66 may be hot enough to kill lacto - I'm not sure but 77 will definitely be*.
*some lacto strains may be tolerant to even these temps - not sure if these strains are present in grain or not. Ultimately if you end up with soured wort, you'll know why.
To work out the heat death for an organism you need to know the D value at a standard temperature (time taken for one log cycle, eg population to reduce to 1/10 initial value, normally at 60 oC for organisms of interest) and the z value (temperature increase required to reduce D value by a factor of 10).
is used as the standard model for spoilage due to thermophilic LABs; as its name implies it's very heat tolerant. Highest D60 value I can find is 8.3 minutes, accompanied by a z value of 6.2oC . Highest z value I can find is 12 oC, accompanied by a D60 of 2.2 minutes.
Taking a worst case scenario and combining D60 = 8.3 minutes and z = 12 oC to get D66 = 2.5 minutes (8.3 x (log (6 oC / 12 oC)), it would take 15 minutes to reach 6 log death, the accepted value for functional sterility.
In case you were wondering, both D and z values are pH dependent but the figures I quoted are at pH 7, at mash pH they will both be much smaller so death will be quicker.
BTW last week I mashed for 8 hours at 60 - 66 oC followed by 2 hours at 68 oC * There was no lactic influence that I could discern.
* It was meant to rise continuously from 62 to 70 over ten hours but the heater turned off while I was asleep.