Evaluation of physical-chemical and microbiological characteristics of freeze-dried and reydrated yogurt

This study aimed to evaluate the physical-chemical characteristics and the viability of lactic acid bacteria during the fermentation process of the yogurt and after the freeze-dried process, in addition to testing three thickener formulations for the rehydration of the yogurt powder. During the fermentation process, the production of lactic acid and the growth of lactic acid bacteria were accompanied. Before and after freeze-dried process, yogurt was analyzed for pH, titratable acidity, carbohydrates, proteins, lipids and viable lactic acid bacteria. After lyophilization, three thickener formulations were tested to evaluate the rehydration of powdered yogurt. At the end of the fermentation process, it was verified that the lactic acid bacteria grew to reach 7.8.10 UFC.g and the acidity obtained was 9.27 g.L. The viable lactic acid bacteria count of freeze-dried and non-freeze-dried yogurt was 5.6.10 CFU.g and 7.8.10 CFU.g, respectively. Non-freeze-dried and freeze-dried yogurts showed a content of 20.8% and 21.0% carbohydrates, 4.0% and 3.6% protein and 3.7% and 2.7% lipids, respectively. The combination of thickeners that provided viscosity similar to commercial yogurts was the guar gum, pectin and maltodextrin mix. Thus, it was possible to verify that the freeze-drying process maintains the physical-chemical characteristics and viability of lactic acid bacteria. In addition, the developed yogurt presented easy reconstitution at the time of consumption.


Introduction
The knowledge of the beneficial effects of dairy products allows the production of a range of yogurts with different flavors, textures, and consistencies in response to consumer preferences (Morell et al., 2015). The yogurt offers nutritional benefits from proteins, lactose, minerals and water-soluble vitamins, important in the human diet (Caleja et al. 2016;Ozturkoglu-Budak et al., 2016). According to the Codex Alimentarius Commission (FAO/WHO, 2011), yogurt is defined as a fermented milk with symbiotic yeasts of lactic bacteria such as Streptococcus thermophilus and Lactobacillus delbrueckii subspecies bulgaricus, which must remain viable in the product until the end of its useful life.
Lactic acid bacteria use lactose as a substrate for fermentation of yogurt, so the final product's low lactose content is advantageous for those who do not digest this carbohydrate Research, Society and Development, v. 9, n. 6, e14962446, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i6.24464 (Silva et al., 2010. In addition, the yogurt is rich in protein, calcium, phosphorus and vitamins and lactic acid bacteria present may exert beneficial effects to the organism (Saad, 2006).
Probiotic microorganisms like Lactobacillus acidophilus and bifidobacteria provide health benefits (Brasil, 2002;Brasil 2008). Probiotics contribute to the control of intestinal infections, stimulation of intestinal motility with consequent relief of intestinal constipation, better absorption of certain nutrients, better use of lactose and relief of symptoms of intolerance to this sugar, decrease in cholesterol levels, anticarcinogenic effect and stimulation of the immune system (Espitia et al., 2016).
During the yogurt processing, the lactic bacteria incorporated into milk are responsible for the acidification of the product until it reaches pH close to 4.0, necessary to avoid the development of undesirable bacteria, to provide specific sensorial characteristics, by modifying flavor, texture, aroma and protein content (Dias & Pulzatto, 2009;Piard et al., 2011). In the fermentation process, the bacterium Lactobacillus bulgaricus releases amino acids and peptides from the milk protein, which stimulates the growth of Streptococcus thermophilus, which grows and releases formic acid and carbon dioxide, thus contributing to the further development of the probiotic Lactobacillus bulgaricus (Oliveira & Damin, 2003).
To preserve the nutritional characteristics of the yogurt and to keep the lactic acid bacteria viable, the drying process can be applied by freeze-dried. According to Mata et al. (2005) and Yamaguchi et al. (2017), the freeze-dried process comprises the removal of water from the product by sublimation, allowing the maintenance of biological, nutritional and sensory characteristics similar to its matrix.
Given the above, the objective of this study was to evaluate the physical-chemical characteristics and the viability of lactic acid bacteria during the fermentation process of the yogurt and after the freeze-dried process, in addition to testing three thickener formulations for the rehydration of the yogurt powder.

Yogurt Preparation
For yogurt formulation whole milk, whole milk powder, sucrose and lactic culture (Lactobacillus acidophilus LA-5®, Bifidobacterium BB-12® and Streptococcus thermophilus) were used. The milk was initially heated to 90 °C for 5 min to promote the Research, Society and Development, v. 9, n. 6, e14962446, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i6.2446 5 growth of the initial culture and partially denature the milk proteins in order to contribute to a better coagulation of the milk and decrease the syneresis after the end of the fermentation (Xu et al., 2008). Subsequently, the milk was cooled to 45 °C and the dried ingredients were added together with the lactic culture previously homogenized. The mixture was incubated at 45 °C for 4 h, cooled to 4 °C and stored in the refrigerator.

Kinetic study of lactic acid production and growth of lactic acid bacteria during yogurt fermentation
During yogurt fermentation, lactic acid production, pH, and lactic acid bactéria counts were analyzed every 30 min and the specific production rates of lactic acid ( ) (1), and cell growth ( ), were determined (2), according to Borzani et al. (2001). (1) (2) : cell concentration; : instantaneous rate of production of lactic acid; : instantaneous rate of cell growth.

Freeze-dried yogurt
For yogurt freeze-dried, 40 g of the yogurt was used, and the samples were frozen for 10 h at -18 °C. The containers were then placed in the freeze dryer (Terroni®, Enterprise II, Brazil) for 24 h at -60 °C.

Yogurt rehydration
Due to freeze-dried, 85% of yogurt moisture was reduced, 85% filtered water at 10 °C was used for the rehydration of the freeze-dried product, which three formulations of Research, Society and Development, v. 9, n. 6, e14962446, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i6.2446 6 thickeners were evaluated at three different concentrations (Table 1). These thickeners were chosen due to their wide use in dairy products. The formulations were developed according to the recommendation of the use of the manufacturers. The thickeners were weighed and mixed into freeze-dried yogurt for further addition of water, with manual stirring.
After rehydration of the yogurt, the viscosity was evaluated and compared to that of a commercial yogurt. The samples (50 mL) were analyzed by viscometer (model Q860A21, Quimis) using number 4 rotor with a speed of 6 rpm. The results were evaluated through analysis of variance and Tukey's test, with a level of 5% of significance, using Statistica 7.0.