Homemade Greek Yogurt Calculations

I recently started making greek yogurt and was frustrated by not knowing the nutritional information, and thought I'd share what I've come up with. I think it's pretty good, but I welcome corrections from anybody who knows more about this stuff.

I'm not including a lot of sources, but am linking to one paper that I think was pretty informative about lactose/glucose/galactose content in milk as it ferments.

First for reference, here are the macros for regular milk (all listed per 100g of milk, we'll scale the numbers as needed later):

fat   carb  protein
----- ----- -----
0.922 5.23  3.38 : 100g 1% milk (nutritiondata.com)
1.94  5.00  3.47 : 100g 2% milk (nutritiondata.com)
3.28  5.23  3.18 : 100g whole milk (nutritiondata.com)

And when we drain off the liquid whey we'll need to subtract off the macros for it (also listed per 100g):

fat   carb  protein
----- ----- -----
0.36  5.14  0.85 : 100g liquid whey (from wikipedia)

The above information will allow us to subtract off the macros for the whey we strain, but there's another more nebulous part to the yogurt puzzle: to what extent does fermentation change the macros of the milk.

As near as I can tell from googling this the carbohydrate reduction from fermentation is often overstated, but there is some reduction happening. My main source on that topic is here:
http://pubmedcentralcanada.ca/pmcc/articles/PMC170011/pdf/aem00006-0101.pdf
In this paper Fig 2 shows the sugar concentrations in milk start at 2.64% glucose, 2.31% galactose, 2.06% lactose and change over time as it ferments. It doesn't give numbers for the final values, but eyeballing the graph I'd say at 5 hours incubation they look like 1.95% glucose, 2.25% galactose, 1.5% lactose. If that's accounting for all the carbs in the milk that's a carb reduction of 18.7% over the 5 hours. The same paper in Fig 5 shows even more directly line (c) the percentage of total carbohydrates consumed by the bacteria. Here it looks like just a bit over 15% after 5 hours. Fig 3 shows (with a different culture and different type of milk) that lactose and glucose keep going down easily up to 24 hours.

So even though the above paper isn't trying to directly answer our question here, I think this suggests 18% isn't an unreasonable estimate for carb reduction of milk while fermenting into yogurt. I'm tempted bo bump that up to 25% for long fermentation times, but I'm not real confident in getting any numbers from that 24hr graph.

So finally here's an example of piecing the above together for a recent batch of yogurt I made from 1% milk:

1. Measure how much milk I started with: 3904g. Scaling up the fat/carb/protein values for that quantity is

fat   carb  protein
----- ----- -----
36.0  204.2 132.0 : for the full gallon of 1% I started with

2. Reduce carbs by 18% to account for fermentation.

fat   carb  protein
----- ----- -----
36.0  167.4 132.0 : for the gallon of 1% after fermenting

3. Measure how much liquid whey was drained off: 2124g, and find the macros for that.

fat   carb  protein
----- ----- -----
7.65  109.2 18.1 : for the liquid whey strained out

4. Subtract to see what's left in the greek yogurt.

fat   carb  protein
----- ----- -----
28.4  58.2  113.9 : macros for the whole batch of greek yogurt

5. Measure the end result: 1618g. Note I think it's normal for the numbers to not quite add up, maybe some water evaporated. Anyway I want to know the data for 100g of the final product, so I divide the above macros into the final 1618g to get

fat   carb  protein
----- ----- -----
1.76  3.60  7.04 : macros for 100g of my homemade greek yogurt

I think the biggest question mark in the above is whether that 18% is a good estimate of the carb reduction from fermentation, but I hope my reading of the paper I quoted justifies that if anything I'm being a little conservative there. And the end result looks plausible to me.

I hope this helps others who are struggling to calculate data for their homemade yogurt, and if I'm wrong in any of the above I'd love corrections to make it better.