## 1 Introduction  
Manufacturing recombined and reconstituted milk and milk products is a technology that appeared at the end of the 20 th century [1]. Recombined and reconstituted milk products provide a nutritious and high-quality source of dairy products in the areas where fresh raw milk is not readily available or is in a short supply [2]. Initially, this technology was applied to obtain fluid milk, but it was followed by the production of recombined evaporated milk and sweetened condensed milk. Nowadays, the recombination industry also includes yogurt, butter, and cheese [3]. The basic types of milk powders used in the recombination industry include skim-milk, whole milk, and buttermilk [4]. These products are the main milk powders in the marketplace [5]. However, the introduction of membrane techniques in the dairy industry has allowed the production of other types of milk powders, containing diverse protein-to-lactose ratios and altered the whey protein to casein ratios, e.g., milk retenate, milk permeate, whey retenate, and whey permeate powders [6; 7]. The use of these powders has enabled the production of recombined dairy products, such as concentrated yogurt, which have large protein and small lactose contents. Numerous investigations were carried out to assess the properties and applications of these powders in the production of dairy products [8; 9; 10; 11; 12; 13; 14; 15; 16]. For instance, several researchers recommended the use of these powders to fortify the milk base during yogurt production [17; 18; 19; 20]. Dairy powders have a very long shelf life, can be stored at ambient temperatures and can be easily transported [21]. However, since the quality of recombined dairy products is directly related to the composition and the physical, chemical, microbiological and sensory standards of the ingredients used, it is of great importance to conserve the physicochemical and microbiological properties of dairy powders during their storage until their final reconstitution [4]. Freeze-dried (direct-to-vat) thermophilic starter cultures are also required to produce yogurt powders. Freeze-dried cultures can contain higher levels of viability than the dried ones obtained by other drying techniques. They can also be stored in a conventional refrigerator and transported at room temperature [22; 23]. These advantages are attributed to the stability of lyophilized cells at room temperature, although their optimal preservation condition is at \(-20\)\({}^{\circ}\)C [22]. The major disadvantages of using freeze-dried concentrate cultures are that, compared to the frozen concentrate cultures, they require a longer lag phase during incubation. Some commercial strains do not survive the process well and, compared to other drying techniques, freeze-drying requires higher costs and larger energy consumption [24; 25; 26; 27]. The viability of lyophilized starter cultures is influenced by several factors, including the growth medium, freezing rate, drying temperature and composition of the freezing medium. Additionally, the subsequent storage conditions, such as temperature, atmosphere, exposure to light and relative humidity, also have a significant impact on the viability of these cultures [28]. Since the starter culture activities significantly affect the rheological and physicochemical characteristics of acid milk gels [29; 30; 31; 32], it is essential to preserve the viability of these cultures during the dry storage of the powder mix, as this will enable the maintenance of the expected rheological and physicochemical characteristics of the final recombined product throughout the storage time. Understanding how the yogurt powder behaves during storage is important because its shelf life is based on whether the recombined product obtained from the dried mix exhibits any of the physical, chemical or sensory characteristics that are unacceptable for consumption [33].