Milk is a liquid mixture of water, protein, lactose, vitamins and minerals secreted by the mammary glands
(Alais, 1971; Agudelo and Bedolla, 2005). Goat’s milk is the third most produced worldwide, after cow’s and buffalo’s milk. Approximately 81% of milk production in the world derives from cattle, followed by milk from other species, such as buffalo (15%). At the same time, goats, sheep and camels contribute to about 4% of total milk production
(FAO, 2019). However, the milk production of small ruminants, including sheep and goats, has grown in recent years, seeking new consumer markets
(Selvaggi et al., 2014a). In addition, milk production doubled in the last decade and could increase up to 53% by 2030
(Pulina et al., 2018). In turn, the cost of maintenance and feeding goats is meager, but the marketing and use of goats are still limited in developing countries
(Kumar et al., 2016). Goat’s milk and its derivatives are gaining interest due to its quality and quantity, its proteins, carbohydrates, fats, vitamins and minerals and its functional properties
(Albenzio et al., 2016).
Protein is a precursor characteristic of many human health benefits in goat’s milk. The percentage of protein in goat’s milk is similar to cow’s milk; however, some differences come from their constituents, such as casein content and structure of the casein micelle, bioactive peptides and the profile of the amino acids, nucleotides and nucleoside
(Kumar et al., 2016). Another characteristic of the goat’s milk is the content of nutraceuticals such as phenolic compounds; this nutrient helps consumers’ health and hence proper and adequate feeding of the goat can modify its quantity
(Chávez-Servín et al., 2018).
The feeding systems for ruminants such as goats have been found to affect the composition of goat milk and can influence antioxidant activity and nutraceuticals compounds
(Keles et al., 2017, Zazharska et al., 2018). Other factors affecting milk composition are the growing season and cutting time of forages and grasses
(Di et al., 2015). This review aims to show the impact of the feeding system on the nutritional and functional characteristics of goat’s milk, as well as on the advantages of goat’s milk as a potentially available product and its benefits to human health. This work was done from 2020 to 2021 at the Engineering Faculty of the Amazcala Campus of the Autonomous University of Querétaro (UAQ) México.
Implications of nutrigenomics
Use of forages on the ruminants feeding
Forages are essential in ruminant diets (cows, goats and sheep); legumes are the most important. The legumes are consumed in quantities about 20 per cent greater than grasses because the legumes contain fewer cell walls
(Rojas et al., 2019). The cell wall quantity in forages determines the rate of digestion and intake in ruminants. The leaves are more easily broken down than stems in the plant because of their cell walls (lignin quantity). The lignification in legumes and grasses is different; in grasses, the lignin is more widely distributed in the plant and it has a more significant inhibitory effect on the rate of digestion. In legumes such as alfalfa, the lignin and protein quantity depends on the cutting time of the plant
(McDonald et al., 2010).
One good example of forage is the genus
Medicago which belongs to the legume subfamily as alfalfa (
Gholani et al., 2014). Alfalfa is the fourth crop with the highest economic value in North America after corn, soy and wheat. It is the most important crop of forage utilized in ruminant feeding globally
(Mielmann, 2013). Many factors, such as the harvest period, influence alfalfa´s nutritionally. Cutting the alfalfa in an optimal growth state can improve the crop’s fields and quality
(Yari et al., 2012). A decrease of crude protein (PC) and an increase in fiber has been reported according to the maturity states
(Lamb et al., 2003). Alfalfa proteins are mainly located in chloroplasts and the change from vegetative growth to reproductive growth increases nutrient production and decreases crude protein content
(Lamb et al., 2012). The protein is related to L-glutamic acid and glutamine content; they are precursors of several amino acids, purines and pyrimidines. L-glutamic acid and glutamine content in mid-flowering alfalfa are significantly lower than in the outbreak period; this is vital for cutting alfalfa for ruminant feeding
(Bezerra et al., 2016).
The rumination and fermentation processes are fundamental in ruminant animals such as goats. These are relatively slow processes and fibrous foods (cell wall resistance) may take a long time in the digestive tract to extract their digestible components. Reducing lignin content in forages can improve digestibility and animal performance
(Barros et al., 2019).
Impact of forage on the quality of Goat’s milk
The feeding system using free-range grazing or permanent confinement has affected the composition of goat’s milk. It may influence the antioxidant activity not only in the milk but also in milk products
(Keles et al., 2017). An additional factor affecting the composition is the year’s season, rainy or dry
(Hilario et al., 2010; Di et al., 2015). Kuhnen et al., (2014) found that the amount of phenolic compounds in milk samples differs among year’s seasons of the year due to the differences in the botanical composition of ingested forages, diversity of plant species and pastures in each season.
Di et al., (2015) show that the consumption of fresh forage diets provides an antioxidant effect in the products elaborated with goat’s milk due to the role of phenolic compounds that directly impact antioxidant activity in milk.
The modification in the composition of feeding has repercussions on milk design. Increasing the proportion of forage in goat’s feeding, the quantity of protein is higher and the fat lower
(Khaskheli et al., 2020). The animals can consume unsaturated fatty acid to increase the synthesis of fatty acid of Novo in the mammary gland. A study on feeding goats and sheep with spring and summer forage had significant content of linoleic acid concerning autumn and winter
(Fernández et al., 2006).
Goat’s feeding and its impact on milk flavor
Goat’s milk flavor is a characteristic of lower lipolysis and is responsible for rancidity
(Chilliard et al., 2003). One possibility to attack the flavor aspect is employing lipid supplementation in the diet to increase the ruminant’s consumption of energy of high interest in the first lactation, increasing milk production and limiting the mobilization of the body lipids
(Chilliard and Bocquier, 1993). Furthermore, the goat’s milk lipolytic system and medium-chain fatty acids could significantly change the content of free fatty acids, playing a significant role in the occurrence of the characteristic goat flavor
(Chilliard et al., 2003).
Fat supplementation in diets could improve goat’s milk composition for greater control of cheese processing and satisfaction of consumer demand. Still, the results show positive or adverse changes in goat dairy products’ physical characteristics and nutritional or dietetic properties
(Chilliard et al., 2003). The addition of flavors carries out the development of new products from goat’s milk; aromas using essences and fruits can modify its taste and contribute to its nutritional value
(Machado et al., 2017).
Goat’s milk and its nutritional and functional properties
In 2016, the United Nations Food and Agriculture Organization (FAO) reported approximately 1 billion goats worldwide, with 15.2 million tons of milk. The significant producers of goat’s milk are India, Bangladesh and Pakistan in Asia; Sudan, South Sudan and Somalia in Africa; France, Spain and Greece are the leading Europe producers
(FAO, 2019).
Cow’s milk is the most consumed worldwide
(Kumar et al., 2016). There is a noticeable difference in fat and protein between cow´s and goat’s milk. The fat content is 4.2% in cow’s milk and 3.8% in goat’s milk
(Ahmad et al., 2013). The casein protein micelles’ size ranged from 100 to 200 nanometers (nm) in goat’s milk and from 60 to 80 nanometers (nm) cow’s milk
(Silanikove et al., 2010). The size of the protein micelles makes a marked difference in their sedimentation rate, solubilization and heat stability
(Haenlein, 2004). These characteristics are essential for making sub-products with goat’s milk as a different cheese or yogurt.
Bioactivity of Goat’s milk-derived peptides
The proteins of goat’s milk are essential nutrients that should be present in diets for infants and young’s children
(FAO, 2019). An
in vitro study using gastric and duodenal juices demonstrated that goat’s milk proteins degraded faster than cow’s milk proteins. This characteristic represents an advantage for digestibility as suggested by
Albenzio et al., (2012). The whey proteins such as a-lactoalbumin, β-lactoglobulin, albumin and Lactoferrin are less susceptible to gastric digestion and enter the intestinal tract as intact protein, where they carry out their biological functions
(Inglingstad et al., 2010) as antimicrobial action
(Liu and Newburg, 2013), cognitive development
(Timby et al., 2014) and immune modulation in the neonate
(Donovan, 2016).
Most bioactive peptides derived from goat’s milk had antimicrobial, antihypertensive, antioxidant and opioid (agonist and antagonist) functionalities. Six peptides have been identified from b-casein (antimicrobial; ACE-inhibitory, immunomodulatory; anti-thrombin), two from as1-CN (antimicrobial; ACE-inhibitory), one from as2-CN (antimicrobial; ACE-inhibitory) and k-CN (antimicrobial)
(Liu and Newburg, 2013). Low numbers of b-Lactoglobulin (b-Lg) peptides (ranging from 0-1.2%) have been found in cow and goat’s milk because b-Lg is resistant to pepsin
(Hodgkinson et al., 2019).
Other peptides released in the β-casein hydrolysis are the tripeptides inhibiting the angiotensin-converting enzyme (ACE), which has the following amino acid sequence: Valine-proline-proline (VPP) and Isoleucine-Proline-Proline (IPP) and they found in goat’s milk or derived products such as cheese, which show cardiovascular and antihypertensive activity
(Murray and Fitz-Gerald, 2007;
Ricci et al., 2010). These peptides are enormously increased by tripeptides hydrophobic chains with aromatic or branched amino acids
(Ricci et al., 2010). Pihlanto-Leppälä et al., 1998 demonstrated that the fermentation of milk with cell crops is not enough to generate the tripeptide of ACE. Digestion with pepsin and trypsin could be necessary. The last explains what can happen under
in vivo conditions or the gastrointestinal tract when fermented products are ingested and generate bioactive peptides.
Gastric and duodenal enzymes degrade goat’s milk proteins faster and more efficiently than camel, cow and sheep milk
(Tagliazucchi et al., 2018). Almaas et al., 2006 found that goat’s milk protein degrades faster than cow milk using human gastrointestinal proteolytic enzymes. (
Espejo-Carpio et al., 2013) reported an increase in the digestibility of goat’s milk proteins as a function of the enzyme-to-substrate ratio. The digestibility of the goat’s milk will be more rapid than in other types of milk and benefit the consumer’s health.
β-casein, another type of casein, is more frequent in goat’s milk, representing between 48-60% of total caseins
(Potocnik et al., 2011). The polymorphic variants of β-casein from the most common dairy cattle breeds are A1, A2 and B. The amino acid at position 67 of the β-casein sequence is critical for releasing β-casomorphin-7. β-casomorphin-7 (BCM-7) is a peptide released by the hydrolytic action of β-casein, associated with a genetic variant related to the breed of goats and cows. In variant A2, the hydrolysis of Isoleucine 66-Proline 67 does not occur or is performed in a low range. For variant A1, the release of BCM-7 is generated by pepsin’s action. Goat’s milk tends to b-casein A2 because the amino acid 67 is a Proline and not developed BCM-7
(De Noni et al., 2009, Muñoz-Salinas et al., 2022).
Nutraceutical characteristics of goat’s milk
In milk and goat cheese, endogenous antioxidants of proteinic origin, such as Lactoferrin and exogenous antioxidants, such as vitamins A, E and C. (
Raynal-Ljutovac et al., 2008). Goat’s milk contains more vitamin A (2,074 IU) than cow’s milk (1,560 IU) due to goats turning all carotene into vitamin A, which is why it’s whiter than cow’s milk. Additionally, goat’s milk is a rich source of riboflavin that acts as a growth factor and has 350% more niacin than cow’s milk
(Pfeuffer, 2000).
The phenolic compounds in foods showed their benefits by reducing pathogenesis or the severity of chronic diseases, including cardiovascular diseases
(Redan et al., 2016). So far, the phenolic compound content in goat’s milk and dairy products like cheese and whey has been little studied
(Hilario et al., 2010). The ruminant’s milk has higher conjugated linoleic acid (CLA) than non-ruminants; this anticancerigen acid decreases atherosclerosis
(Pfeuffer, 2000). The content of CLA is significant when feeding goat with grasses
(Fernández et al., 2006,
Kumar et al., 2016).
The oligosaccharides present in milk can act as prebiotics, which helps to maintain the gastrointestinal tract healthy by promoting the growth of beneficial bacteria and preventing the development of pathogenic bacteria. More than 95% of oligosaccharides in human milk are resistant to digestion and used by intestinal bacteria. The oligosaccharides from goat’s milk share a similar structure to human milk
(Urashima et al., 2013). Goat’s milk contains between 250 to 300 mg/L oligosaccharides, 4 or 5 times higher than cow’s milk
(Martínez-Ferez et al., 2005). The goat’s milk showed higher levels of mannose-6-phosphate. It participated in the glycolytic pathway and was implicated in synthesizing oligosaccharides, biosynthesis of glycoproteins and glycophospholipids
(Kalhan, 2009).
Selenium plays a cofactor in the activity of glutathione peroxidase. It prevents lipid peroxidation in mammals
(Crespo et al., 1995) and has a protective effect on events such as atherogenicity
(Wojcicki et al., 1991). In human, goat and camel’s milk, selenium is about 50% higher than cow’s milk
(Debski et al., 1987). Higher selenium content could be because the goat’s and camel’s milk improves oxidative stress in experimental type-I diabetic rats
(Meena et al., 2015).
