Physicochemical properties, antioxidant and antidiabetic activities of different hydrolysates of goat milk protein.

There is growing interest in the origin, preparation, and application of bioactive peptides. This study investigated the impact of 6 enzymes on the structural, physicochemical properties, antioxidant activities, and antidiabetic potential of defatted fresh goat milk. Structural and functional changes resulting from enzymatic hydrolysis were assessed using gel electrophoresis, laser particle size analysis, multi-spectroscopy, and evaluations of foaming and emulsification properties. Antioxidant capacity was determined through free radical scavenging, Fe2+ chelation, and reducing ability experiments. Additionally, the inhibitory effects of the hydrolysates on α-glucosidase and α-amylase were measured to evaluate antidiabetic activity. Results showed that enzymatic hydrolysis disrupted the spatial structure of goat milk protein and reduced its molecular weight. Papain hydrolysate exhibited the highest degree of hydrolysis (32.87 ± 0.11%) and smallest particle size (294.75 ± 3.33 nm), followed by alcalase hydrolysate (29.12 ± 0.09%, 302.03 ± 7.28 nm). Alcalase hydrolysate showed the best foaming properties, while papain hydrolysate demonstrated the strongest DPPH and hydroxyl radical scavenging activity, Fe2+ chelation, and antidiabetic potential. These findings provide solid theoretical basis for utilizing defatted goat milk as functional ingredients or excipients in the food, medical, and cosmetic industries.

Genetic and environmental factors shaping goat milk oligosaccharide composition.

Oligosaccharides (OS) in milk have been suggested to influence the health and development of the newborn by promoting growth of beneficial gut bacteria, stimulating brain development, and enhancing immune functions. Goat milk is a natural source of specific OS, which could be a potential beneficial ingredient for infant formula. In this study, goat milk oligosaccharide (gMOS) content from approximately 1,000 dairy goats across 18 commercial farms was studied. A genomic relationship matrix was used to unravel genetic and environmental factors shaping gMOS content. The most abundant gMOS identified was 3'-NGL, with a concentration of 32.05 mg/kg, while 3-FL exhibited the lowest concentration at 1.85 mg/kg. Acidic OS had a notably higher content (81.67 mg/kg) than neutral OS (24.88 mg/kg). High variability in gMOS content was observed among individual goats, which could for a large extent be attributed to genetic differences. Heritability estimates ranged from 31% for 3'-GL to 85% for 3-FL. High positive genetic correlations (>0.57) were estimated between 3'-SL and 6'-SL, and between 6'-GL and 3'-GL. The contribution of differences between farms to variation in milk OS content varied from 3% for 3'-NGL to 45% for 6'-SL. While gMOS like 3'-GL, 6'-GL, and 6'-NGL, were significantly influenced by systematic environmental factors such as the lactation stage, the impact of these factors was relatively minor compared with the importance of genetic and farm effects. This research, which stands out due to its relatively large sample size, underscores the pivotal role of genetics, and to a smaller extent farm practices like feed ration, in determining gMOS composition.

Rapid determination of volatile benzene derivatives and chlorobenzenes in goat's milk by HS-SPME-GC-MS/MS.

A method for the determination of eight benzenes (BTEXs) and twelve chlorobenzenes (CBs) in goat's milk by headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS/MS) was developed. The study investigated the impact of various factors such as extraction fiber type, salt amount, equilibrium conditions, and desorption conditions on the outcomes. Target analytes were separated on a DB-HeavyWAX column and quantified using the external standard method. The results showed that the target compounds had a good linear relationship in the range of 0.01 ∼ 50 µg/L (R2 > 0.997), the limit of detection (LOD) was 0.003 ∼ 0.150 µg/L, and the limit of quantification (LOQ) was 0.01 ∼ 0.50 µg/L. The average recoveries were 82%-116% and the relative standard deviation (RSD) was 0.8%-17.3% under the three addition levels of 1×, 2×, and 10 × LOQ. In a survey of twenty goat's milk samples, only ethylbenzene, xylenes, cumene, chlorobenzene, and 1,4-dichlorobenzene were detected at levels exceeding their respective limits of quantification. The method was evaluated using two ecological scales (Eco-Scale), GAPI and AGREEN, to verify its environmental friendliness and applicability. This method is simple, green, and efficient, which provides a certain theoretical basis for the production and quality safety evaluation of dairy products.

Goat Milk Allergy and a Potential Role for Goat Milk in Cow's Milk Allergy.

In many parts of the world, goat milk has been part of the human diet for millennia. Allergy to goat's milk, not associated with allergy to cow's milk, is a rare disorder, although some cases have been described. Goat milk proteins have substantial homology with cow's milk proteins and even show cross-reactivity; therefore, they are not advised as an alternative to cow's milk for infants with IgE-mediated cow's milk allergies. However, there are indications that, due to the composition of the goat milk proteins, goat milk proteins show lower allergenicity than cow's milk due to a lower αS1-casein content. For this reason, goat milk might be a better choice over cow's milk as a first source of protein when breastfeeding is not possible or after the breastfeeding period. Additionally, some studies show that goat milk could play a role in specific types of non-IgE-mediated cow milk allergy or even in the prevention of sensitization to cow's milk proteins. This review discusses a possible role of goat milk in non-IgE mediated allergy and the prevention or oral tolerance induction of milk allergy.

Prophylactic effects of Tibetan goat kefir on depression-like behaviors in chronic unpredictable stress model through the gut-brain axis.

BACKGROUND: Depression is a common psychological disorder, and traditional therapeutic drugs often result in side effects such as emesis, dry mouth, headache, dysentery and constipation. Probiotics and goat milk have garnered widespread attention for their ability to modulate immune function and regulate the endocrine system, and for their anti-inflammatory effects. In this work, the effects of Tibetan goat kefir on the behavior, immune status, neuroendocrine response and gut microbiological composition of chronic unpredictable mild stress (CUMS) mouse models were evaluated. RESULTS: The results indicated that Tibetan kefir goat milk significantly alleviated behavioral despair in mice. Furthermore, the results demonstrated that Tibetan kefir goat milk mitigated the inflammatory response in the mice and moderated the hyperactivity of the hypothalamic-pituitary-adrenal axis and the expression of brain-derived neurotrophic factor. Meanwhile, chronic stress-induced gut microbial abnormalities were restored. In addition, the correlation between gut microbiota and nervous system was evaluated. CONCLUSION: These results explained the potential mechanism of Tibetan kefir in the antidepressant effect on the CUMS model and enriched diets for depressed patients. © 2024 Society of Chemical Industry.

Label-free quantitative proteomic analysis of functional changes of goat milk whey proteins subject to heat treatments of ultra-high-temperature and the common low-temperature.

This work investigated the functional changes in whey proteins obtained from goat milk subject to various temperature treatments. Ultra-high temperature instantaneous sterilization (UHTIS) caused less damage than the common low-temperature, whereas spray-drying treatment had the opposite effect. A total of 426 proteins were identified in UHTIS and control treatment groups, including 386 common proteins and 16 and 14 unique proteins. The UHTIS treatment upregulated 55 whey proteins while down-regulated 98. The UHTIS-treated whey proteins may upregulate three metabolic pathways but downregulate one. Overall, UHTIS only slightly impacted the composition and functions of whey proteins from goat milk compared to the common low-temperature treatments.

Identification and quantification of goat milk adulteration using mid-infrared spectroscopy and chemometrics.

The fraudulent adulteration of goat milk with cheaper and more available milk of other species such as cow milk is occurrence. The aims of the present study were to investigate the effect of goat milk adulteration with cow milk on the mid-infrared (MIR) spectrum and further evaluate the potential of MIR spectroscopy to identify and quantify the goat milk adulterated. Goat milk was adulterated with cow milk at 5 different levels including 10%, 20%, 30%, 40%, and 50%. Statistical analysis showed that the adulteration had significant effect on the majority of the spectral wavenumbers. Then, the spectrum was preprocessed with standard normal variate (SNV), multiplicative scattering correction (MSC), Savitzky-Golay smoothing (SG), SG plus SNV, and SG plus MSC, and partial least squares discriminant analysis (PLS-DA) and partial least squares regression (PLSR) were used to establish classification and regression models, respectively. PLS-DA models obtained good results with all the sensitivity and specificity over 0.96 in the cross-validation set. Regression models using raw spectrum obtained the best result, with coefficient of determination (R2), root mean square error (RMSE), and the ratio of performance to deviation (RPD) of cross-validation set were 0.98, 2.01, and 8.49, respectively. The results preliminarily indicate that the MIR spectroscopy is an effective technique to detect the goat milk adulteration with cow milk. In future, milk samples from different origins and different breeds of goats and cows should be collected, and more sophisticated adulteration at low levels should be further studied to explore the potential and effectiveness of milk mid-infrared spectroscopy and chemometrics.

Goat milk as the connecting link in dengue and brucella coinfection.

In areas with widespread prevalence of myth of goat milk as a platelet booster, the goat milk can be connecting link in dengue and brucella coinfection.

Investigating the impact of milk protein, inulin, and honey on quality attributes of goat milk yoghurt.

In this present study, a three-factor Box-Behnken, response surface methodology (RSM) design was employed to optimize the skimmed milk powder (SMP)/whey protein concentrate (WPC) ratio (0.25-0.75%w/v) as a source of milk protein, inulin (1-2%w/v), and honey (4-6%w/v) for production of high-quality goat milk yoghurt (GMY). The resulting ANOVA and response surface equations revealed the significant effect (p < 0.05) of these variables on the various attributes such as total solid (%), pH, titratable acidity [(LA) % by weight], syneresis (%), DPPH (% inhibition), viscosity (m.Pa⋅s), whiteness index (WI), and overall acceptability (OA). The coefficient of determination (R2) for all response variables ranged from 0.88 to 0.99. Lack-of-fit tests resulted in non-significant F-values. The optimal conditions were determined as SMP/WPC at 0.36%w/v, inulin at 1.00%w/v, and honey at 6.00%w/v. The optimum values for total solid, pH, titratable acidity, syneresis, DPPH, viscosity, WI, and OA were 22.03, 4.46, 0.77, 6.34, 25.20, 182.30, 76.29 and 8.37, respectively with desirability value of 0.95.

Analysis of Human Milk Microbiota in Northern Greece by Comparative 16S rRNA Sequencing vs. Local Dairy Animals.

Milk is a biological fluid with a dynamic composition of micronutrients and bioactive molecules that serves as a vital nutrient source for infants. Milk composition is affected by multiple factors, including genetics, geographical location, environmental conditions, lactation phase, and maternal nutrition, and plays a key role in dictating its microbiome. This study addresses a less-explored aspect, comparing the microbial communities in human breast milk with those in mature milk from species that are used for milk consumption. Since mature animal milk is used as a supplement for both the infant (formula) and the child/adolescent, our main aim was to identify shared microbial communities in colostrum and mature human milk. Using 16S rRNA metagenomic sequencing, we focused on characterizing the milk microbiota in the Northern Greek population by identifying shared microbial communities across samples and comparing the relative abundance of prevalent genera. We analyzed ten human milk samples (from five mothers), with five collected three days postpartum (colostrum) and five collected thirty to forty days postpartum (mature milk) from corresponding mothers. To perform an interspecies comparison of human milk microbiota, we analyzed five goat and five bovine milk samples from a local dairy industry, collected fifty to seventy days after birth. Alpha diversity analysis indicated moderate diversity and stability in bovine milk, high richness in goat milk, and constrained diversity in breast milk. Beta diversity analysis revealed significant distinctions among mammalian species, emphasizing both presence/absence and abundance-based clustering. Despite noticeable differences, shared microbial components underscore fundamental aspects across all mammalian species, highlighting the presence of a core microbiota predominantly comprising the Proteobacteria, Firmicutes, and Actinobacteriota phyla. At the genus level, Acinetobacter, Gemella, and Sphingobium exhibit significant higher abundance in human milk compared to bovine and goat milk, while Pseudomonas and Atopostipes are more prevalent in animal milk. Our comparative analysis revealed differences and commonalities in the microbial communities of various mammalian milks and unraveled the existence of a common fundamental milk core microbiome. We thus revealed both species-specific and conserved microbial communities in human, bovine, and goat milk. The existence of a common core microbiome with conserved differences between colostrum and mature human milk underscores fundamental similarities in the microbiota of milk across mammalian species, which could offer valuable implications for optimizing the nutritional quality and safety of dairy products as well as supplements for infant health.

Revealing the mechanism of flavor improvement of fermented goat milk based on lipid changes.

The mechanism of goat milk (GM) flavor improvement based on lipid changes requires understanding. According to sensory evaluation results, the texture, taste, appearance, aroma, and overall acceptability score of Guishan fermented goat milk (GMF) were higher than those of GM. In total, 779 lipid molecules and 121 volatile compounds were formed from the metabolite-lipid level in the GM and GMF, as determined through lipidomics and gas chromatography-mass spectrometry. The key volatile flavor compounds in the GMF were (E,E)-2,4-decadienal, ethyl acetate, acetoin, 2,3-pentanedione, acetic acid, and 2,3-butanedione. Of them, 60 lipids significantly contributed to the flavor profiles of the GMF, based on the correlation analysis. The triacylglycerides (TAGs) 12:0_14:0_16:0 and 13:0_13:0_18:2 contributed to aroma retention, while TAG and phosphatidylethanolamine were identified as key substrates for flavor compound formation during fermentation. Lipids associated with glycerophospholipid and linoleic acid metabolism pathways significantly affected volatile compound formation in the GMF. This study provides an in-depth understanding of the lipids and flavors of the GMF, and this information will be useful for the development of specific GMF products.

Comparative analysis of the fatty acid profiles in goat milk during different lactation periods and their interactions with volatile compounds and metabolites.

This study aimed to compare the composition of fatty acids in goat milk during lactation with human milk, as well as analyze the differences in their interaction with odor and metabolites. Polyunsaturated fatty acids content was higher in human milk, while odd-chain, branched-chain, and monounsaturated fatty acids content were higher in goat milk with a decreasing trend during lactation. PUFAs in human milk undergo auto-oxidation to produce aldehydes (hexanal), giving it a mild aroma. Butyric acid in goat colostrum mediates the synthesis and auto-oxidation of PUFA, while taurine mediated the hydrolysis of amino acids. They produce a furanone compound (2(5H)-furanone) with a buttery flavor. The presence of butyric acid in goat transitional milk had an impact on flavor and metabolites. The medium chain fatty acid composition of the goat mature milk was affected by nucleic acid compounds, which then oxidized to produce methyl ketone (2-nonanone), giving it an unpleasant flavor.

Analysis of milk adulteration by means of a potentiometric electronic tongue.

The adulteration of milk presents significant challenges in the food industry, promoting the need for efficient detection methods. This study introduces a potentiometric electronic tongue for rapid and accurate milk adulteration detection. Utilizing polymeric membranes integrated with various additives, the electronic tongue distinguishes between different milk types and detects common adulterants. Experimental results demonstrate its effectiveness in discriminating raw, pasteurized, and medicated cow milk, as well as goat milk. Moreover, it successfully identifies adulterants like water and bovine milk in goat milk samples. Chemometric analyses, including Principal Component Analysis and Partial Least Squares regression, correlate sensor responses with traditional milk parameters such as fat, protein, and lactose content with up to a 0.97 R2 on the validation step. Strong correlations validate the electronic tongue's potential for rapid milk quality assessment. This innovative approach offers a cost-effective, reliable solution for detecting milk adulteration in contrast with current techniques that require numerous, time consuming experiments.

Screening of lactic acid bacteria with the ability to reduce goaty flavor related fatty acids in goat milk.

In this study, we conducted sensory evaluation and gas chromatography-mass spectrometry analysis on fermented goat milk samples prepared by 12 strains of lactic acid bacteria (LAB) isolated from goat milk to screen for strains with the ability to reduce the goaty flavor. The bacterial counts of fermented goat milk was 7.07-9.01 log CFU/mL. The electronic nose distinguished fresh goat milk (FGM) and fermented goat milk, and the electronic tongue results showed that Leuconostoc citreum 1, 4, 20, 22, 32, and 57, Latilactobacillus curvatus 144 and 147 imparted fermented goat milk a taste different from FGM. Overall, Leuconostoc citreum 57, Leuconostoc citreum 126, Latilactobacillus curvatus 142, Latilactobacillus curvatus 143, and Latilactobacillus curvatus 147 were screened with the ability to improve the flavor of goat milk. They gave fermented goat milk a goat flavor score lower than or equal to FGM. And the fermented goat milk samples 57, 126, 142, 143, and 147 contained 25, 22, 15, 24, and 17 volatile flavor compounds, respectively, with a greater variety and content of ketones and aldehydes and lower levels of hexanoic acid, octanoic acid, and decanoic acid than FGM. However, the pH and WHC results indicated that the application of these strains as secondary cultures is necessary. Our finding provides basic research data to improve the flavor of goat milk products.

Integrated analysis of genomics and transcriptomics revealed the genetic basis for goaty flavor formation in goat milk.

Goat milk exhibits a robust and distinctive "goaty" flavor. However, the underlying genetic basis of goaty flavor remains elusive and requires further elucidation at the genomic level. Through comparative genomics analysis, we identified divergent signatures of certain proteins in goat, sheep, and cow. MMUT has undergone a goat-specific mutation in the B12 binding domain. We observed the goat FASN exhibits nonsynonymous mutations in the acyltransferase domain. Structural variations in these key proteins may enhance the capacity for synthesizing goaty flavor compounds in goat. Integrated omics analysis revealed the catabolism of branched-chain amino acids contributed to the goat milk flavor. Furthermore, we uncovered a regulatory mechanism in which the transcription factor ZNF281 suppresses the expression of the ECHDC1 gene may play a pivotal role in the accumulation of flavor substances in goat milk. These findings provide insights into the genetic basis underlying the formation of goaty flavor in goat milk. STATEMENT OF SIGNIFICANCE: Branched-chain fatty acids (BCFAs) play a crucial role in generating the distinctive "goaty" flavor of goat milk. Whether there is an underlying genetic basis associated with goaty flavor is unknown. To begin deciphering mechanisms of goat milk flavor development, we collected transcriptomic data from mammary tissue of goat, sheep, cow, and buffalo at peak lactation for cross-species transcriptome analysis and downloaded nine publicly available genomes for comparative genomic analysis. Our data indicate that the catabolic pathway of branched-chain amino acids (BCAAs) is under positive selection in the goat genome, and most genes involved in this pathway exhibit significantly higher expression levels in goat mammary tissue compared to other species, which contributes to the development of flavor in goat milk. Furthermore, we have elucidated the regulatory mechanism by which the transcription factor ZNF281 suppresses ECHDC1 gene expression, thereby exerting an important influence on the accumulation of flavor compounds in goat milk. These findings provide insights into the genetic mechanisms underlying flavor formation in goat milk and suggest further research to manipulate the flavor of animal products.

Differences in proteomic profiles and immunomodulatory activity of goat and cow milk fat globule membrane.

This study aimed to clarify the composition and bioactivity differences between goat and cow milk fat globule membrane (MFGM) protein by proteomic, and the immunomodulatory activity of MFGM proteins was further evaluated by using mouse splenic lymphocytes in vitro. A total of 257 MFGM proteins showed significant differences between goat and cow milk. The upregulated and unique MFGM proteins in goat milk were significantly enriched in the positive regulation of immune response, negative regulation of Interleukin-5 (IL-5) secretion, and involved in nucleotide-binding oligomerization domain (NOD)-like receptor signaling. The contents of IL-2 and Interferon-γ in the supernatant of spleen lymphocytes treated with goat MFGM proteins were much higher than those of IL-4 and IL-5, suggesting a Th1-skewed immune response. These results revealed that goat MFGM proteins could possess better immunomodulatory effects as compared to cow milk. Our findings may provide new insights to elucidate the physiological functions and nutritional of goat milk.

Machine learning methods for unveiling the potential of antioxidant short peptides in goat milk-derived proteins during in vitro gastrointestinal digestion.

Milk serves as an important dietary source of bioactive peptides, offering notable benefits to individuals. Among the antioxidant short peptides (di- and tripeptides) generated from gastrointestinal digestion are characterized by enhanced bioavailability and bioaccessibility, while assessing them individually presents a labor-intensive and expensive challenge. Based on 4 distinct types of amino acid descriptors (physicochemical, 3D structural, quantum, and topological attributes) and genetic algorithms for feature selection, 1 and 4 machine learning predicted models separately for di- and tripeptides with ABTS radical scavenging capacity exhibited excellent fitting and prediction ability with random forest regression as machine learning algorithm. Intriguingly, the electronic properties of N-terminal amino acid were considered as only factor affecting the antioxidant capacity of dipeptides containing both tyrosine and tryptophan. Four peptides from the potential di- and tripeptides exhibited highly predicted values by the constructed predicted models. Subsequently, a total of 45 dipeptides and 52 tripeptides were screened by a customized workflow in goat milk during in vitro simulated digestion. In addition to 5 known antioxidant dipeptides, 9 peptides were quantified during digestion, falling within the range of 0.04 to 1.78 mg L-1. Particularly noteworthy was the promising in vivo functionality of antioxidant dipeptides with N-terminal tyrosine, supported by in silico assays. Overall, this investigation explored crucial molecular properties influencing antioxidant short peptides and high-throughput screening potential peptides with antioxidant activity from goat milk aided by machine learning, thereby facilitating the identification of novel bioactive peptides from milk-derived proteins and paving the way for understanding their metabolites during digestion.

Quality characteristics of goat milk powder produced by freeze drying followed by UV-C radiation sterilization.

Goat milk was directly freeze-dried into milk powder after freezing and then sterilized using UV-C radiation to produce low-dose, medium-dose and high-dose UV-C radiation sterilized freeze-dried goat milk powder (LGP, MGP and HGP). UV-C sterilization effectively reduced the total bacteria count and coliform bacteria in the goat milk powder while preserving the active proteins, and maintaining the color unchanged. Additionally, LGP, MGP, and HGP all exhibited a moisture content below 5 g/100 g and water activity below 0.5. Upon reconstitution, the milk powder formed uniform and stable emulsion. During accelerated storage tests, the increased Aw did not compromise the microbial quality of milk powder, and there were no significant changes in active proteins as confirmed via SDS-PAGE results. Furthermore, the color parameters (a*, b* and ΔE) showed a strong correlation with hydroxymethyl furfural levels.

Lactiplantibacillusplantarum JS19-adjunctly fermented goat milk alleviates D-galactose-induced aging by modulating oxidative stress and intestinal microbiota in mice.

Oxidative stress is a crucial factor in the age-related decline in physiological, genomic, metabolic, and immunological functions. We screened Lactiplantibacillus plantarum JS19 (L. plantarum JS19), which has been shown to possess therapeutic properties in mice with ulcerative colitis. In this study, L. plantarum JS19-adjunctly fermented goat milk (LAF) was employed to alleviate D-galactose-induced aging and regulate intestinal flora in an aging mouse model. The oral administration of LAF effectively improved the health of spleen and kidney in mice, while mitigating the hepatocyte and oxidative damage induced by D-galactose. Additionally, LAF alleviated D-galactose-induced dysbiosis of the intestinal flora by reducing the abundance of harmful bacteria Desulfovibrio and Helicobacter, while greatly promoting the growth of beneficial Rikenellaceae_RC9_gut_group and Eubacterium. Biomarker 5-hydroxyindole-3-acetic acid was found to be positively linked with those harmful bacteria, while bio-active metabolites were strongly correlated with the beneficial genus. These observations suggest that LAF possesses the capability to mitigate the effects of D-galactose-induced aging in a mouse model through the regulation of oxidative stress, the gut microbiota composition, and levels of fecal metabolites. Consequently, these findings shed light on the potential of LAF as a functional food with anti-aging properties.

Comparison of the Lipid Composition of Milk Fat Globules in Goat (Capra hircus) Milk during Different Lactations and Human Milk.

Goat milk is considered the optimal substitute for human milk and is characterized by variations in the lipid composition of its fat globules across lactation phases. Therefore, the objective of this study was to thoroughly analyze the differences between goat milk during different lactations and human milk, aiming to offer scientific guidance for the production of functional dairy products. Compared with transitional and mature milk, the findings indicated that the total membrane protein content in goat colostrum exhibited greater similarity to that found in human milk. Additionally, goat milk exhibited higher milk fat globule size, as well as a higher total lipid and protein content than human milk. A total of 1461 lipid molecules across 61 subclasses were identified in goat milk and human milk. The contents of glycerides and glycerophospholipids were higher in goat colostrum, whereas sphingolipids and fatty acids were more abundant in human milk. Meanwhile, the compositions of lipid subclasses were inconsistent. There were 584 differentially expressed lipids identified between human and goat milk, including 47 subclasses that were primarily involved in the metabolism of glycerophospholipids, sphingolipids, and triglycerides. In summary, for both the membrane protein and the lipid composition, there were differences between the milk of different goat lactations and human milk.