Researchers from Victoria University looked at the effect of seven different strains to ferment soymilk and found that Lactobacillus acidophilus ATCC 4962 and L. casei ASCC 290 increased calcium solubility the most, with 89 and 87 per cent increases reported.
"Fermentation of calcium-fortified soymilk with selected probiotics might therefore be a promising way to enhance calcium bioavailability from calcium-fortified soymilk increasing its potential to improve bone health," wrote lead author in the Journal of Food Science .
Limiting bone loss could ease the burden of osteoporosis, estimated to affect about 75m people in Europe, the USA and Japan.
According to the International Osteoporosis Foundation, the total direct cost of osteoporotic fractures is €31.7bn in Europe, and $17.5bn in the US (2002 figure).
The total annual cost of osteoporosis in the UK alone is over £1.7bn (€2.5bn), equivalent to £5m (€7.3m) each day.
"As cows' milk is high in calcium while soymilk is not a naturally rich source, manufacturers often fortify soymilk with similar levels of calcium as in cows' milk (approximately 120 mg/100 mL) by adding calcium phosphate, calcium carbonate, or calcium chloride," explained the researchers.
"However, not all calcium fortificants are equal, and some forms of calcium may have poor solubility.
The bioavailability rather than the total content of calcium in soymilk is thus an important issue," they stated.
Indeed, experts have found fault with many soymilks that have been enriched with calcium.
Studies have revealed sedimentation of the calcium in several products, raising doubts about their effectiveness.
For example, a study carried out at Creighton University and published in Nutrition Today in February 2005 found that the calcium actually available in some soy and rice drinks can be as much as 85 per cent lower than the amount on the product label, owing to the mineral settling at the bottom of the pack.
The researchers fermented calcium-fortified milk with seven Lactobacillus strains, including L. acidophilus ATCC 4962, ATCC33200, ATCC 4356, ATCC4461, L. casei ASCC290, L. plantarum ASCC276, and L. fermentum VRI-003.
The solubility of calcium from the resulting soymilk was measured using atomic absorption spectrophotometry (AA), with the greatest increases found for L. acidophilus ATCC 4962 and L. casei ASCC 290 after 24 hours.
The researchers also report that fermentation significantly increased the conversion of isoflavones from their natural glucoside form into the biologically active aglycone form.
During fermentation isoflavones change from the glucoside form to the aglycone form, said to be more bioavailable.
A study by Yakult last year ( The Journal of Nutrition , Vol. 136, pp. 2291-2296) stated that it made sense that aglycones are more bioavailable since they are smaller molecules and are more hydrophobic, and so do not dissolve as easily in water and thus rapidly excreted.
The viability of the probiotic bacteria was also maintained above 7 log10 colony forming units (CFU) per millilitre after fermentation and storage, indicating that the soymilks would offer probiotic activity when consumed.
"Our results show that fermenting calcium-fortified soymilk with the selected probiotics can potentially enhance the calcium bioavailability of calcium-fortified soymilk due to increased calcium solubility and bioactive isoflavone aglycone enrichment," concluded the researchers.
Source: Journal of Food Science Published on-line ahead of print, doi: 10.1111/j.1750-3841.2007.00520.x "Fermentation of Calcium-Fortified Soymilk with Lactobacillus: Effects on Calcium Solubility, Isoflavone Conversion, and Production of Organic Acids" Authors: A.L. Tang, N.P. Shah, G. Wilcox, K.Z.Walker, L. Stojanovska