**1. Introduction**

Different types of lactic acid fermentation are the base of more than 3500 traditional fermented foods found worldwide [1]. After sourdough products and vegetables, dairy products are the third most desired and consumed products in Bulgaria (**Table 1**). Yoghurt and yoghurt-like products are the most famous of all dairy products. Their consumption is the highest in the countries of the Mediterranean, Asia, and Central Europe. The consistency, taste, and aroma differ from region to region and from that of other fermented products. In some areas, yoghurt is produced in the form of a very thick liquid, while in other countries, it is in the form of a softer gel.


#### **Table 1.**

*Average dairy product consumption per family member in Bulgaria, 2012–2021.\**

Yoghurt and yoghurt-like products can also be prepared in the form of a dessert or as a drink.

The wide consumption of fermented milk products in our country is not only the result of a centuries-old tradition. They are widely accepted as a native food with a naturally balanced composition of essential nutrients (proteins, carbohydrates, fats, mineral salts, vitamins, and enzymes), which are easily digestible and necessary, especially for growing and aging organisms [2]. In addition, lactic acid fermentation with selected bacteria repeatedly increases the functional and biological significance of milk-based foods.

#### **1.1 Traditional Bulgarian yoghurt**

Bulgarian yoghurt is a traditional food produced by microbial lactic acid fermentation of pasteurized milk. Its traces date as early as 8000 BCE when the domestication of milk-producing animals like sheep, cows, and goats began [3]. Before the commercialization of dairy production in Bulgaria, yoghurt was mainly for family use and mostly made from ewe milk.

Nowadays, both industrial and artisanal technologies of yoghurt production are similar. The processes start with the filtration of the raw milk for the removal of solid particles (**Figure 1**). A common "technological" step is heating the milk for some time and subsequent storage at a suitable temperature. Regardless of the way of production (**Figure 1**), the process is carried out by inoculation with two symbiotically connected bacterial species – *Lactobacillus delbrueckii* subsp. *bulgaricus* and *Streptococcus salivarius* subsp. *thermophilus*. The symbiotic relationship of the starter results in the shortened fermentation process and stable characteristics of the final product [4].

The main difference between the artisanal and industrial processes is the step of homogenization (**Figure 1**). It affects the texture and body of the product by reducing whey syneresis (whey separation) and densification of the coagulum, resulting in a thicker yoghurt with improved consistency. In homemade and artisanal yoghurts, the milk fat floats to the surface, forming a creamy layer, called "kajmak". The taste and aroma of Bulgarian yoghurt are specific and also dependent on the raw milk used and the flavoring properties of the starter cultures [5, 6].

*From Traditional Bulgarian Dairy Products to Functional Foods DOI: http://dx.doi.org/10.5772/intechopen.108998*

#### **Figure 1.**

Yoghurt is evaluated by five organoleptic qualities – appearance, color, aroma, consistency, and taste, according to the International Dairy Federation (IDF) Standard 99A (1987) [7] – and some extra qualities for Bulgarian yoghurt include consistency after stirring, body, and texture, according to the Bulgarian National Standard (12:2010) [8].

One of the rarest and almost forgotten Bulgarian dairy products is called *brano mliako.* "It is made in the region of the Rhodope Mountains by an ancient recipe that has remained unchanged for centuries. It is known to be one of the most unique organic foods in the world because of its qualities. "*Brano mliako*" is made from ewe milk and very much resembles yoghurt. The specificity is that it is made only at the end of summer. The technology starts by collecting raw milk in wooden containers, where it is filtered and thickened. The dehydrated milk then ferments spontaneously, or a starter culture can be added to it, mainly sheep yoghurt. After that, a thin layer of goat or sheep tallow is poured on top, so that the product can be "sealed". This anaerobic preservation makes "*brano mliako*" "suitable to consume for 3 to 4 months.

Gruev (1970) [9] developed a laboratory technology and obtained the same final dairy product. The raw milk is twofold concentrated at reduced pressure at 45–50°C, sterilized by the Koch method for 30 min, and cooled to 45°C. Then, the milk is inoculated with 1% yoghurt starter culture and left for the fermentation process until achieving approximately 190°T acidity. The addition of a 2% yeast-based starter culture, which has been isolated from "*brano mliako*" and cultured in grape must, continues the fermentation. At the end of the yeast fermentation, the obtained dairy product is put in glass containers, hermetically sealed, and stored at 8–10°C for 4–5 months (Gruev, 1970) [9].
