**5. Sheep and goat yogurt**

In yogurt made from sheep milk as with those produced with cow milk, homogenisation increases product firmness and reduces product serum separation. Sheep yogurt is characterised by higher values of hardness, adhesiveness and extrusion, such factors are explained [31] by its high solids content.

 A research related with the analyses of the microstructure of goat, cow and sheep yogurts observed that in sheep yogurt the protein matrix of milk consists mainly of chains of large individual casein micelles. There were also small and regular voids. As a result, sheep milk produces a stronger gel that is more resistant to deformation [32]. Another study reports that sheep milk has a high viscosity that will influence the firmness of the product and may be caused by an increase in water binding capacity provided by its milk proteins [33]. Moreover, an investigation carried out by [34] shows that sheep milk is used in the production of mixed yogurts—goat milk was also used in the cases depicted—due to the higher amount of protein found in sheep milk that will improve the consistency of goat yogurt. On the other hand, some research [35] found out that goat yogurt has a poorer consistency, hardness and stability when compared, for example, with sheep and cow yogurt. In the case of the protein matrix, casein micelles of small size are bound in thick chains presenting large agglomerates as well as large empty spaces filled with serum or occupied by yogurt bacteria. This yogurt reveals a less compact gel and is therefore more delicate, brittle and less resistant to deformation. These properties affect the results of the instrumental analysis conducted on yogurt texture. This yogurt will then present low texture values and high syneresis values [30]. Several studies have been carried out on the low consistency caused by the presence of goat milk and [36], in a study based on samples of cow and goat milk yogurt and a mixture of the two, found out that as goat milk was added the firmness and consistency of the gel decreased.

The higher porosity of the protein network observed through the analysis of the microstructure and the subsequent lower degree of micellar aggregation also contribute to the mechanics observed, since a gel with low porosity is characterised by a compact matrix, which would contribute to increase the firmness and consistency of the gel. The poor firmness of the gel can still be influenced by the size of the fat globules and their mechanical properties as explained by [37] who reports a positive correlation between poor firmness and the smaller size of the globules.

Goat milk yogurt is less commercially produced, although it has high digestibility and good nutritional and organoleptic properties [6].
