This work was performed to study the microbiota of raw goat’s milk (67 samples) collected in different areas of Sardinia, in order to select autochthonous lactic acid bacteria (LAB) strains for use in goat cheese manufacturing. Total mesophilic bacteria ranged between 105 and 107 cfu/mL; mean counts of
Goat milk production is a crucial contributor to rural economy in many countries, especially in the Mediterranean and Middle East regions. Overall France, Greece, Italy, and Spain produce 49.2% of goat's milk in the Mediterranean region and 9.4% of the world goat's milk (FAOSTAT, 2018). In Italy, the population of dairy goats is reared mainly in extensive or semi-intensive systems. The size of the company is, on average, very small (36 goats/farm), with higher concentrations in Sicily (26.0%) and Sardinia (21.5%), followed by Piedmont, Lombardy, and Veneto, which together represent 22.2% of the total [
Due to the growing consumer interest in functional foods, goat's milk has gained popularity mainly because its high digestibility, high nutritional quality, low allergenicity, and potential nutraceutical properties [
Goat milk presents a rich and complex autochthonous microbiota, and its detailed knowledge is essential for the diversification of productions. This microbiota is responsible for the peculiar characteristics presented by fermented goat milk products and is composed by a wide range of microorganisms with different characteristics that can be potentially considered for use by the dairy industry. According to previous surveys, the main components of the autochthonous microbiota are lactic acid bacteria (LAB) belonging to the genera
For safeguarding and promoting the traditional Sardinian goat dairy products, information on the microbial diversity of raw milk is the preliminary step to monitor the autochthonous microflora that certainly contribute to the typical organoleptic and quality characteristics of cheeses, in order to search for and select new strains with distinctive patterns of technological properties to be used in cheese production.
The objective of this study was to isolate and identify the dominant microbiota associated with raw goat’s milk samples collected from dairy farms located in different areas of Sardinia. The strains belonging to the predominant LAB species were also analyzed by determining some technological properties relevant to their use as adjunct/starter cultures.
A total of 67 samples of raw goat’s milk collected from dairy farms located in different areas of Sardinia were analyzed. Each sample represented the pooled milk from one single milking of each herd. Samples were transported to the laboratory under refrigeration and analyzed on the same day.
Ten milliliters of milk was transferred to a sterile tube containing 90 mL of 2% (w/v) sodium-citrate solution. Decimal dilutions were prepared in sterile solution of 2% (w/v) sodium citrate and plated in duplicate on specific media to enumerate microbial groups.
Total mesophilic bacteria (TMB),
The presence of foodborne pathogens Salmonella spp. and
Yeasts were enumerated on potato dextrose agar (PDA, Microbiol) with chloramphenicol (0.01%) after incubation at 25°C for 5 days.
Several technological properties were studied on 289 LAB isolates. Caseinolytic, lipolytic, and acidifying activity, citrate utilization, and acetoin production were evaluated as reported by Cosentino et al. [
Finally, the LAB strains were screened for the antibacterial activity against the following indicator strains:
Microbial counts were calculated as the number of colony-forming units (cfu) per milliliter of sample and reported as log10 cfu/mL. Calculations of standard deviations (SD) were also performed. The mean microbial counts of milk samples collected from two geographical areas of Sardinia were analyzed by Student’s t-test using the software GraphPad Prism Statistics vs. 3.00. The significant level of test was set at P<0.05.
Figure
Mean counts ± SD of main microbial groups isolated from raw goat's milk samples collected from dairy farms located in North and South Sardinia.
In relation to the presence of pathogenic species, it is important to note that neither
The numbers of total LAB were in the range from 104 to 107 cfu/mL, and significantly higher (p< 0.05) counts were detected in raw milk samples collected in north Sardinia. Enterococci counts ranged between 103 and 105 cfu/mL. The numbers of presumptive lactococci were higher than those of presumptive lactobacilli. Similar LAB counts were reported by Perin and Nero [
No significant difference was detected in the means of total yeast counts, ranging between 103 and 105 cfu/mL. Even if few studies have been carried out on the total count of yeasts in goat's milk, our data confirmed their low presence [
The distribution of LAB species found in raw goat’s milk samples is summarized in Table
Distribution of LAB species in raw goat’s milk samples.
Genus | Species | Frequency (%) | N. of strains |
---|---|---|---|
| | 15.6 | 45 |
| 6.2 | 18 | |
| 5.9 | 17 | |
| 1.7 | 5 | |
| |||
| | 14.5 | 42 |
| 6.9 | 20 | |
| 5.9 | 17 | |
| 5.2 | 15 | |
| |||
| | 7.3 | 21 |
| |||
| | 3.1 | 9 |
| 1.7 | 5 | |
| |||
| | 2.4 | 7 |
| |||
| | 11.4 | 33 |
| 7.6 | 22 | |
| 2.8 | 8 | |
| 1.7 | 5 |
Of the 308 isolates obtained from M17, MRS, and Kenner Fecal
Table
Functional characteristics of technological interest for prevalent LAB species isolated from goat milk.
Species (% of positive strains) | ||||
---|---|---|---|---|
| | | ||
Casein hydrolysis | 37.7 | 27.2 | 28.6 | |
| ||||
Citrate utilization | 20 | 30.3 | 88 | |
| ||||
Lipolytic activity | 0 | 0 | 0 | |
| ||||
Acetoin production | 0 | 6.1 | 21.4 | |
| ||||
Acidifying activity | ∆pH (0-1) | 37.7 | 42.4 | 83.3 |
∆pH (1-2) | 6.7 | 51.5 | 16.7 | |
∆pH (> 2) | 55.5 | 6.1 | 0 | |
| ||||
Milk coagulation | 73.3 | 18.1 | 21.4 | |
| ||||
Β-galactosidase activity | 100 | 75.7 | 100 |
As for the acidifying ability, expressed as decrease in pH with respect to the value of noninoculated control milk (pH 6.5) after 24 h of incubation at 30°C in RSM, the majority of
None of the strains produced lipolytic reactions on tributyrin agar, in agreement with the results of Meng et al. [
The antagonistic effect of LAB dairy strains on pathogenic microorganisms could be used for expanding the range of healthful dairy foods. LAB originally isolated from raw milk or artisanal dairy products are probably the best candidate for improving the microbiological safety of these foods, because they are well adapted to the conditions of the substrate.
In this study the 120 strains belonging to the predominant species were preliminarily screened for antimicrobial activity against five indicator strains, including two well recognized foodborne pathogens, by means of an agar spot method (Table
Distribution of antibacterial activity in prevalent LAB species isolated from goat milk (% of strains).
| | | |||||||
Target strains | 0 | 3-0.5 | 6-4 | 0 | 3-0.5 | 6-4 | 0 | 3-0.5 | 6-4 |
diameter of inhibition zone (mm) | |||||||||
| |||||||||
| 51.1 | 20 | 28.9 | 63.6 | 15.2 | 21.2 | 0 | 0 | 100 |
| |||||||||
| 22.3 | 37.7 | 40 | 33.3 | 51.5 | 15.2 | 42.8 | 4.8 | 52.4 |
| |||||||||
| 68.9 | 31.1 | 0 | 6 | 6 | 88 | 71.4 | 28.6 | 0 |
| |||||||||
| 84.4 | 8.9 | 6.7 | 0 | 3 | 97 | 78.6 | 21.4 | 0 |
| |||||||||
| 89 | 11 | 0 | 0 | 3 | 97 | 66.7 | 33.3 | 0 |
Although the characterization of raw goat’s milk in terms of LAB composition and technological activity has been carried out by several authors in different parts of the world [
This work was performed to study the microbiota of raw goat’s milk collected in different areas of Sardinia, in order to select LAB strains for use as adjunct or starter cultures in the manufacturing of both artisanal and industrial goat’s milk cheeses. This local raw milk could serve as source for natural LAB strains, well adapted to milk and cheese environment which can contribute to the safety, quality, and development of typical taste and flavor of the final product.
Predominance of
Further studies will be carried out to determine the growth dynamics and behaviour of the selected strains in the cheese environment and their potential probiotic properties.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interests regarding the publication of this paper.
This work was partially supported by grants from Università di Cagliari (Progetti FIR, 2016).