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JSM Microbiology

Cite this article: Lajnaf R, Gharsallah H, Attia H, Ayadi MA (2021) Antimicrobial properties of native and digested camel milk proteins-A review. JSM Mi-crobiology 8(1): 1054.

*Corresponding authorRoua Lajnaf, National Engineering School of Sfax, Sfax Tunisia, University of Montpellier, Montpellier France Tel: +216 74 675761, Email id: roua_lajnaf@yahoo.fr ; roua.lajnef@enis.tn

Submitted: 25 July 2021

Accepted: 08 August 2021

Published: 11 August 2021

Copyright© 2021 Lajnaf R, et al.

OPEN ACCESS

Keywords•Camel milk; Proteins; Antimicrobial activities;

Enzymatic digestion

Review Article

Antimicrobial properties of native and digested camel milk proteins-A reviewRoua Lajnaf1,2*, Houda Gharsallah1,3, Hamadi Attia1, M.A. Ayadi1

1Alimentary Analysis Unit, National Engineering School of Sfax, BPW 3038, Sfax, Tunisia2Montpellier University, UMR IATE, Place E. Bataillon, 34095 Montpellier Cedex 5, France3Tunisian Olive Institute, University of Sfax, Tunisia

Abstract

A comprehensive review on camel milk protein composition in comparison with cow milk proteins and an overview of antibacterial properties of native and enzymatically hydrolyzed proteins are provided. Native camel milk protein fractions as caseins and whey proteins were found to present significant antibacterial activities especially against Escherichia coli which is one of the most common contaminating bacteria in the food chain. While purified proteins as α-lactalbumin and β-casein exhibited attractive antifungal activities against Aspergillus species. Detailed investigations on the effect of enzymatic digestion on antimicrobial properties of camel milk proteins against Gram-positive and Gram-negative bacteria were also investigated for the improvement of of camel caseins and whey hydrolysates production and consumption as nutritional components or additives worldwide.

INTRODUCTIONCamel milk is an important food source for human nutrition

in the arid parts of the world for centuries. This milk is reported to contain all the essential nutritious components which are already found in bovine milk. It is even richer in vitamin C, iron, and other bioactive substances such as lactoferrin and lysozyme leading to various therapeutic properties and health benefits (1). Thus, there has been a growing interest recently in this milk as an potential alternative to bovine milk due to its exceptional nutritional value and therapeutic effects (2).

According to the latest Food and Agriculture Organization (FAO) statistics, camel (both species) milk production in the world is reported to be about 3.1 million tons per year representing 0.4% of the total milk production of the world, while the cow milk production represents 81.1% of total milk production (715.9 million tons per year). Kenya is currently expected to be the biggest producer of camel milk worldwide followed by Somalia and Mali (3). In Tunisia, total camel milk production is estimated to be around 1095 tons per year, representing only 0.1% of total milk production in Tunisia (1,3).

Recent studies conducted on camel milk have mainly concentrated on its physico-chemical characteristics and composition in comparison to bovine milk. Indeed, this milk is distinguished by its ability to last for more than 10 days when it is pasteurized (4). Furthermore, camel milk was found to remain stable for the longest time at room temperature among milk samples from other mammalian species. Camel milk took 8 hours to reach a pH value of 5.8 at 30°C, whereas bovine milk took 3 hours to reach a pH level of 5.7 at the same temperature, leading

to note that camel milk is characterized by a higher conservation when compared to cow milk (5). This behavior was explained by the presence a greater content of antimicrobial components such as lysozyme, lactoferrin and immunoglobulins in camel milk than other mammalian milks (6-8).

Proteins in camel milk are a collection are classically classified according to their solubility into caseins and whey protein representing respectively 73.1% and 26.9% of total proteins (9). First, camel caseins are characterized by different proportions of the known caseins α-, β- and κ- when compared to and caseins from bovine and other mammalian species. For instance, Camel milk is distinguishing by its higher amounts in β-casein representing respectively 44% and 37.4% of camel and bovine milk proteins. The high percentage of β-casein leads to higher digestibility rate of camel caseins as well as lower incidence of allergy to infants (10). Furthermore, camel caseins contain lower amounts of κ-casein and αS1-casein amounts than those of cow milk (9). The κ-casein and α-casein represent respectively 7.1% and 24.7% in cow milk. For camel milk the α-casein represent 29.1% of total milk proteins, while no peaks were detected for the camel κ-casein in the chromatograms due to its low concentration making it obscured by the other caseins (9). Camel whey is distinguished by being totally devoid of the β-lactoglobulin which is the major whey protein in cow milk representing more than 55% of total bovine whey proteins (11-14). Therefore, the α-lactalbumin is the main camel whey protein representing approximately 72.8% of total proteins in camel whey followed by Camel Serum albumin (7.8%) and lactoferrin (4.1%) (14-17). The absence of the β-lactoglobulin in camel milk leads scientists to suggest this milk as a suitable alternative for children suffering

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from cow milk proteins allergy as the β-lactoglobulin is one of the main milk allergens. Camel whey contains also specific minor proteins which are not comparable to any protein in bovine milk as CWBP (Camel Whey Basic Protein), PGRP (Peptidoglycan recognition protein) and WAP (Whey Acidic Protein) (14, 17-19).

The contamination of food products by pathogenic bacteria is of significant issue in the food industry. Thus, to enhance the safety of food products and their shelf life several methods have been employed including the use of natural antimicrobial agents as well as synthetic agents. Due to the detrimental impact of synthetic agents on the human health and the environment, there has been a growing interest to the natural sources of antimicrobial agents in the food. Therefore, there is a need for novel antimicrobial agents as food proteins (20).

This review summarizes the available information of camel milk proteins in both native and digested states, with emphasis on their antibacterial properties.

ANTIMICROBIAL PROPERTIES OF NATIVE CAMEL MILK PROTEINS

Antimicrobial properties of proteins in their mixture

Several studies have shown that camel milk proteins are important nutritional and functional sources and could provide particular biological activities such as antimicrobial properties either in the pure state or as a mixture (1,16,21,22). Jrad et al. (22) studied the antibacterial activities of whole native camel milk caseins against three Gram-positive bacteria (B. cereus, L. innocua and S. aureus) and 2 Gram-negative bacteria (E. coli and P. aeruginosa). These authors found that cell growth was significantly inhibited only E. coli strain in the presence of camel caseins at a protein concentration of 20 g/L. Indeed, a decrease of 19.3% ± 0.02 of E. coli, among the tested bacteria, was observed in the presence of undigested camel caseins at a concentration of 20 mg/mL.

However, the growth of the strains of L. innocua, B. cereus, S. aureus and P. aeruginosa was not significantly affected at the same protein concentration. On the other hand, the growth of Listeria innocua was significantly inhibited in the presence of higher camel casein concentrations (40 g/L) (22).

The antimicrobial activities of the soluble fraction in camel milk were also carried out by Salami et al. (23). These authors reported that native camel whey proteins presented higher antimicrobial activities toward Escherichia coli when compared to their bovine equivalent at a protein concentration of 0.5 mg/mL. These findings were explained by the higher content of antimicrobial factors in camel whey such as lactoferrin, lysozyme and immunoglobulins. Indeed, immunoglobulins in camel milk (IgGs) are quite unique and exceptional in the animal world leading in the neutralization of bacterial and viral enzymes (23,24).

Antimicrobial properties of pure proteins

Purified camel milk proteins showed an important antimicrobial activity in both soluble or colloïdal phases. For instance, the purified camel β-casein exerted strong antifungal activities towards Aspergillus tamarii and Aspergillus sclerotiorum

at a protein concentration of 5 g/L with inhibition zones of 20 ± 0.9 and 30 ± 1.5 mm, respectively, while its bovine counterpart no bactericidal activities (25). However, camel β-casein didn’t exhibit any antibacterial activity even at a protein concentration of 5 g/L. Indeed, camel β-casein did not exhibit any bactericidal activity against Enterococcus faecalis, Escherichia coli Pseudomonas aeruginosa and Staphylococcus aureus. Meanwhile, this protein had no antifungal activity against Aspergillus protuberus and Penicillium bilaiae even at a concentration of 5 g/L (21).

For whey proteins, the purified lactoferrin from camel milk, which is one of the major whey proteins representing 4.1% of total camel whey proteins, exhibited promising antibacterial activity against Staphylococcus aureus and E. coli. This protein showed the best bactericidal activity at a protein concentration of 4 mg/mL with a higher effect against E. coli compared to that of Staphylococcus aureus. The inhibition zones at this concentration are 28±1.7 mm and 20±0.57mm for E. coli and Staphylococcus aureus, respectively (26).

Camel α-lactalbumin exhibited significant antibacterial and antifungal activities in its apo (calcium-depleted) form because it particular molecular structure and amino acid residues composition when compared to its bovine counterpart (16, 27). Indeed, camel apo α-lactalbumin had important antimicrobial activities in vitro against Gram (-) bacteria (Pseudomonas aeruginosa) and against three fungal pathogens species (Aspergillus sclerotiorum, Aspergillus tamarii and Penicillium bilaiae) at a protein concentration of 1 g/L with inhibition zones ranging between 9 and 16 mm. While, bovine α-lactalbumin didn’t show any bactericidal activity in this study (16). However, camel α-lactalbumine in its holo (calcium loaded) had no bactericidal activity against Pseudomonas aeruginosa, Enterococcus faecalis, Staphylococcus aureus and Escherichia coli and no antifungal activity against Aspergillus tamari, Aspergillus sclerotiorum, Aspergillus protuberus and Penicillium bilaiae at a protein concentration of 1 g/L (16).

ANTIMICROBIAL PROPERTIES OF DIGESTED CAMEL MILK PROTIENS

Previous works showed a few strategies to improve the antimicrobial activities of proteins such as enzymatic hydrolysis (20, 28). Indeed, milk protein-derived bioactive peptides are inactive within the protein sequence. However, they can be released either in vitro by enzymatic hydrolysis or in vivo by digestive proteases. Once bioactive peptides obtained after digestion are released, they can act as regulatory compounds with specific activities such as antimicrobial activities (21, 29) Hayes et al. (30) studied the hydrolysis of casein proteins during fermentation with proteolytic bacterial strains to produce antimicrobial peptides. Kumar et al. (31) evaluated the antibacterial activities enzymatic digested camel casein samples with Alcalase, α-Chymotrypsin and Papain against Gram-negative (Escherichia coli) bacteria and Gram-positive (Staphylococcus aureus, Listeria monocytogenes and Bacillus cereus). The antimicrobial activity (measured as the zone of inhibition) was found to be higher for whole obtained hydrolysates with all enzymes as compared to their fractions with almost comparable inhibition zones with inhibition zones ranging between 12.46

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and 19.11 mm depending on the tested bacteria and the used enzyme. Overall, Alcalase and α-Chymotrypsin produce peptides characterized with higher antimicrobial activities (31). Meanwhile, whole hydrolysates obtained with α-Chymotrypsin exhibited higher inhibition effect against S. aureus than that of Papain and Alcalase. On the other hand, casein hydrolysates exhibited an inhibitory effect against E. coli which is one of the most common contaminating bacteria in the food chain, with better results of peptides generated by Alcalase and α-Chymotrypsin than those obtained with Papain. The higher antimicrobial activities of whole hydrolysates obtained with camel caseins are explained by the presence of different peptides characterized by various sizes and charges leading to a significant synergic contribution to their activity (31) According to the study of Jrad et al. (22), the antibacterial activities of camel milk caseins against E. coli was increased when compared with those of native casein after hydrolysis by pepsin and pancreatin enzymes. Indeed, after successive hydrolyses by pepsin and pancreatin, camel milk casein hydrolysates exhibited anti-bacterial activity against E. coli with a decrease of 19.73±0.01% under the same conditions.

In addition to caseins, camel whey proteins presented significant antimicrobial activities in their hydrolyzed state. Ineed, Salami et al. (23) found that both of bovine and camel whey proteins, and their hydrolysates, inhibit the growth of E. coli. These authors reported that limited proteolysis of camel and bovine whey proteins improved their antimicrobial effects against E. coli with the highest inhibition of growth obtained with hidrolyzed camel whey proteins using proteinase K followed by ultrafiltration on an ultrafiltration membrane with a 3 kDa molecular mass cut-off (23). However, the degree of hydrolysis with trypsin and chymotrypsin was low, and thus the antimicrobial activities were not significantly enhanced (20, 23).

The antibacterial activities of Trypsin-hydrolyzed camel whey proteins were studied recently by Wang et al. (32) In this study, both of camel and cow milk hydrolyzed whey proteins presented strong antibacterial activity against gram-negative bacteria (E. coli and Salmonella typhimurium) and gram-positive bacteria (Streptococcus mutans and Staphylococcus aureus). The inhibition zones of hydrolyzed camel whey proteins ranged between 9.96 mm (Streptococcus mutans) and 26.9 mm (E.coli). Meanwhile, the inhibition zones of hydrolyzed bovine whey proteins ranged between 11.13 mm (Streptococcus mutans) and 24.53 mm (E. coli). The antibacterial effect of camel whey Trypsin generated bacteriostatic peptides was stronger than that of their bovine counterpart (32).

SUMMARY AND CONCLUSIONResearch proven that camel milk proteins have promising

antimicrobial activities in their native and hydrolyzed sates. First, native camel milk proteins in their natural mixture (caseins and whey proteins) exhibited significant effect towards pathogenic bacteria especially E. coli. Pure camel proteins such as lactoferrin had important antimicrobial activities in vitro against both Gram (-) and Gram (+) bacteria. On the other hand, camel β-casein and α-lactalbumin, which are the main camel milk proteins, showed strong antifungal activities against two species of Aspergillus. Previous results in this review showed also that camel milk can

be used as a source of natural protein to produce hydrolysates with interesting antibacterial activities. Once digested, camel milk proteins release different bioactive peptides which are necessary to fulfill antimicrobial properties. These results will encourage the future development of camel caseins and whey hydrolysates as nutritional components or additives of antimicrobial drugs. Finally, the results revealed the potential of camel milk proteins as well as their hydrolysates as antimicrobial agents for human consumption, and as food ingredients in health foods to improve their functionality and shelf life.

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Lajnaf R, Gharsallah H, Attia H, Ayadi MA (2021) Antimicrobial properties of native and digested camel milk proteins-A review. JSM Microbiology 8(1): 1054.

Cite this article

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