6 MJ/m\(^3\), but it burns cleanly and can run an engine. Biogas is the quite different product of anaerobic digestion, and flue gas is the fully oxidised exhaust of ordinary combustion, which will not burn at all.

Answer: B

Correct answer: single cell

Single cell protein is microbial biomass - yeast, bacteria or fungi - grown on a cheap substrate and harvested for its protein. Molasses, whey and hydrolysates all serve, which is what makes it a by-product route rather than a primary process.

The organisms multiply quickly and are indifferent to climate and season, but the nucleic acid content of microbial cells limits how much can safely be included in a diet, which is the constraint that has kept the technology to feed rather than food in most applications.

Answer: C

Correct answer: molasses

Molasses is the syrup remaining when no further sucrose can economically be crystallised from cane or beet juice. It still holds around 50 per cent fermentable sugar, which is why it is worth so much more than a waste.

Its outlets are fermentation to ethanol, yeast and other products, and use as a feed ingredient, where its sugars and its palatability both count. Bagasse is the fibrous residue from crushing, press mud the filter cake from juice clarification, and vinasse the spent wash left after distillation.

Answer: D

Correct answer: pectin

Pectin is a polysaccharide of the plant cell wall, extracted commercially from citrus peel and apple pomace where it is most abundant. It gels in the presence of sugar and acid, which is what makes jams set, and it is also used as a thickener and stabiliser.

It is present at only a few per cent of the dry peel, but its value per kilogram is high enough to support the extraction. Cellulose and lignin are structural and do not gel; starch is a storage polysaccharide and gels quite differently.

Answer: A

Correct answer: Rice husk, being highly silicified

Rice husk is roughly 18 per cent silica and almost entirely indigestible. Feeding it supplies essentially no nutrition, and the abrasive silica damages the digestive tract of the animal, so it has no place in a ration.

The other three all have feed value: rice bran carries oil and, once defatted, protein and fibre; oilseed cake at 40 per cent protein is a concentrate; and bagasse, though poor, can be fed to ruminants after treatment. Husk is the one whose key constituent is of no nutritional use whatever.

Answer: B

Correct answer: Briquetting transforms energy per cubic metre only

Compare the two rows: the calorific value rises only from 14 to 15 MJ/kg, essentially unchanged, while the energy density rises from 1400 to 9750 MJ/m\(^3\) - a factor of seven.

The whole of the gain is in volume, because briquetting compacts the material without altering its chemistry. That is exactly the problem it is meant to solve: loose husk fills a lorry long before it approaches the weight limit, so transport cost per tonne is what makes distant use uneconomic, and density is what fixes it.