[QUOTE=doreen T]
Um, you seem to be missing the point of that Megazyme booklet ... The description is for determining the lactose content only, not the total carbohydrate content.

Correct. As I said, I'm not an expert on this. But I would assume the sugar content of milk is well known, and that lactose is the main sugar. Hence disappearance of lactose should equate to disappearance of significant carbohydrate.

Because bacterial beta-galactosidase is contained within the bacteria, it wouldn't break down lactose outside the bacteria. So if you spun out the bacteria and measured the lactose in the supernatant, you'd have an indication of how much lactose the bacteria had eaten.

Yes, there might be some lactose and glucose and galactose and numerous other sugars that are involved in living processes within the bacteria. If the bacteria remain whole until they reach the colon, this wouldn't be significant. Even if they don't, this might not be significant. I suspect it's never been studied.

The protein we eat as well as the glycerine part of fats can also be converted to carbohydrate and we generally ignore these when calculating carbs in LC diets, although people with diabetes who are injecting insulin do take them into account.

Another possibility is that some of the bacteria in the yogurt are dying and breaking down, releasing beta-galactosidase into the whey. In this case they could create glucose and galactose from remaining lactose. But the galactose or glucose (whichever was measured in the assay) would be included in the results.

>The galactose content in yogurt is a known health concern for some people who lack the liver enzyme which converts galactose into glucose (glycogen).

Yes. There are many carbohydrate diseases. And people with this particular disease shouldn't eat lactose. This doesn't mean that healthy people shouldn't eat carbohydrates. Glucose is not synonymous with glycogen. Glycogen is a polymer made up of many glucose units. It's human starch.

>Only glucose is utilised by the bacteria to be converted to lactic acid. The galactose remains unconverted.

Glucose and galactose are interconverted in most species via UDP-glucose and UDP-galactose. In some bacteria, the presence of galactose in the medium induces the formation of an enzyme called "galactozymase," which ferments galactose.

>Yes, the lactose content may go as low as 4 g per cup (depending on amount of fermentation or sourness), but there is still the free glucose and galactose to consider as part of the total carbohydrate content in yogurt.

Because the assay for lactose involves measuring the glucose and/or galactose, these will be measured by the assay.

>Yogurt is made very successfully from Lactaid or similar lactose-reduced milks, using the same bacteria as for regular yogurt. Since the step of splitting the lactose into its two monosaccharide components has already taken place, Lactaid-type milk actually ferments much more quickly than regular milk because the glucose is readily available.

And what is the point of paying extra for Lactaid? I'm not particularly interested in the speed with which my yogurt ferments. I usually leave it overnight.

As I noted, I'm not an expert in this, and I don't want to debate it ad infinitum. I just wanted to provide some information. For those who are able, perhaps eating yogurt and measuring the resulting blood sugar levels would be the best indication of how the particular yogurt you eats is affecting you.