London | Researchers have manipulated the metabolic properties of yogurt-producing bacteria to sweeten the yogurt naturally and reducing sugar content in the final product.
Lactose was also eliminated using microbiological methods, so that those with lactose intolerance could enjoy the yogurt. The goal was to engineer the yogurt bacteria not to consume glucose, a fermentation product that is a particularly sweet form of sugar, said Eric Johansen from Chr Hansen Holding A/S – a global bioscience company in Denmark.
In certain countries, yogurt contains live cultures of bacteria – Streptococcus thermophilus and Lactobacillus delbrueckii subsp bulgaricus. Normally, when grown in milk, the two bacterial species break down lactose, a disaccharide, into its monosaccharide components, glucose and galactose.
They consume the glucose and secrete the galactose, researchers said. We wanted to change them so that they would eat the galactose and spit out the glucose. That required a number of changes in metabolism, said Johansen. In the first step, the investigators grew S thermophilus on a medium where galactose was the sole food source.
Thus, individual bacteria had to consume galactose in order to grow. A few mutants were capable of doing so, and the investigators cultured these, he said.
The next steps were to modify the bacteria so that they would no longer consume glucose, and would no longer even transport glucose into the cell. Researchers grew the bacteria in a medium containing a glucose analog called 2-deoxyglucose, which is toxic to cells. The few mutants that survived in this medium lacked the ability to metabolise glucose.
A second round of selection, with higher levels of 2-deoxyglucose, resulted in survival of mutants lacking the glucose transport mechanism. Researchers also used 2-deoxyglucose to isolate mutants of Lactobacillus bulgaricus, to select for mutants that were unable to transport glucose into the cell. This prevented them from consuming the glucose produced by S thermophilus.
The yoghurt was now made with the modified bacteria. The yogurt had very little lactose, and not much galactose. But it was high in glucose – and sweet. Researchers were able to reduce added sucrose by 20 per cent while maintaining the desired sweetness.
The sugar content of food is of increasing concern to health-conscious consumers, and dairy products are often criticised due to the presence of added sugar sucrose, said Johansen.
We reasoned that since glucose is considerably sweeter than lactose or galactose, bacteria that release glucose into the product could allow for a reduction in of added sugar while maintaining the desired sweetness in the yogurt, he said. The findings were published in the journal Applied and Environmental Microbiology.