**1. Introduction**

Goats and sheep were the first animals to be domesticated by humans for livestock husbandry, about 10,000 years ago [1, 2]. These animals were raised around the world in hundreds of different breeds. There are currently more than 750 million goats and 1000 million sheep [3].

Portugal is one of the largest producers of sheep milk worldwide. However, a substantial part of the goat and sheep milk production comes from family-scale farms and is normally intended for the owners' own consumption.

Even so, a significant part of the national milk production is already fully industrialised and has already generated some products that have been awarded with a protected designation of origin (PDO). Such designation grants them a high economic impact.

This is the case of the famous Serra da Estrela cheese and of the Transmontano goat cheese made of the milk obtained from Serra da Estrela sheep and Serrana goats, respectively.

Milk has a high nutritional value; however, small differences in the composition of the different types of milk may generate large nutritional and technological differences in milk processing industry [4]. Goat milk is quite important since it has high biological value and important nutritional qualities. Its higher digestibility, alkalinity and dietary characteristics make it highly recommended for infant feeding and for adults who are sensitive or allergic to cow milk [5, 6]. These benefits can be attributed to the micellar structure of casein protein in goat milk and to the fact that it contains a large quantity of fatty acids with higher digestibility [5, 7]. On the other hand, sheep milk is characterised not only by its higher total solids, fat, protein and caseins but also by its larger amount of minerals and vitamins [5, 8].

 Yogurt has been known to mankind for over 6,000 years. The word "yogurt" seems to be derived from the Turkish word "jugurt" which first appeared in the eighth century [9]. The same author mentions that yogurt comes from the Middle East, where milk was scarce due to the desert environment.

Moreover, in milk technology yogurt and its derivatives are called fermented milk products. This process results from the development of certain microorganisms that modify the normal components of milk. Lactose is partially transformed into lactic acid. In certain milks, it also produces ethyl alcohol. Furthermore, proteins may suffer peptonisation, which improves digestibility [8].

 The Food and Agriculture Organisation (1984) defined yogurt as "the coagulated milk product obtained by lactic acid fermentation through the action of *Lactobacillus delbrueckii ssp. bulgaricus* (*Lb. bulgaricus*) and *Streptococcus thermophilus* from milk and milk products. The microorganisms in the final product must be viable and abundant". The *Codex Alimentarius* (Codex STAN 243-2003) specifies that yogurt should contain a minimum of 2.7% (m/m) milk proteins, a maximum of 15% milk fate, a minimum of 0.6% titratable acidity (expressed as % of lactic acid) and a minimum of 107 CFU/g of microorganisms (total microorganisms in the starter culture).

 The objective of this work was to address the issue of producing yogurts from goat and/or sheep milk with high consumer acceptability and high nutritional value so customers could be offered a new and alternative product in the competitive market of fermented milk and in a context where cow milk yogurt has the largest market share. In order to support the findings observed in this study, a complementary characterisation of the goat, sheep and cow milks was also carried out.
