There's a popular myth around that yogurt is much lower in carbs than stated on the label -- that the bacterial cultures "eat" the lactose, converting it to lactic acid thus reducing the carb content by 8 grams per cup.
I have been studying and researching this to death, trying to separate fact from myth.
I really, really wanted this to be true, because I LOVE yogurt, and missed having it every day.
Here's what I've found so far ..
Lactose is a disaccharide .. meaning it's composed of 2 simple sugar molecules bonded together. The 2 simple or monosaccharide sugars that make up lactose are glucose and galactose.
Only glucose is converted to lactic acid. The process to ferment galactose is much more complicated and bacteria will only resort to using it when glucose has been completely used up. However the yogurt would be very, very sour by this point .. unpalatable to most tastes.
Most commercial yogurts are fermented until approximately 20 - 25% of the lactose has been converted. Then the yogurt is rapidly chilled to halt the fermentation. 20 - 25% of 12g carbs is NOT 8g .. it's 2.4 to 3g. So, commercial yogurt made from whole milk, starting out at 12g carbs per cup, will most likley have no less than 9g residual carbs in it --
not 4g. During storage, as long as the bacterial cultures truly are "live", they will continue to very slowly ferment some of the residual glucose .. but most manufacturers report that yogurt fermented beyond 30% is quite sour and considered unpalatable.
I have read the article from LowCarb Luxury a zillion times ... it's been posted and quoted all over the internet. I've lurked at the
GO-diet's discussion board ... and it's stated that 4g lactose remaining per cup of yogurt applies to
home-made yogurt that has been allowed to come to end-fermentation. ie -- it must be allowed to become quite sour. And I could not see anywhere that residual glucose and galactose sugar had been considered -- only lactose was tested for.
Here's some info. from the US Dairy Export Council (USDEC):
Quote:
About 20 to 30% of the lactose in the yogurt base is broken down to glucose and galactose, and the glucose is converted to lactic acid during yogurt fermentation. Bacterial enzymes in the intestinal tract can help break down the remaining lactose (into glucose and galactose). However, gastric acids in the stomach can destroy probiotic culture organisms and the bacterial ß-galactosidase enzymes that break down the lactose.
http://www.usdec.org/files/pdfs/5yogurt.pdf
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note - the link is to a pdf file ... you'll need to have Acrobat Reader installed to view it.
My conclusion is that the amount of carbohydrate which ultimately gets converted to lactic acid is much less than 8g per cup .. likely no more than 2 or 3g per cup. The residual carbs in finished yogurt consist of unsplit lactose molecules + unfermented glucose + galactose sugar.
Yogurt is a good source of calcium, and the live bacteria have many health benefits. But it is not as low-carb as some would like to believe
Doreen