Published in Applied Research
Manipulating yogurt bacteria to reduce sugar content without reducing sweetness
Additional ingredients are often added to yogurt to improve consumer acceptability of appearance, sweetness, texture and flavour. This also includes the addition of sucrose, sweeteners, texturising agents and fruit preparations which can result in sugar content being similar to that found in soft drinks. 
A study published in Applied and Environmental Microbiology has manipulated the metabolic properties of yogurt-producing bacteria (S.thermophilus and Lb. delbrueckii subsp. bulgaricus) to sweeten the yogurt naturally, while reducing sugar in the final product. Johansen et al., from a Danish company, state that normally the two bacterial species break down lactose, a disaccharide, into its monosaccharide components, glucose, and galactose.  The species consume the glucose and secrete the galactose.
Johansen et al. grew S. thermophilus on a medium where galactose was the sole food source.  The individual bacteria had to consume galactose so that it could grow.  The researchers cultured the mutants that were capable of doing this and modified the bacteria so it wouldn’t consume and transport glucose into the cell.  The team did this by growing the bacteria on 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.
Johansen et al. also used 2-deoxyglucose to isolate mutants of Lactobacillus bulgaricus, to select mutants that were unable to transport glucose into the cell. This prevented them from consuming the glucose produced by S. thermophilus.
The team then prepared yogurt using the cultures.  The team used a milk base with 1.0% fat, 4.5% protein and 0.1% sucrose, and adjusted the protein to 4.5% by adding skimmed milk powder and sucrose at a concentration of 0.1%.  The milk base was then heat treated at 98oC for 30 minutes before cooling to fermentation temperature and inoculated with 0.0024% vol/vol Pre Innoculation Material.  The inoculation strain contained 90% s.thermophilus and 10% Lb.delbrueckii subsp. bulgaricus. Following fermentation, sucrose was added at various concentrations up to 5% wt/vol. 
Johansen et al. used a sensory panel of 10 trained assessors to rank the products for sweetness.  The panellists report that the yogurt produced with the manipulated cultures as being sweeter than the yogurt using the wild-type strains with no added sugar.  Johansen et al. state “the yogurt produced with the wild type strains containing 5% wt/vol added sucrose was not distinguishable from yogurt produced with the mutant strains containing 4% wt/vol added sucrose.”  They note that the mutant strain can reduce the amount of added sugar by up to 10 grams per litre and still provide the desired taste.  As the yogurt is also very low in lactose, this would also be suitable for those with lactose intolerance.

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