The global demand for poultrymeat is growing but consumers are becoming more discerning in their dietary preferences. The fat content of poultrymeat has increased as genetic progress has delivered faster rates of bodyweight gain and also fat deposition, to which it is closely correlated. The rise in further processing for fast-food poultry products has resulted in growing birds to greater bodyweights, and another tendency towards more body fat. Fast growing broilers require more energy to maximise their genetic potential, although much of the energy is ‘wasted’ in additional body heat as the result of their high metabolic rate, and the metabolism of proteins and amino acids, particularly in hot climates. In many cases, the first limiting nutrient is not protein and amino acids, but an overall reduction in energy intake.
Improving energy utilisation
Feed manufacturers are continually looking for ways to improve the utilisation of nutrients, especially energy. This can be achieved by:
• the addition of fat to diets in order to slow down the rate of food passage in the gut and thus allow more time for the action of the digestive enzymes
• the inclusion of feed enzyme products to help break down poorly digested dietary components
• mixing saturated and unsaturated fats to enhance fatty acid absorption
• the use of synthetic essential amino acids to give a better balanced lower protein diet and thus reduce nitrogen excretion - a high energy cost function
• precision grinding of cereal grains to increase surface area for the activity of digestive enzymes
• steam pelleting and conditioning to increase nutrient utilisation.
The energy value in fat is 2.25 times higher than from carbohydrates because of its chemical structure and the higher content of hydrogen relative to oxygen. Fats and oils – often from rendering operations or yellow grease from the catering and restaurant industries – are common ingredients in poultry feeds to improve growth rate and feed conversion. However, yellow grease particularly tends to have a significant content of trans fatty acids (‘trans fats’), which tend to accumulate in animal products and reduce productivity. They have also been linked to health problems in humans. As a result, nutritionists have been looking for alternative energy sources for animal feeds.
Sugars in feeds
There has been growing interest in feeding sugars to animals in the last few years as a source of dietary energy but there is little published data on how sugars fit with other feed components, e.g. protein, fibre, starch and pectins, what nutrients they provide and the levels of total sugars in feeds.
Sugars are defined as monosaccharides (simple sugars), disaccharides and oligosaccharides. These carbohydrates can be distinguished from polysaccharides (long chains of monosaccharides) by their solubility in 80% ethanol. Sugars are non-neutral detergent fibre carbohydrates, as well as non-structural carbohydrates because they are not included in the neutral detergent fibre fraction found in the cell contents.
The chemical structures of various sugars are shown in Figure 1. Glucose and fructose are the simple sugars most commonly found in plants. The most abundant disaccharide in plants is sucrose, which is a molecule of glucose bonded to fructose. Lactose (glucose + galactose) is found in milk. Maltose is a disaccharide with the same glucose to glucose alpha-linkage as starch. Oligosaccharides are the chains of up to 20 monosaccharide units. They include stachyose and raffinose found in soybeans. Generally, plants contain little oligosaccharide. Except for oligosaccharides, sugars are broken down by the digestive enzymes in mammals.
The sugar content of feedstuffs is variable. Mature grains such as maize and oats contain little sugar because most has been converted to storage polysaccharides. Forages such as pasture or hay may have relatively greater amounts of sugars. By-product feeds such as molasses, bakery waste, citrus pulp and almond hulls tend to have a high sugar content, which is affected by processing method. By-products such as distillers grains or brewers grains have little glucose, fructose or sucrose because these are used up during the fermentation process.
Digestion of sugar
The saliva and crop of the chicken contain some alpha-amylase but little starch digestion has been demonstrated in the crop and proventriculus gizzard. The digestion of most carbohydrates (polysaccharides) into monosaccharides and their subsequent absorption take place in the small intestine. Alpha-amylase is secreted from the pancreas into the duodenum and this hydrolyses the 1,4’ alpha-linkages on both sides of the 1,6’ branching points in starch, producing mainly maltose and some branched oligosaccharides (isomaltose). The enzyme maltase, also called a-glucosidase, splits maltose. At the same time, oligo-1,6’-glucosidase (isomaltase) produced by the intestinal mucosa hydrolyses the branched oligosaccharides into glucose. The brush border membrane of the jejunum produces other disaccharidases that complete the digestion of complex dietary polysaccharides into monosaccharides. Sucrose is hydrolysed by sucrase into glucose and fructose, while lactase converts any lactose into glucose and galactose.
Several researchers have demonstrated that sugars, particularly sucrose, are more digestible than starch. The greatest maltase activity has been shown in the jejunum, followed by the ileum, while the lowest value was seen in the duodenum. Table 1 shows that more of the energy is metabolised in sugar than in starch.
Sugar syrup and sugar feed supplement
Al Khaleej Sugar Co LLC, Dubai is the largest stand-alone sugar refinery in the Middle East. It has developed a new approach to feeding sugar to animals in the form of sugar syrup. This is derived from the crystallisation of sucrose, which is separated to leave the syrup. Sugar syrup contains 70-74% invert sugar. The chemical composition of sugar syrup is compared to maize in Table 2.
Sugar syrup is an energy-rich material that is valuable as a feed ingredient for poultry. Its energy content is similar to maize and it is highly digestible, offering instant energy. It also adds aroma and palatability. Unlike feed fats, sugar syrup does not contribute to the cholesterol content of poultrymeat or eggs.
Sugar syrup in liquid form cannot be included at levels higher than 10% in feeds because of handling difficulties, but Al Khaleej Sugar has developed a process to produce a sugar powder, known as sugar feed supplement, which is suitable for animal feeds.
Cereal grains still comprise 60% of poultry feeds but they bring associated problems. Price fluctuations in the grain markets directly affect feed prices, and cereal grains add to both the fibre content of the feed and the risks of aflatoxins and pesticide residues. While maize has a metabolisable energy content of around 13MJ/kg, sugar feed supplement contributes 18MJ/kg, without significantly increasing the cost.
The sugar product is a free-flowing powder that can be stored in bags for two years. It is easy to mix with other feed ingredients, without the need for special equipment, and it is free of aflatoxins.
Sugar syrup and the sugar feed supplement can contribute to the future of poultry nutrition.
Characteristics of sugar feed supplement
• As an instant energy ingredient
• Substitute for starch and grains
• No deleterious effects of sucrose
• Easy to handle, transport and store
• No physical limitations on incorporation in feed
• Non-hygroscopic and non-corrosive
• Maintains the binding effect
• Adds aroma and palatability
• Can be incorporated in mash feeds
• Less chance of insect infestation
Hot room pasteurisation of dried egg albumen has been used by the egg processing industry for a number of years. This practice to assure Salmonella-free egg albumen is time-consuming and requires considerable space.
Recent studies by Hammershoj and colleagues (Journal of Food Science and Technology , 2006, 41: 249-261 and 263-274) propose a fluidised bed process to pasteurise dried egg albumen. The process forces hot air or gas through a bed of albumen solids at a velocity to overcome gravity and suspend the particles in a fluidised manner. The close contact with the hot air would provide high heat transfer and allow for faster heat treatment. Fluidised beds are already widely used in the food processing industry to dry, cool, freeze and agglomerate food products.
The first paper reports the effects of fluidised bed process on bacterial, physical and chemical properties. In the first experiment, the researchers investigated temperatures of 90 or 130°C at a relative humidity of 2-3%. In experiment 2, they used two different levels of relative humidity – 2 and 20% – and a fixed temperature of 113°C. The higher relative humidity gave much better elimination of bacteria and increased surface hydrophobicity. This second characteristic indicates more protein unfolding, which is likely to improve functional properties. Using temperatures of 115-130°C for 1-2 hours at high moisture levels gave total plate counts comparable to those achieved with traditional pasteurisation, indicating a similar degree of bacterial elimination. The researchers noted some discoloration of the albumen powder. It was darker and more yellow, which may have been the result of a Maillard reaction, possibly indicating incomplete de-sugaring. An increase in particle size was also noted, particularly as the high relative humidity, which may have been due to agglomeration.
The second published paper evaluates the effects of fluidised bed treatment on functional properties. Gels prepared from powders treated at higher temperatures and higher relative humidity had higher stress and strain and better water-holding capacity. Foaming capacity was improved at the higher air moisture level. Foam stability was improved by increasing surface pressure.
The authors of the papers concluded that fluidised bed technology offers potential for the pasteurisation of dried egg albumen. There is a need to investigate further the effects of this process on colour and its effectiveness in eliminating Salmonella. Other processing times and temperatures also merit further study.