hello Gretchen, welcome!
Interesting post .. especially the reference links, which are the exact ones I used (among others) when doing my own research into the matter.
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Originally Posted by Gritchen
Re the methods: This is I think what Dr. Goldberg used:
Enzymic methods for the measurement of lactose are well known ..... by determination of either D-galactose or D-glucose........ and measurement of either released D-galactose or D-glucose
http://216.239.51.104/search?q=cach...ctosidase&hl=en
See above for full details. |
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. Lactose content is determined
indirectly by measuring the free galactose ... note that the article states:
"The measurement of lactose as D-galactose liberated is more generally reliable than measurement as D-glucose liberated because preparations generally contain more free D-glucose than free D-galactose."
As I noted in my essay
The Truth about carbs in YOGURT, finished yogurt, whether commercial or home-made, contains residual lactose + glucose + galactose sugars. These all must be taken into account when considering the total carbohydrate content, not just lactose alone.
The galactose content in yogurt is a known health concern for some people who lack the liver enzyme which converts galactose into glucose (glycogen). Galactose can build up in the bloodstream leading to a condition known as
galactosemia ... which is associated with impaired brain development in children, and eye damage/cataracts in adults.
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Originally Posted by Gritchen
The important thing to understand is that the bacteria take up the lactose whole. Then the enzymes inside the bacteria split the lactose into glucose and galactose and metabolize them into lactic acid. Glucose and galactose can be interconverted in many species. How much conversion goes on probably depends on the species of bacterium, which varies a lot from brand to bran.
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This is incorrect. Yogurt's bacteria secrete the lactase enzyme (beta-galactosidase) which hydrolyses the lactose ... ie, splits it into glucose and galactose. Only glucose is utilised by the bacteria to be converted to lactic acid. The galactose remains unconverted. From the
US Dairy Export Council:
"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..."
The reason is because galactose requires an
alkaline environment in order to be fermented into lactic acid. If you refer to the
Metazyme booklet which you linked to ... right below the bit you quoted about "Enzymic methods for the measurement of lactose are well known" ... you'll see in the chemical equation where:
The β-D-galactose is then oxidised .....at pH 8.6
Yogurt is hardly alkaline with pH approx. 4.2. Even liquid milk starts at a pH of approx. 6.5. To achieve an alkaline pH, sodium hydroxide (NaOH) must be added before galactose will ferment (oxidise) to lactic acid.
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Originally Posted by Gritchen
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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.
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Originally Posted by Gritchen
Note that this is different from Lactaid milk, in which you use the same (or a similar) enzyme to break the lactose into glucose and galactose, but there are no bacteria to break those sugars down into lactic acid so the carb count doesn't change.
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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. You can read more about this
here.
Doreen