Essential Oils in Animal Diets to Improve the Fatty Acids Composition of Meat and Milk Quality in Ruminant

Abstract

Adding essential oils to the diet of ruminants is a novel strategy that improves milk and meat quality by enhancing production and fatty acid content. Including essential oils has various effects, such as modifying the biohydrogenation of unsaturated fatty acids. As a result, the fatty acid profile leaving the rumen can be affected, which in turn can affect the levels of important fatty acids in the milk and meat produced by ruminants. In the rumen, microorganisms convert unsaturated fatty acids to mostly saturated fatty acids and some unsaturated fatty acids through biohydrogenation. Added essential oils can shift the rumen microbiota, followed by changes in the fatty acid profile. The impact of essential oils on the biohydrogenation of fatty acids depends on various factors such as the type of essential oil used, its chemical composition, interactions with nutrients present in the feed, the ability of ruminal microbes to adapt to essential oils, and type of animal. Studies have indicated that various essential oils can influence ruminal fermentation and biohydrogenation of dietary fatty acids, and thus, affect the presence of polyunsaturated fatty acids in milk and meat, which is associated with positive effects on human health.

1. Introduction

Feed utilization is strongly affected by rumen fermentation, so nutritionists have focused on the physiology of the rumen and its microbiome. The entire rumen microbial community cooperates in the digestion and utilization of the feed, rather than the efforts of individual groups or strains to perform specific metabolic pathways such as fatty acids biohydrogenation. The shifts in the composition and abundance of ruminal microorganisms are affected not only by feed type [1] but also by the use of rumen modifiers such as plant secondary metabolites, including phenolic compounds and essential oils Essential oils (EOs) [2]. EOs are volatile, complex compounds extracted from different parts of the plants to protect plants against attacks by bacteria, fungi, or insects. Common EOs fall under two chemical groups: terpenoids and phenylpropanoids, which are synthesized through secondary metabolism in plants [3]. Chemically, extracted EOs vary due to genetic determinants of the plant, stage of growth, and environmental factors [4]. Supplementing ruminants with EOs can improve their productive performance, rumen fermentation, health, and product quality (such as milk and meat) [3, 5, 6, 7].

As a result of the prohibition of antibiotics in the animal production sector, scientists have used plant EOs and phytochemicals as available alternatives to antibiotics [8]. The EOs generally have antibacterial activity against both gram-negative and gram-positive bacteria [9, 10]. The mode of action of EOs is based on their ability to disrupt cell walls and cytoplasmic membranes, leading to lysis and leakage of intracellular compounds [11].

Many gram-positive bacteria in the rumen are involved in biohydrogenation reactions of unsaturated fatty acids (FA) [12]. Manipulating the rumen fermentation process is an important function of EOs. [13], and impeding the biohydrogenation of n-6 and n-3 fatty acids in the rumen [14], which inhibits the biohydrogenation process in the rumen and increases the passage of polyunsaturated fatty acids (PUFA) into milk and meat, and this has positive effects on human health. Extensive literature supports the use of EOs as rumen modifiers; however, more studies are urgently needed to improve the quality of milk and meat by improving their PUFA content. In this chapter, we discussed the effect of EOs on the rumen microbiome and the characteristics of dairy and meat final products.

2. Biohydrogenation in the rumen

Forages and concentrate contain abundant PUFA such as α-linolenic acid (18:3n-3), linoleic acid (18:2n-6), and oleic acid (cis-9 18:1) [15] in the form of dietary lipid. In the rumen, dietary lipids are hydrolyzed and released nonesterified FA. Then, 18:3n-3, 18:2n-6, and cis-9 18:1 are converted to saturated FA via cis-trans isomerization to trans-FA intermediates, followed by hydrogenation of the double bonds [12]. This process is called biohydrogenation. However, a small percentage of PUFA passes into milk and meat. PUFA is toxic to bacteria, so it is believed that the bacteria perform the biohydrogenation process to reduce the toxicity of PUFA [16]. Bacteria prefer saturated FA for synthesizing their cell membranes because the double bonds in unsaturated FA distort their molecular shape and disrupt the structure of the lipid bilayer [17].

In general, the bacteria responsible for biohydrogenation are classified into groups A and B [18]. Group A bacteria hydrogenate 18:2n-6 and 18:3n-3 to trans-11 18:1 and related isomers, while group B bacteria convert those 18:1 isomers to 18:0. The Butyrivibrio group contains the most active bacterial species involved in the biohydrogenation of C18 unsaturated FA [19, 20], including the genera Butyrivibrio and Pseudobutyrivibrio, and Butyrivibrio proteoclasticus [21]. Within the complex ecosystem of the rumen, some bacteria could not be exclusively classified into one of the two groups. B. proteoclasticus, for instance, converts 18:1 isomers not only to 18:0 but also hydrogenates 18:2n-6 to 18:0 [22].

3. Effects of EOs on rumen microbiota

The activity of EOs can alter the rumen microbiota. Early studies on the effects of dietary EOs focused mostly on the effect of EOs on feed utilization efficiency and ruminant performance. There have been only a limited number of studies conducted on the impact of EOs on rumen microbiota. These effects typically involve regulating ruminal fermentation processes thus, it affects the final products. Benchaar et al. [23] reported that the addition of EOs affected the bacteria responsible for ammonium production, the proteolytic bacteria Provotella, and the methane-producing archaea. EOs can reduce ruminal ammonia-N production, as they can inhibit ammonia-producing bacteria Clostridium sticklandii, Peptostreptococcus anaerobius, lostridium sticklandii, and Peptostreptococcus anaerobius and reduce the decomposition of amino acids in the rumen [24].

Depending on the type of EOs, EOs may have inhibitory effects on rumen microbiota with a decrease in rumen microbial abundance, and stimulatory effects with an increase in rumen microbial abundance [25]. EOs addition can modulate rumen fermentation by altering volatile fatty acid concentration and decreasing methane emissions by broadly altering the rumen bacterial community [26]. In beef cattle, the addition of EOs increased the relative abundance of Parabacteroides distasonis and Bacteroides thetaiotaomicron bacteria, which is associated with increased propionate concentration in the rumen [27]. Additionally, in vitro studies conducted by Zhou et al. [26] revealed that the addition of EOs reduced the abundance of Succiniclasticum bacteria, which converts the succinate to propionate, indicating a negative impact on propionate concentration. EOs can effectively reduce the presence of methanogens in the rumen, and may also have a negative impact on the relative abundance of Ruminococcus flavefaciens, Ruminococcus albus, and Fibrobacter succinogenes [28] and Ruminococcus flavefaciens in dairy goats [29]. In beef cattle, EOs have been found to increase the relative abundance of the bacterial family Succinivibrionaceae [26, 27, 28, 29, 30]. This increase is strongly negatively correlated with the relative abundance of Methanobacteriaceae microorganisms [31].

On the other hand, adding EOs to ruminant diets can have varying effects on the rumen microbiota. The effects are dependent on the components of the EO, which should be obtained from the same plant species, these components can also vary depending on the geographical location and the season of harvest [32]. Furthermore, the impact of the same EO on the rumen microbiota can vary depending on the dosage used. The impacts of EOs on rumen microbiota seems also more affected by ruminal pH. The pH level can affect the dissociation of EO molecules and their final form. Some studies suggest that the undissociated hydrophobic form of the active EO molecules is more effective in antimicrobial activity as it dissolves better in the bacterial membrane’s lipid bilayer, this effect has been observed in low rumen pH conditions [33, 34].

4. Effect of EOs on the microbiota operating the biohydrogenation of fatty acids

Altering rumen populations with EOs may also have an effect on some rumen microorganisms responsible for the biohydrogenation of PUFA, which causes a modification of the rumen FA profile that passes into the milk and meat of ruminants [35]. The use of EO supplements suitable for ruminants must be well planned in terms of the type of EOs and dosage. A proper choice has a positive effect on feed efficiency, digestion, rumen fermentation, and meat and milk production, while an inappropriate choice can lead to negative effects, including reducing feeding efficiency and increasing methane production and resistance of rumen bacteria to the effects of EOs. The antibacterial properties of EOs may cause bacteria to become resistant to EOs. However, very few studies have researched bacterial resistance to EOs. EOs are a mixture of chemical compounds that have antimicrobial activities [8], making it hard for bacteria to develop resistance mechanisms [36]. However, it has been found that Staphylococcus aureus has developed resistance to EOs [37, 38].

5. Effects of EOs on milk fatty acids profile

Milk fat content and FA composition are primarily determined by lipid metabolism in the rumen and the mammary gland [39]. EO supplementation can reduce the levels of C8 and C12 fatty acids. This occurs because EOs can produce potent inhibitors that hinder the synthesis of de novo FA in the mammary gland [40, 41].

The rate and extent of FA biohydrogenation could also impact variations among animal species [42]. In small ruminants, supplementation of EOs is more effective in the biohydrogenation process. For instance, Juniper EO addition enhanced the presence of n-3 fatty acid in goat milk, while anise, clove, and juniper had minimal impact on milk components [43]. Cinnamon oil elevated the concentration of unsaturated FA, n3 linolenic acid, and conjugated linoleic acid [44]. The inclusion of garlic or juniper EO in the diet of dairy cows resulted in an increased proportion of conjugated linoleic acid in milk [45]. Clove and thyme EO were found to be more effective in decreasing the biohydrogenation process and increasing the concentration of C18:3 n-3, C18:4 n-3, and n-3 FA in milk fat and decreased the concentrations of C18:3 n-6, C20:4 n-6, and n-6 FA in the milk of lactating goats [6].

Dairy cattle fed a blend of EOs (capsaicin, carvacrol, cinnamaldehyde, and eugenol) secreted more CLA, suggesting that EO influenced the ruminal biohydrogenation of PUFA [46]. Supplementation with 750 mg of a mixture of EOs had no change in milk FA profile [47]. However, supplementing 2 g/d of the same mixture increased the concentration of CLA in milk fat [48].

6. Effects of EOs on meat fatty acid profile

Meat typically has a high content of saturated fatty acids (SFA) and a low content of PUFA/SFA [49]. Therefore, enhancing the PUFA levels in animal diets, either by incorporating a source rich in n-6 or n-3 PUFA, or by supplementing with EOs to inhibit biohydrogenation, generally leads to an improvement in the PUFA/SFA ratio. It has been found that the meat obtained from animals that are fed on pasture has a lower n-6/n-3 ratio compared to the meat from animals that are fed on grains because of the higher levels of α-linolenic acid found in pasture [50].

There are limited studies on the impact of supplementation of EOs to beef on its FA composition. Supplementing rosemary oil had a limited effect on the FA composition in various tissues and did not result in a significant alteration in the overall profile [51, 52, 53]. However, according to Nieto et al. [54] rosemary oil has a significant impact on the level of C18:0 content while causing a decrease in C16:0 fatty acid. The addition of anise, clove, and thyme EO enhanced the ratio of PUFA to SFA. It also increased the levels of n-3 fatty acids while decreasing the levels of n-6 fatty acids [5].

7. Conclusion

Changes in the FA profile of meat and milk resulting from EO addition have been shown to be variable and unpredictable in terms of n-3 fatty acids. Furthermore, to enhance our understanding in this area, it seems important to precisely characterize the effects of various environmental elements on the major rumen microbial species involved in the biohydrogenation process and to study how these effects are influenced by diet and ruminal environment. Further studies are needed on the factors that influence the biohydrogenation bacteria in the rumen so that we can propose new rational dietary modifications that will ultimately lead to healthier ruminant products for human consumption.

Conflict of interest

The author declares no conflict of interest.