Kefir is a popular fermented dairy beverage, created by culturing milk with a symbiotic colony of bacteria and yeasts known as kefir grains. This food has gained attention for its probiotic content, but also for specific nutritional enhancements that occur during fermentation. One such nutrient is Vitamin K2, a fat-soluble compound of growing interest due to its distinct physiological roles. This article addresses whether kefir contains Vitamin K2 and explores the biological process that makes this possible.
The Direct Answer: K2 in Kefir
Yes, kefir does contain Vitamin K2, a nutrient that is not readily available in the raw milk used to make it. The presence of this vitamin is a direct result of the metabolic activity of the diverse microbial community within the kefir grains. These microorganisms actively synthesize menaquinones, the compounds that make up Vitamin K2, during the fermentation process.
The primary form of Vitamin K2 found in kefir is menaquinone-7 (MK-7), although other longer-chain menaquinones like MK-8, MK-9, and MK-11 have also been detected. This synthesis begins as the mesophilic bacteria and yeasts consume the milk’s nutrients in a cool, room-temperature environment. Studies have shown that the concentration of menaquinones increases continuously as fermentation progresses, distinguishing kefir from non-fermented dairy.
For instance, some kefir samples made with natural grains have been measured to contain up to 4.82 µg of MK-7 per 100 grams. This K2 is a byproduct of the grains’ life cycle, making kefir a reliable dietary source of bacterially produced menaquinones.
Understanding Vitamin K2
Vitamin K2 belongs to a group of fat-soluble compounds called menaquinones, which are structurally defined by a naphthoquinone ring and a side chain of varying lengths. The length of this side chain determines the specific menaquinone form, designated as MK-n. The two most studied forms are MK-4 and MK-7, which differ significantly in their dietary sources and biological half-lives.
MK-4 is the shortest form and is primarily found in animal products like meat, egg yolks, and high-fat dairy, and it has a relatively short lifespan in the bloodstream. In contrast, MK-7 is a longer-chain menaquinone that is predominantly synthesized by bacteria, making it abundant in fermented foods. The larger molecular structure of MK-7 allows it to remain in circulation for a much longer period, offering sustained biological activity.
The main physiological function of Vitamin K2 is to activate specific proteins through a process called carboxylation. This includes Matrix Gla Protein (MGP), which prevents calcium from depositing in soft tissues like arteries. It also activates osteocalcin, a protein that directs calcium into the bone matrix, promoting proper bone mineralization and density.
Factors Influencing K2 Content
The final concentration of Vitamin K2 in kefir is highly variable, depending on several factors related to the production process. The specific starter culture, or the composition of the kefir grains used, is a major determinant of K2 output. Different strains of bacteria and yeast possess varying efficiencies for menaquinone biosynthesis, with certain mesophilic bacteria strains being high producers of MK-7.
The conditions of the fermentation itself, including time and temperature, also play a significant role in K2 generation. Kefir is typically fermented using mesophilic cultures at cooler temperatures, generally between 20°C and 24°C, which promotes K2 synthesis. Longer fermentation times, often around 20 hours, allow the microorganisms more opportunity to synthesize and accumulate menaquinones.
The source and composition of the milk used as the substrate also influence the final K2 concentration. Because K2 is a fat-soluble vitamin, higher-fat milk naturally leads to a higher K2 content in the resulting kefir, as the vitamin is sequestered in the fat globules. Furthermore, the initial Vitamin K1 content in the milk, which is higher in grass-fed dairy, provides the raw material that some bacteria can convert into menaquinones during fermentation.