Nutrition Newsletters
Grain Processing - Does it Destroy Nutrient Value?
Oats, barley, triticale, corn, rice, rye, sorghum, wheat... Whole, cracked, crushed, ground, expanded, extruded, micronised, steam rolled, steam flaked, pelleted… Cereal grains are presented to us in our horse feeds in so many different ways that the question “What is best for my horse?” is on the lips of all horse owners. So what’s the difference between all of these methods of grain presentation? Does grain processing destroy a grains nutrient value or does it enhance it? Should you feed processed or unprocessed grains? And what happens when grains are fed unprocessed? This article will answer all of your grain processing questions and will clear the murky waters of grain based feed selection.Why do we process cereal grains?
To answer this question, we need to ask ourselves, why do we feed cereal grains to our horse? Cereal grains are fed primarily as a source of energy in a horse’s diet. The energy is derived from the fibre and starch. To extract this energy, the horse must digest this fibre and starch in its gastrointestinal tract. The fibrous part of a cereal grain is fermented by bacteria in the horse’s hindgut to provide the horse with volatile fatty acids (VFAs), which it absorbs from the hindgut and burns within the body for energy. This process is the same as that used to extract energy from pasture and hay. It is efficient and will not cause digestive upsets in the horse.
In contrast, starch, which constitutes the major source of energy in a cereal grain, must be digested by enzymes in the small intestine. Starch is simply many glucose molecules all bonded together and bundled up into ‘starch granules’. These starch granules are then embedded amongst protein in a structure known as the protein matrix (Figure 1).
Figure 1: A scanning electron micrograph of the endosperm of barley showing the starch granules embedded within the protein matrix, adapted from Black (2001)
The starch granules, embedded in the protein matrix are then encased within individual ‘endosperm’ cells and protected by a cell wall. Then many of these cells are packed tightly within the cereal grains ‘starchy endosperm’, found in the middle of a grain. The endosperm is then protected by the aleurone layer and finally the entire structure is covered by the seed coat (Figure 2). Now from the plants perspective, all of this packaging is absolutely critical for its survival and is designed to protect the plant embryo and its stored sources of energy and protein to ensure it will be able to grow and survive for the first few days following germination.
Figure 2: The location of starch granules (stained black) within the endosperm cells of barley grain. The protein matrix (stained green) surrounds the starch granules within the endosperm cells. The aleurone layer surrounds the endosperm cells which make up the grains starchy endosperm (images produced by P. Littlefield,
So, the packaging is clever and essential from the plants perspective, however for the horse, all of this packaging is just a nuisance and prevents the horse from being able to digest and extract the energy from the grain. In fact, this packaging was actually specifically designed to allow a grain to pass through the gastrointestinal tract of an animal undamaged so it may germinate when it is excreted in the manure.
The packaging can be likened to a security system at a casino which prevents the thieves (or in this case the enzymes) from stealing the cash (the starch). To digest the starch the enzymes in the horse’s small intestine must first breech the seed coat, then penetrate the aleurone layer. Following this they need to be able to make their way through the endosperm cell walls (these are the cells that contain the starch), then burrow through the sometimes impenetrable vault of the protein matrix before finally reaching the starch granule. Then, in a cruel twist of fate, if the enzyme reaches this far, it will find that the starch is bundled so tightly into a ball that the enzymes cannot digest it. So the horse is presented with a difficult hurdle – just how does it go about extracting the energy held in the starch of cereal grains?
Enter Grain Processing!
It has been recognised for many years now that to effectively digest cereal grains, horses need some assistance. We provide that assistance through grain processing. Grain processing is used to break down the barriers the enzymes have to face in reaching and digesting cereal grain starch.
Grain processing can take on two forms:
1. Physical Processing – this can include the cracking, rolling, crushing and grinding of grains.
2. Heat/Moisture/Pressure Processing – this group of processing methods can include extrusion, micronisation, steam rolling/flaking and pelleting.
How do these processes help?
Physical processing is the same process as chewing and aims only to change the physical structure of the grain, breaking the seed coat and reducing the grains particle size to give the enzymes better access to the starchy endosperm within the centre of the grain. While the seed coat and aleurone layer barriers are removed, physical processing only causes minor damage to the endosperm cell walls and leaves a majority of the protein matrix and starch granule structure intact, meaning only small improvements to starch digestion will be made. Work conducted in horses showed that cracking corn only improved its digestibility in the small intestine of the horse by 1%. So while physical processing can get an enzyme through the front doors of the casino, gives them access to some of the cash floating around at the tables, and maybe even gets them into the strong room, it leaves the enzymes without a key, security code or set of explosives to get it into the vault. To do this, we need to disrupt the tile like structure of the protein matrix and alter the structure of the starch granule. And to achieve this, we need to cook the grain.
When grains are processed using a combination of heat, moisture and pressure, the structure of the protein matrix is physically disrupted, compromising its ability to protect the starch granules. In addition, these processes also turn the ordered and tightly packed structure of the starch granule into an open and vulnerable structure which can be easily attacked by enzymes in a process known as gelatinisation. Once the enzymes get into the starch vault, they can go about their work of cutting up the starch into single glucose molecules, which the horse then absorbs from the small intestine into the body, where it is burnt for energy.
What happens if cereal grains are fed without being processed using heat, moisture and pressure?
Starch within grains which are fed either whole or physically processed will remain largely undigested as it passes through the small intestine and will eventually be delivered to the hindgut. This is where the trouble begins. The bacteria in the hindgut do not face the same barriers as the enzymes in the small intestine, and they are able to reach and rapidly ferment the starch contained in unprocessed or physically processed grains. This rapid fermentation of starch causes excessive production of VFAs and lactic acid, which then accumulates in the hindgut and lowers the hindgut pH (the hindgut contents become acidic). Low pH in the horse’s hindgut causes a multitude of diseases and behavioural disturbances including laminitis/founder, colic, endotoxaemia, systemic acidosis, reduced fibre fermentation, poor appetite, wood chewing and the eating of bedding as well as deficiencies in the B-group vitamins (including biotin) and vitamin K.
What about oats?
The general consensus is that oats can be fed unprocessed. As it is a larger grain, horses are capable of chewing the grain enough to break its seed coat, negating the need for physical processing. Studies have also found that oat starch is far easier to digest than corn or barley starch in an uncooked form, thus negating the need for processing which will destroy the protein matrix and starch granule structures. However, whether oats can be fed unprocessed needs to be decided on a horse by horse basis. Observe your horses manure closely when you are feeding him oats. If you observe whole oat grains in his manure, whole oats is not a suitable feed for this horse. It is important to make sure the oats you are observing in the manure are whole and not just undigested hulls. Do this by taking them from the manure and squeezing them. If they are whole you will observe the white starch oozing from the centre. If you want to feed oats specifically, but your horse doesn’t digest them well, cracked, steam rolled and micronised oats are available in
And the Moral of the Story?
Don’t feed cereal grains unless they have been processed using a combination of heat, moisture and pressure, with the exception of oats for some horses. If you feed whole, uncooked cereal grains, your horse will get little benefit from them and they have a good chance of causing disease and behavioural problems. Remember, the reason you feed cereal grains is to provide your horse with a source of energy. Most of this energy is held within the grains starch. If the horse can’t digest this starch, then you are better off not feeding the grain at all.
Next time you go to buy feed for your horse, read the label carefully and choose feeds that contain extruded, micronised, steam flaked/rolled or pelleted components. Your horse will thank you for it!