i am gona post here a nice email from my friend Peter, who was speaking about gouda cheese in cheese mailing list: (very intersting) read
Ideally any lactose left in the curds will be consumed by the still active bacteria in the curds, Atleast to the point where the pH reaches 4.6 or so ( this is where the acid present in the product starts inhibiting the bacterial action )
Ideally you want your cheese to end up with a pH around 5.25 12 hours after pressing.. This parameter can be controlled by adjusting the adjusting the amount of lactose left in the curds.. The key to doing this is replacing whey with warm water when cooking the curds.. In large scale cheese production ( wich I was involved in until recently ) you typically remove 30-60% of the whey and replace with water.. The lactose is dissolved in the whey and by replacing the whey with water you remove 30-60% of the lactose while keeping the total quantity constant..
Example:
100 kg of milk makes roughly 10 kg of cheese and 90 kg of whey..
Milk contains approx. 4.5% lactose
IOW your cheese vat contains 4.5 kg of lactose ( lactose content doesnt change notably for the pH drop you see before you start cooking )
Remove 60 kg of whey ( 2.7 kg lactose ) and add 60 kg of water
Lactose content is now 1.8%
Once you start pressing you remove 90 kg of whey ( 1.62 kg lactose )
Lactose content in the cheese is now 1.08%
When cooling the cheese it will lose another kg of whey ( 10.8 g )
Total lactose content in the 9 kg of cheese is now 169.2 g or 1.88%
Most of this will be consumed by the bacteria from the starter culture, but its hard to tell exactly how much without expensive lab equipment..