In the last few years, the number of goat farms (in particular operating under organic farming practices) has been increasing not only in the Czech Republic but also worldwide leading to greater production of goat milk (Josrová, 2018). Therefore, farmers and dairy producers are actively seeking to develop new products and formulas which would attract consumers’ attention. Goat milk represents a welcome alternative to products made from cow’s milk. For instance, products made from goat milk are considered to be healthier because goat milk shows better nutritional characteristics like digestibility due to smaller diameter of fat globules (3.5 μm in goat milk compared to 4.5 μm in cow’s milk) which causes the fat to be better dispersed in the milk (Park et al., 2007). Goat milk contains also a higher amounts of caproic, caprylic and capric acids (in total 15 – 18% compared to 5 – 9% in cow’s milk) that were shown to be beneficial to humans (Sanz Sampelayo et al., 2007). These short-chain fatty acids have been proven to help patients with malabsorption syndrome, intestinal disorders, coronary heart diseases, cystic fibrosis and other conditions (Haenlein, 2004; Ribeiro and Ribeiro, 2010). Furthermore, it is possible to increase the content of omega-3 fatty acids in goat milk by supplementing regular goat feed with moderate amounts of linseed oil which is rich in α-linolenic acid as described in Borková et al. (2018). Differences in milk casein content were also found to impact human health. Goat α_S1-casein content is lower (typically 10 – 13% of total casein content) compared to cow’s milk and depends on the particular genetic variant for α_S1-casein. The presence of null alleles for α_S1-casein such as 0₁, 0₂, 0₄ and N cause the absence of α_S1-casein in goat milk (Caboni et al., 2016) which is an important factor in decreasing the allergic sensitation of α_S1-casein (Ballabio et al., 2011; Hochwallner et al., 2014).

Many more possibilities are available to boost nutritional impact of goat milk products as using milk high in omega- 3 essential fatty acids, probiotic bacteria and prebiotics. Probiotic bacteria are well known for their antimicrobial, antioxidant and immunomodulatory effects (Lee et al., 2011; Tomáška et al., 2015). Prebiotics are more or less indigestible components of foods comprising fructooligosaccharides (FOS) which positively influence the growth and activity of probiotic bacteria in the gastrointestinal system (Valcheva and Dieleman, 2016). FOS are oligosaccharides consisting of D-fructose chains connected by β-(1→2) glycosidic bonds and capped by glucose. The yacon (Smallanthus sonchifolius) plant belongs to the Asteraceae family and grows in eastern Andes from Venezuela to northern Argentina (Caetano et al., 2016). Yacon tuber is a great source of FOS (Topolska et al., 2015) which do not undergo hydrolysis by human intestinal enzymes and are instead selectively fermented by bacteria in the colon leading to a more balanced composition of gut microbiota. Dry yacon tubers contain up to 50% (very rarely up to 70%) of FOS (Velez et al., 2013).

The aim of this study was to increase the nutritional value of goat milk by producing a probiotic yoghurt drink made from goat milk with elevated omega-3 fatty acids and enriched with natural yacon prebiotics.

Our previous work (Borková et al., 2018) revealed that feed supplemented either with linseed oil or linseed extrudate increased the amounts of omega-3 fatty acids in goat milk which can be successfully used to produce yoghurt drinks with added value. We found out that feed supplemented with linseed oil yields better results than linseed extrudate. Therefore, we decided to modify the previous experiment using feed supplementation with linseed oil to get enriched omega-3 goat milk which was used to produce yoghurt drinks with added yacon prebiotics to boost the drink’s health benefits. We also increased the length of linseed oil supplementation to goats to eight weeks to prove that it has a long-lasting effects on omega-3 fatty acid elevation (and other quality indicators) in goat milk. Our last experiment published in 2018 found a minor (but statistically insignificant) increase in milk yield, milk fat and total milk solids in milk samples from goats fed linseed oil. Results from the current study revealed that after four weeks of linseed oil supplementation the milk yield has increased (p <0.05) and this effect lasted until the end of experiment. The linseed oil supplementation did not affect other quality parameters like fat and protein content or total solids (Table 2). Thus, the increased milk yield and quality (see below) after linseed oil supplementation may positively impact farm’s economy offsetting the costs for feed supplementation.

From Table 3 (listing some major and nutritionally important minor fatty acids) it is evident that linseed oil supplementation (ILY) changes fatty acid profile in the final product. To sum up the results, saturated fatty acid (SFA) levels were significantly decreased (p <0.001) while the levels of monosaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) increased (p <0.01, p <0.001 resp.). Contrary to our 2018 study, linseed oil supplementation was able to reduce short-chain SFAs and C12:0 in yoghurt drinks (in particular C6:0, p <0.05; C8:0, p <0.05; C10:0, p <0.01; C12:0, p <0.05). These are interesting results because the data in scientific papers so far either found an increase of short-chain SFA levels as in Bernard et al. (2009) or no change at all (Martínez Marín et al., 2011) after linseed oil supplementation. In the case of medium-chain SFAs (C14:0 and C16:0), we saw their levels to decrease (p <0.01) after linseed oil supplementation which is in accordance with the data published in scientific literature (Bernard et al., 2009; Martínez Marín et al., 2011) and our last paper (Borková et al., 2018). According to Martínez Marín et al. (2011) the observed reduction of short and medium-chain SFAs was likely caused by the increased intake of PUFAs from linseed oil added to the feed which might have affected some crucial enzymes in the de novo fatty acid synthesis pathway such as acetyl-CoA carboxylase and fatty acid synthase. The de novo synthesis in the mammary gland is the major source of fatty acids with less than sixteen carbon atoms. Unlike our previous short-term supplementation experiment, the long-term supplementation with linseed oil decreased the levels of C14:0, C16:0 and also the levels of C6:0 to C12:0.

The increased amounts of trans-C18:1 and trans-C18:2 isomers, ALA and conjugated linoleic acid contributed to the elevated levels of MUFAs and PUFAs. These results are in accordance with findings of Martínez Marín et al. (2011) and Bernard et al. (2009) who also observed such increases after linseed oil supplementation. The increase of the unsaturated fatty acids mentioned above in yoghurt drinks was caused by the ALA present in linseed oil. Fatty acids longer than eighteen carbons get into milk from lipoproteins present in blood plasma (which themselves originate from food or animal’s fat depots). The PUFAs present in feed undergo biohydrogenation in rumen during digestion which leads to the formation of C18:0 and other isomers like t11-C18:1 (vaccenic acid) and t11,c15-C18:2 (Chilliard et al., 2007). These fatty acids are subsequently absorbed in the colon and transported through blood into the mammary gland where they might be desaturated by stearoyl-CoA desaturase activity. Contrary to our previous report in Borková et al. (2018), the linseed supplementation did not decreased c9-C18:1 in yoghurt drinks. The PUFAs biohydrogenation in goat rumen is a very complex process influenced by many external and internal factors. The composition of metabolites resulting from biohydrogenation process is dependent on the supplement type and feed itself which both impact the lipogenesis in the mammary gland and the activity of key enzymes. Unlike our previous experiment, we did not see the desaturase activity to drop significantly in the yoghurt drinks (see the desaturase index in Table 3) made from the experimental group (ILY). This fact is the likely cause why we did not observe the c9-C18:1 fatty acid to decrease. It is worth to mention that the present study confirmed again that ALA levels in yoghurt drinks remained increased.

From the nutritional perspective it is important to underscore the increased content of trans-fatty acids in the ILY drinks. As a matter of fact, trans-fatty acids in food can negatively influence consumers’ health. However, this notion is not true for all trans-fatty acids (Anadón et al., 2010; Jacome-Sosa et al., 2014; Ganguly and Pierce, 2015) like vaccenic acid (t11-C18:1) which is the main biohydrogenation intermediate of ALA.

All our yoghurt drinks (with or without the addition of yacon tuber powder) made from enriched omega-3 milk (LO goat group) have met the requirements of the Regulation no. 397/2016 Col. of Czech Republic specifying that all yoghurt products must contain at least 7 log CFU.g^-1 (which equals to 10⁷ CFU.g^-1). Yoghurt drinks containing yacon tuber powder (ILY) showed significant increase in the number of bifidobacteria than yoghurt drinks without the yacon addition (IL) (Table 4). This result is likely due to the presence of fructooligosaccharides in the yoghurt drinks which are a welcomed substrate for the probiotic bacteria leading to their increased counts.

Two pooled milk samples (one from the experimental, the other from control group) were created to mimic standard practices on the farm because farmers usually do not have access to individual milk samples but only to the pooled milk from the herd. Both pooled samples were used to manufacture yoghurt drinks with yacon tuber powder (one drink per each pooled sample denoted as PLY and PCY) because we were interested if there is any difference in fatty acid profile in yoghurt drinks made from the pooled sample versus yoghurt drinks made from individual milk samples. The results for both groups are shown in Table 5. The yoghurt drinks made from pooled milk samples from goats supplemented with linseed oil (PLY) had increased contents of MUFAs, PUFAs and a lower content of SFAs similar to findings for ILY yoghurt drinks. PLY yoghurt drinks had 66% higher ALA content (compared to the PCY group used as a control). The same comparison for yoghurt drinks from individual milk samples (ILY and ICY) has shown only 53% increase in ALA content. The higher amount of ALA in the pooled samples (66%) was caused by some goats having higher milk yield with the highest ALA content (unpublished results). Fructooligosaccharide content in yoghurt drinks from the pooled samples was 0.73 – 0.81 g.100 g^-1 of yoghurt drink thus these yoghurt drinks can be a valuable source of prebiotics, probiotics and α-linolenic acid.

Goat yoghurt drinks with bifidobacteria and yacon tuber powder are novel products that might be interesting for health conscious consumers providing them with added value. These drinks are a good source of fructooligosaccharides and bifidobacteria compared to regular products without any added prebiotics. In case of the milk produced by goats fed linseed oil it is possible to manufacture a product which has an increased omega-3 fatty acid content and decreased content of saturated fatty acids. These yoghurt drinks eventually contained significantly higher amounts of α-linolenic acid and lower levels of lauric, myristic and palmitic acid.