By adding a novel genetically engineered bacterial strain to dairy, the fermentation process is limited to converting lactose to glucose, a technique that could remove the need to add sweeteners to dairy products.
The research, part of the EU-funded Nutra Cells project (QLK1-CT-2000-01376), also has implications for lactose intolerance because, by converting the lactose in the dairy to glucose, the final lactose content of the dairy is significantly reduced.
"The resulting strain can be used for in situ production of glucose, circumventing the need to add sweeteners as additional ingredients to dairy products," wrote lead author Wietske Pool from the University of Groningen. "Moreover, the enhanced removal of lactose achieved by this strain could be very useful in the manufacture of products for lactose intolerant individuals."
The researchers, led by Professor Oscar Kuipers from the University of Groningen, screened numerous strains and plasmids of directly engineered Lactococcus lactis bacteria. The tests were performed on L. lactis strain NZ9000, constructed from the L. lactis strains LL108, LL302 and pORI280.
Lactococcus lactis is used extensively in the dairy industry in the production of fermented milk products, and has a relatively simple carbon metabolism pathway. By directly engineering this strain of Lactococcus lactis the researchers were able to delete genes that coded for glucose metabolism by the bacteria. This was achieved by disrupting the main sugar transport system, the so-called sugar phosphotransferase system (PTS).
Lactose is a disaccharide, meaning it is made up of two basic sugar molecules, a galactose molecule bonded to a glucose molecule. By deleting certain genes in the lactose-PTS, it was possible to create a bacterium (NZ9000Glc-Lac+) that selectively fermented the galactose part of the lactose, but not the glucose part. The glucose then accumulated outside of the cell, in the dairy product.
One of the genes deleted has only recently been discovered - glucose-PTS EII-cel (ptcBAC) - while the others are known - glucokinase (glk) and EIIman/glc (ptnABCD).
Professor Kuipers told that the fundamental research used a lab strain and that some extra work would be needed to take it to an industrial setting, but one of the main challenges is not technical but industrial and consumer attitudes and regulations regarding genetically modified organisms (GMO), particularly in Europe.
It may be, said Professor Kuipers, that this kind of bacteria would pass EU regulations, since it is based on gene knockout, and recombinant DNA, and could be considered as self-cloning. No non-bacterial DNA is added.
And in the US, where industrial, regulatory, and consumer issues regarding GMOs are different to Europe, the research "could be of interest."
"It is our opinion that GMO is much safer than a randomly mutated strain. They are much more controlled, with much more predictable side effects," he said.
The viability of the strain was tested both under synthetic laboratory conditions, as well as in skim milk with a lactose content of about five per cent. In both sets of tests, NZ9000Glc-Lac+ increased the glucose content, and decreased the lactose content. The fermentation process was stopped when the pH of the milk fell to 4.2, as occurs in milk fermentation under natural conditions.
"L. lactis NZ9000Glc−Lac+ could be used to produce glucose from lactose, which could serve as a natural sweetener in fermented dairy products. This in situ produced glucose could replace, at least in part, the frequent addition of other sweeteners to dairy products, as the sweetness of glucose is about 60 per cent of that of sucrose," wrote the researchers in the September issue of the journal Metabolic Engineering (Vol. 456-464).
The technique could also impact on the taste of the final dairy product, they said, since glucose production in combination with less acidification of the product would produce a milder taste.
And since the strain uses lactose to produce the glucose, there is more effective removal of lactose from the dairy product.
"Therefore, the strain could be used as a nutraceutical to produce milk fermentation products with lower residual lactose concentrations, which would be suitable in a diet for individuals suffering from lactose intolerance."
