Role and functions of emulsifier as a feed additive in poultry and pigs

The major cost of feed formulations is energy. Hence, from an economic point of view, enhancing the energy efficiency of these raw materials is of great importance. Fats and oils are important energy sources in animal diets due to their high energy value and energy density. The amount of energy that animals can receive from dietary fat mainly depends on the digestibility of the fat. Fat digestibility is considerably reduced when emulsifying molecules like bile salts are absent.

In Ho Kim
Department of Animal Resources and Science
Dankook University, Cheonan, Republic of Korea

The recent increase in feed prices has prompted scientists to find an effective strategy to reduce feed costs without compromising the performance and health of animals. It is well documented that young animals have physiological limitations in absorbing fat because of low levels of natural lipase production and a low rate of bile salt production. The incomplete fat digestibility has consequently led to a greater requirement for energy sources, resulting in a hike in feed prices.

Figure 1. Formation of an oil-in-water (O/W) emulsion

The major cost of feed formulations is energy. Hence, from an economic point of view, enhancing the energy efficiency of these raw materials is of great importance. Fats and oils are important energy sources in animal diets due to their high energy value and energy density (Figure 1). The amount of energy that animals can receive from dietary fat mainly depends on the digestibility of the fat. Fat digestibility is considerably reduced when emulsifying molecules like bile salts are absent. Studies are focused on ways to improve fat digestibility to overcome the issue of inefficient energy utilization by monogastric animals. A particular strategy that is recently being considered by nutritionists for efficient energy utilization is the use of exogenous surfactants, often called “emulsifiers”, which have been effectively digested by lipase. Emulsifiers stabilize a mixture of two substance that do not combine well together, such as oil and water, and stop the coalescence of the globules in the dispersed phase. Thus, lipid has to be emulsified first. The emulsified small fat droplets are formed to increase the contact area for the lipase enzyme, which is produced by the pancreas to aid in the breakdown of fat. The natural or synthetic emulsifiers are used to improve fat digestibility and enhance energy efficiency, thereby reducing feed costs.

Emulsifiers can facilitate the digestion and absorption of lipids because they can well disperse the oil droplets in the emulsion. Based on the hydrophilic-lipophilic balance principle, an emulsion type is selected systematically, and the right mix of emulsifiers can increase stability. Emulsifiers can get to the protein-stabilized interface through small gaps in the entangled protein layer, which causes surface pressure and the creation of emulsifier-rich domains at the interface (Figure 2). A hydrophilic head and a hydrophobic tail make them up. The hydrophobic tail is oriented towards the oil phase, whereas the hydrophilic head is directed towards the watery phase. The lipophilic portion is made up of fatty acids produced from fats and oils such as soybean oil, rapeseed oil, coconut oil, and palm kernel oils, whereas the hydrophilic portion can be made up of sucrose, propylene, glycol, glycerol, sorbitol, or polyglycerol.

Figure 2. The effects of competitive adsorption of emulsifiers, including glyceryl monooleate and sucrose ester with milk proteins and emulsifier-fat interactions on the fat crystallization behavior and stability of whipped-frozen emulsions (Jinju et al., 2020).

IMPACT OF EMULSIFIER ON POULTRY AND PIG PERFORMANCE AND NUTRIENT DIGESTIBILITY
Natural emulsifiers are produced in the animal body e.g. bile and phospholipids, as well as from plants such as soy lecithin. Various emulsifiers are currently used in poultry and pig diets and their effects are shown in Table 1. In the earlier study, Jin et al. (1998) stated that weanling pig-fed basal diets having tallow as an energy source and supplemented with lecithin showed higher average daily gain (ADG), average daily feed intake (ADFI), and gain: feed ratio (G: F) than pigs fed basal diet without emulsifier addition. In addition, Jones et al. (1992) found that adding emulsifiers such lecithin or lysolecithin to nursery diets that contained soybean oil or tallow as an energy source enhanced fat digestibility but not in diets that contained lard. Moreover, Kim et al. (2008) found that feeding lecithin supplements to finishing pigs raised ADG and lowered feed conversion ratio (FCR). Xing et al. (2004) reported that lysophospholipid (LPL) supplementation to diets containing 5% lard improved ADG during days 15– 35 and the overall period (d 0–35). The positive effects on growth performance could be the result of increased palatability with the addition of LPL or lecithin to the diets, which caused a greater feed and energy intake (Overland and Sundstol, 1995). In sows, the inclusion of 3% LPL in the diets decreased BW loss and backfat thickness loss while improving apparent nutrient digestibility and milk nutrient concentration (Zhao et al., 2017). Sometimes they can enhance the absorption of other nutrients, such as protein. According to Zhao et al. (2015), the apparent total tract digestibility of DM, GE, and crude fat in pigs fed LPL diets rose throughout the first two and subsequent three weeks after weaning. Jin et al. (1998) agreed that the lecithin emulsifier in the diet increased the ATTD of DM, GE, and N in weanling pigs. Similar results were found when LPL was supplied to growing pigs (Dierick and Decuypere, 2004). Overland et al. (1993a) observed that the addition of soy oil improved the ATTD of DM, GE, crude fat, and CP. On the contrary, the ATTD of DM, CP, or GE was linearly decreased by lecithin (Overland and Sundstol, 1995) or LPL (Xing et al.,2004) supplementation in weanling pigs. Soares and Lopez-Bote (2002) found no evidence that dietary LPL supplementation had any effects on weanling pigs’ ability to digest DM, CP, or crude fibre during the first two weeks after weaning or the following two weeks. Additionally, when tallow and lecithin were employed, ADG significantly increased from d 0 to 7 post-weaning, according to Jones et al. (1992), but as pigs neared d 35 post-weaning, the beneficial effect appeared to fade. This trend means the beneficial effect on growth performance was weakened according to the age of pigs increased. Thus, the effects of emulsifiers are influenced by the age of the animal and the types of fat used in the diet.

The most important characteristic of meat quality is tenderness. Hence, increasing the softness of pork is crucial to raising consumer pleasure. The supplementation of lecithin (0, 0.4, 2, and 8%) in the diet has been reported to improve chewiness and dressing percentage, and reduce pork color (Akit et al., 2014). The improved chewiness was suggested due to a decrease in collagen content by the inclusion of an emulsifier in the diet. In addition, the hardness and chewiness of the m. Longissimus thoracis were reduced by the 1.5 and 7.5% lecithin treatments and not the 0.3% lecithin treatment group. Furthermore, the supplementation of lecithin to a basal diet containing lard as an energy source had lower serum cholesterol concentration in weaning pigs than those fed diet containing tallow as an energy source. On the other hand, pigs fed lard plus lysolecithin had higher cholesterol than those fed tallow plus lysolecithin (Jones et al., 1992). It indicated that there was an interaction between emulsifiers and energy sources on total cholesterol concentration. The concentrations of HDL or triglyceride were not affected by dietary fat sources (soybean oil, poultry fat, and a blend of 50% soybean oil and 50% poultry fat or emulsifier addition) (Guerreiro Neto et al., 2011). Zhao et al. (2015) found that the addition of 0.05 and 0.10% LPL had no discernible impact on LDL cholesterol. Fat sources and emulsifier types potentially have an influential role in the improvement of serum lipids in animals.