Yogurt, defined as the product of milk fermentation by Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, has a long history of beneficial impact on the well-being of humans. Yogurt bacteria have been shown to improve lactose digestion in lactose-intolerant individuals, to affect the intestinal transit time, and to stimulate the gut immune system1. Bacteria used to ferment milk to obtain yogurt belong to thermophilic, bile-sensitive species of lactic acid bacteria, which are not ideally suited for survival into the human gut. However, assessing the viability of these bacteria through the digestive tract may be relevant to evaluate their potential to deliver some beneficial effects for the well-being of the consumer2.
The aim of the study was to determine the survival of bacteria from yoghurt with the addition of sucrose in model digestive juices. The material tested was yoghurt obtained in laboratory conditions by fermenting UHT milk using an ACIDIFIX starter culture (Chr. Hansen, Denmark). The yoghurts were made in 6 variants - in a natural form and with the addition of a 50% sucrose solution in the amount of 1%, 2%, 3%, 4%, and 5%. The optimal addition of sucrose to yoghurt was determined on the basis of the active acidity analysis during the 5-hour fermentation at 45 °C, and then 28-day storage period at 6 °C. The yogurt with the optimal sucrose addition was analyzed for the survival of yoghurt bacteria in the model gastric juice, and then in the model intestinal juice. The viable population of yoghurt bacteria was measured using the droplet method on Petri dishes to determine the population of Lactobacillus delbrueckii subsp. bulgaricus (on MRS agar) and Streptococcus thermophilus (on M17 agar). Petri dishes were incubated anaerobically for MRS media or aerobically for M17 media at 37 °C for 72 hours.
The microflora of the ACIDIFIX yoghurt culture does not show the ability to ferment milk without the addition an additional source of carbohydrates. For the study of the survival of yoghurt microflora in the model digestive juices, yogurt with 5% sucrose was selected because such an addition allowed to achieve the optimal pH in the shortest fermentation time and keeping this value during storage. In the tested model gastric juice, the bacteria S. thermophilus and L. delbrueckii subsp. bulgaricus showed the ability to survive in the conditions prevailing in the stomach, and thus resistance to the effects of low pH. L. delbrueckii subsp. bulgaricus showed greater survival in the environment of the model intestinal juice than S. thermophilus. The studied yoghurt microflora shows a high sensitivity to the conditions of the model intestinal juice compared to the conditions of the model gastric juice.