The effects of high hydrostatic pressure (HHP) and heat treatments on antimicrobial protein stability and on the physico-chemical, microbiological, rheological and shelf-life properties of donkey milk were investigated. Although heat treatment at 75°C for 2 min resulted in 1.50 log CFU ml−1 microbial inactivation, losses in activities of lysozyme (58%) and lactoferrin (82%) were observed due to whey protein denaturation. By contrast, HHP application at 400 MPa caused lower enzyme activity losses (22 and 37% respectively) whilst maintaining a significant reduction of microbial load (1.80 log CFU ml−1). Color analyses showed that the lightness values of all samples decreased during storage. Higher flow consistency (viscosity) and lower flow behavior indexes were observed in heat-treated samples compared to untreated and HHP-treated ones, which can be explained by advanced protein denaturation during heat-treatment. The results suggest that HHP is a more suitable process than heat treatment for preservation of donkey milk within the conditions studied.

Microbial transglutaminase (MTGase) is an enzyme widely used in the dairy sector to improve the functional properties of protein-based products via the formation of a network between protein molecules. The aim of this study involving cheese from the milk of donkeys was to evaluate the effects of treatment with MTGase at the concentrations of 0 (control), 5, 8 and 10 U/g milk protein on the cheese-making process parameters, as well as the physical and chemical characteristics of the resulting cheese. MTGase influenced the time of gel formation from rennet addition (P < 0.05), with a delay at the two highest concentrations, accompanied by a lower (P < 0.01) pH of cheese and the lowest (P < 0.01) loss in cheese weight at 24 h of storage. The highest gel viscosity (P < 0.01) was observed at the highest concentration of the enzyme, reaching the value of 70 mPa⋅sec after 60 min. The chemical composition and color of the cheeses were not significantly affected by the inclusion of MTGase, regardless of the enzyme concentration. These findings may be of relevance in adapting the cheese-making process and might help in the design of new dairy products from donkey's milk.

The work described in this Research Communication addressed the hypothesis that it is possible to produce cheese from donkey milk by appropriate adjusting of the cheesemaking parameters. A series of coagulation trials were performed on donkey milk, alone or fortified with goat milk (85/15 and 70/30, v/v), using calf rennet under different technological conditions. The parameters that changed were pH and concentration of soluble calcium, amount of rennet added and temperature of coagulation. Donkey milk gave rise to sufficiently firm curd only at ‘extreme’ technological conditions and, as expected, addition of goat milk improved coagulation. A cheesemaking protocol was developed for producing fresh cheese prototypes, which were checked for microbiological safety, chemical composition and sensory characteristics. Pure donkey milk gave 5·9% yield, cheese having 6·12 pH, 32·4% dry matter, 2·1% fat and 18·5% protein. The electrophoretic analysis ascertained that β-casein was the most abundant compound in donkey cheese, but whey proteins were also present in non-negligible amounts. Finally, the sensory evaluation demonstrated that all cheeses were acceptable and provided detailed description of their flavour characteristics.

During the last years the interest in donkey milk has increased significantly mainly because of its compelling functional elements. Even if the composition and nutritional properties of donkey milk are known, its microbiota is less studied. This Research Communication aimed to provide a comprehensive characterisation of the lactic acid bacteria in raw donkey milk. RAPD-PCR assay combined with 16S rDNA sequencing analysis were used to describe the microbial diversity of several donkey farms in the North West part of Italy. The more frequently detected species were: Lactobacillus paracasei, Lactococcus lactis and Carnobacterium maltaromaticum. Less abundant genera were Leuconostoc, Enterococcus and Streptococcus. The yeast Kluyveromyces marxianus was also isolated. The bacterial and biotype distribution notably diverged among the farms. Several of the found species, not previously detected in donkey milk, could have an important probiotic activity and biotechnological potential. This study represents an important insight to the ample diversity of the microorganisms present in the highly selective ecosystem of raw donkey milk.

The traditional utilization of donkeys (Equus asinus) as dairy animals has recently attracted substantial scientific interest with regard to human nutrition. Donkey milk is well tolerated by infants with cows’ milk allergy, useful in the treatment of human immune-related diseases, in the prevention of atherosclerosis, and in-vitro studies showed an anti-proliferative effect. Active 3-3′-5-triiodothyronine (T3) in colostrum and milk could play different physiological roles, systemic and paracrine, for both the mother and the suckling offspring. The aim was to evaluate whether thyroid hormones (TH) concentrations in milk and blood of lactating donkeys change with the advancing lactation and whether they can be affected by dietary supplementation with several trace elements, some of them directly involved with TH synthesis (I), metabolism (Se) and action (Zn). Sixteen lactating jennies were divided into two groups (CTL and TE). Mixed feed for TE was added with Zn, Fe, Cu, Mn, I, Se and Co. Every 2 weeks milk and blood samples were collected at 11·00. Total concentrations of T3 in milk (T3M) and T3 and T4 in plasma (T3P and T4P) were assayed using ELISA kits, validated for the donkey species. T3M was not correlated with TH concentrations in blood, did not change with the stage of lactation, and was significantly higher in TE (4·09±0·07 ng/ml, mean±se) than in CTL group (3·89±0·08 ng/ml). T4P (81·8±5·2 ng/ml) and T3P (15·2±1 ng/ml) significantly changed with time, but were not significantly affected by dietary treatment. T3P/T4P ratio was significantly lower in TE group. This study indicates that in donkey milk the concentration of T3, a human-like bioactive compound, can be affected by trace elements intake.

Stereospecific analysis is an important tool for the characterization of lipid fraction of food matrices, and also of milk samples. The results of a chemical-enzymatic-chromatographic analytical method were elaborated by chemometric procedures such as linear discriminant analysis (LDA) and artificial neural network (ANN). According to the total composition and intrapositional fatty acid distribution in the triacylglycerol (TAG) backbone, the obtained results were able to characterize pure milk samples and milk mixtures with 1, 3, 5% cow milk added to donkey milk. The resulting score was very satisfactory. Totally correct classified samples were obtained when the TAG stereospecific results of all the considered milk mixtures (donkey-cow) were elaborated by LDA and ANN chemometric procedures.

For its characteristics, donkey milk has been proposed as an alternative to goat or artificial milk to feed allergic infants. Therefore, it is important to increase our knowledge on health and immunological characteristics of donkey milk. Ten donkeys, bred as companion animals, were enrolled in this study and sampled once a month, for eight months. Milk (10 ml) was collected from each half udder for somatic cell count (SCC), bacteriological analysis and total bacteria count (TBC). The major pathogens were tested for antimicrobial susceptibility, and Staphylococcus aureus isolates were further genotyped by nanoarray analysis. Whey lysozyme and NAGase (NAG) activities were also assessed. Overall, 101 half-udder milk samples were taken. They showed very low values of TBC (<250 cfu/ml) and SCC (<50 000 cells/ml) and a minor prevalence of pathogens: Staph. aureus was isolated only from 5 milk samples (3 animals), Streptococcus equi from 2 samples and Str. equisimilis from a single sample. All the isolates were sensitive to all antibiotic classes used in veterinary medicine. None of the Staph. aureus isolates were shown to harbour genes coding for any enterotoxin, toxic-shock syndrome toxin or antibiotic resistance. Lysozyme levels were always very high (4000–5000 U/ml), while NAG values were mostly low (<50 U/ml), out of the last part of lactation. The results of this study confirmed the low prevalence of intramammary infections in donkey and the absence of food-borne pathogens, suggesting that donkey milk could be a safe food, if the mammary gland is healthy and the animals are milked in proper hygienic conditions.