Energy is a crucial & essential constituent of poultry ration affecting consumption, performance and profitability of the farming business. Despite of precision in formulation many factors are responsible that can lead to variation in energy value of the poultry feed. Formulating the high energy diets like broiler finisher makes it mandatory to use oil in order to match the nutritional requirements of modern fast-growing evolved breeds. This crucial balance of energy-protein ratio mainly depends on quality concentrated ingredients that are considered while formulating the ration. Tricky job of animal nutritionist is to match the physical quality of feed ingredients available at the feed mill with nutritive value of the ingredients that is considered during the formulation. The minor error or overlooking at this point may lead to variation in energy that can affect the performance of the birds & profitability of the poultry farming business as well. Oil used in poultry feed and its quality is one of the critically important factors which can prevent this variation and associated losses as well.
Oil & fats used in poultry ration-
There are different sources of oil used in poultry feed which mainly are subjected to availability and price.
Commonly used soybean oil to coconut oil many such sources are used with minimum 2.5 to 4% level of inclusion. The reason of its significance is with lowest use of 2.5% of the oil contributes around 7 to 10 percent of the energy in the ration. Therefore, the quality of these oils used must be optimum.
Fact-finding of energy from oil source in poultry feed
Fats are used as high energy sources in broiler rations. The energy yielding potential of lipids is determined by degree of saturation and chain length (Wiseman, 2003). The location of the fatty acid in the glycerol molecule and the proportion of free fatty acids affect its energy value. Saturated fatty acids are less absorbable and have a lower energy value than unsaturated fatty acids because unsaturated fatty acids are polar solutes and are therefore readily incorporated into micelles and absorbed (Wiseman, 2003). According to Rose (2001), animal fat or vegetable oils may be used as a source of energy up to a maximum of 6%. Above this level it becomes difficult to maintain pellet quality or to mechanically move the sticky feed when it is not pelleted.
The composition of a source determines the amount of energy that source can provide to the animal. Most dietary fat sources contain more unsaturated fatty acids than saturated fatty acids. Based on the knowledge of the fatty acid composition, it is possible to manipulate the amount of energy to be provided to the animal. The energy yielding potential of lipids is determined by degree of saturation and chain length (Wiseman, 2003). The location of the fatty acid in the glycerol molecule and the proportion of free fatty acids affect its energy value. Saturated fatty acids are less absorbable and have a lower energy value than unsaturated fatty acids because unsaturated fatty acids are polar solutes and are therefore readily incorporated into micelles and absorbed (Wiseman, 2003).
Properties of the oil used in poultry feed and its evaluation
It is evident from above two research experiments that oil oxidation due to loss of calorific value along with effect on physical parameters of animal feed negatively affects the performance parameters. This indicates at most care to be taken to prevent the oxidation of oil. Let’s understand in detail about the oxidation and its prevention.
What is oil oxidation?
Oil oxidation is an undesirable series of chemical reactions involving oxygen that degrades the quality of an oil. Oxidation eventually produces rancidity in oil, with accompanying off flavours and smells. All oil is in a state of oxidation - you cannot stop it completely - but there are ways to reduce it. Attempts should therefore be made to reduce oxidation at each stage of oil manufacture.
Oxidation is not one single reaction, but a complex series of reactions. When oil oxidizes it produces a series of breakdown products in stages, starting with primary oxidation products (peroxides, dienes, free fatty acids), then secondary products (carbonyls, aldehydes, trienes) and finally tertiary products.
Oxidation progresses at different rates depending on factors such as temperature, light, availability of oxygen, and the presence of moisture and metals (such as iron). The type of oil also influences the rate of oxidation. Marine oils (including fish, mussel) are highly susceptible to oxidation due to the large number of polyunsaturated fatty acids (PUFA) they contain. These unsaturated fatty acids have reactive double bonds between their carbon atoms, whereas saturated fats have no double bonds so they oxidise more slowly.
How do we measure oxidation?
Measuring oxidation involves testing for the primary and secondary breakdown products. The most common test is peroxide value (PV). However, very rancid oils can have a reduced PV therefore the anisidine value (AV) and a Totox value are used to show the whole oxidation story (see Figure 1). Other measurements of oxidation are the acid value (free fatty acid FFA), thiobarbituric acid value (TBA) and iodine value (IV). Head space volatiles (smell) can also be tested using artificial nose technologies.
Peroxide value (PV)
Primary oxidation processes in oil mainly form hydroperoxides, which are measured by the PV. In general, the lower the PV, the better the quality of the oil. However PV decreases as secondary oxidation products appear (Figure 1). Most customers will require a PV of less than 10 in marine oils, but PV may need to be as low as 2, depending on the market.
The PV test is a good way to measure the amount of primary oxidation products in fresh oils. Oils with significant levels of peroxides may still be odourless if secondary oxidation has not begun. If oxidation is more advanced, the PV may be relatively low but the oil will be obviously rancid.
Anisidine value (AV)
The secondary stage of oxidation occurs when the hydro- peroxides decompose to form carbonyls and other compounds, in particular aldehydes. These are what gives the oil a rancid smell, and they are measured by the AV. The lower the AV, the better the quality of the oil. Most customers will require an AV of less than 30 in marine oils but AV may need to be as low as 10, depending on the market.
The AV test is a good way to measure secondary oxidation products and should be used together with a primary test like PV.
The Totox value is calculated by the formula AV + 2PV to indicate an oil’s overall oxidation state. The lower the Totox value, the better the quality of oil.
The acid value (which is twice the free fatty acid (FFA) value) measures how many fatty acids (a component of oil) are cleaved from their parent molecules (triglycerides or phospholipids). Cleavage of a free fatty acid from a parent molecule shows hydrolytic breakdown and is often used in whole biological systems as an indication of stress. The test reveals enzymatic activity due to microorganisms in the raw material.
Thiobarbituric acid (TBA) value
The TBA value is a method to investigate secondary oxidative aldehyde products, usually in PUFA. The TBA test is unsuitable for complex food and biological systems. The test is not specific and can interact with other non-oil minor components (DNA, sugars).
Iodine value (IV)
The IV (“iodine adsorption value” or “iodine number” or “iodine index”) measures the number of reactive double bonds present in an oil. A higher IV number indicates more double bonds in the sample and therefore that greater care will be needed to slow down oxidation. IV can range from 10 for coconut oil, 94- 120 for rapeseed oil, and 117-143 for soybean oil, up to 185 for fish oil. IV is not a measure of quality but is an indicator of oil composition.
Several different factors can be controlled to reduce the amount of oxidation that occurs in your oil. These include:
A) Temperature - the number one contributing factor of oxidation in oil. If possible reduce the temperature to the lowest possible throughout processing, shipping and manufacture.
B) Oxygen exposure (in the air) will be a catalyst for production of free radicals. To reduce oxidation, seal all containers with the smallest possible headspace, reduce the area of the oil in contact with air and/or cover the oil with an inert gas (like nitrogen) at all possible points.
C) Light (UV) can trigger the oxidative degenerative cascade. Reduce the exposure of the oil to direct light by using brown glass/plastic containers or black plastic bags.
D) Moisture in combination with these other factors can accelerate oxidation. If possible limit the amount of water in the oil to less than 0.2%.
E) Transition metals, in particular transition metals like iron and copper, can act as pro-oxidants. If possible, you may need to change equipment (e.g. no brass valves), limit time in contact with and/or remove transition metals from the oil to reduce oxidation.
F) Antioxidants (AOX) terminate the oxidation chain reactions and inhibit other oxidation reactions by being oxidized themselves. AOX do not stop oxidation but do slow down the rate at which oxidation occurs.
There are many different ways AOX can reduce oxidation, so it is important to choose the right AOX for your application/product. AOX can be synthetic or extracted from natural plant sources. Common AOX are:
A) Tocopherols (vitamin E) Carotenoids & ?-carotene
BHT- butylated hydroxytoluene
BHA - butylated hydroxy anisole
TBHQ - tert-Butylhydroquinone
C) Natural extracts Flavonoids
Rosemary and spice extracts Tea catechins
E) Retinol (vitamin A)
F) Preventive Antioxidants
Citric acid, phosphoric acid, ascorbic acid (vitamin C), ethylene diamine tetra acetic acid (EDTA), uric acid