milk questions - two issues to answer please!

[scasnerkay]

My favorite cow has just finished (perhaps un-necessarily) a course of antibiotics for a mild staph infection (normal skin-type staph). The time has passed for all clear now but I am wondering if there is some residual effect on the milk.
I made a Caerphilly on Tuesday. At the beginning of the make the pH was low at 6.55 even after cleaning and recalibrating the meter. I went ahead, and the pH at renneting was 6.5, with a flocculation of 7 mins using a smaller than usual amount of rennet. That tells me the pH meter is pretty close to accurate. Other than that everything seemed pretty normal, except…
The make stalled after draining, and never got below 6.35, even after all night. I never got to milling the curd – it is chicken food!
My guess is that there was some residual antibiotic effect! Any other guesses???
Another question… Why the heck is her pH so low? Do some cows just have low pH milk? The highest I have ever measured hers is 6.6. Her milk is absolutely beautiful with a really thick cream layer that has a lovely yellow cast. She is eating actual grass (who would guess - here in California!!) and I wonder does that effect pH?

[Kern]

The simplest explanation is likely the correct one (Occam's Razor in action):  The lactose fermentation stalled because of residual antibiotics killing or weakening the bacteria.  The only other possible explanation would be that the milk ran out of lactose which would be highly doubtful.   

A pH of 6.55 is a little low but not horribly low.  The raw Jersey milk I used this winter (pellet fed cows, no silage) jumped up to about 6.75, give or take from about 6.5 late last spring.  I expect that the pH will drop when the animals start eating grass again in the next several weeks.  I am using some of this milk tomorrow and will post the initial pH after I measure it.

Update:  The pH of the milk I used today was 6.71.  This is down about a tenth from milk from the same herd about two months ago.  I suspect that the cows are starting to eat the spring grass shoots.

[scasnerkay]

Kern, when you have a lower starting pH, do you use less rennet?
After calving, this cow's pH was quite low (6.4 to 6.45) for quite a while, finally coming up to more like 6.55 or 6.6. I have done about 7 makes since she stabilized with pH, and flocc times have been consistently about 7 mins, even though I am using less rennet.

[Kern]

Susan,

You ask a good question.  If you've followed my work with Belper Knolle you'll note that adding rennet at really low milk pH results in a very fast "gel time".  Still, I was using levels equivalent to about 1/8 tsp per gallon of milk - a pretty standard amount.  I'm sure that if I had cut this in half that the "gel time" would have lengthened somewhat but I don't know what would have happened to the "curd strength".  I think that you are probably going to be better off using a standard amount of rennet and adjusting your cut time through the product of the floc time and multiple for the cheese you are making.  Otherwise you might have a weak curd unsuitable for the type of cheese you hope to make.

[scasnerkay]

Working on a 2 gallon make of a tomme... starting pH was 6.55, and pH at renneting was 6.5. I used 0.6ml of veg rennet - less than the "standard" of 0.75ml for that volume of milk. Flocculation was 6.5 mins... Maybe just as well, since a long flocculation would potentially mean too much acidity developing during the coagulation. Curd was quite strong.
Why would a lower amount of rennet yield a weaker curd?

Hopefully the pH will develop normally on this make - I will post back when we are further along!

[scasnerkay]

Must have been residual antibiotic in the milk on Tuesday. Milk collected on Thursday PM is now well on its way to a lovely Tomme, right on schedule with pH of 5.3 at 4 hours of pressing.

Still have an unresolved question of why the starting pH is so low at 6.5 to 6.55 for this lovely cow...

[Kern]

I lower amount may not yield a weaker curd.  I stated that I was unsure of the effect of a lower amount of rennet on curd strength.  The certain think is that a lower amount will lengthen the floc time.