Growth kinetics of E.coli on media containing lactose and casein

Question

I did this experiment last month using E.coli isolated from raw milk and have following data via uv-spec:

time    OD 66o nm
0  -    0;
2  -    -0.091;
4  -    -0.009;
6  -    0.08;
8  -    0.215;
10 -    0.47;
12 -    0.735;
14 -    0.93;
16 -    1.29;
18 -    1.29;
20 -    1.29;
22 -    0.96;
24 -    0.88;

The control and the growth media were Luria broth with the addition of casein and lactose 2% w/v. Data triplicated; average used.

I would like to know the reasons about this phenomenon. My inferences were:

1. Exhaustion of the casein and lactose by bacteria during lag phase.
2. Instrumental and procedural errors.

I was planning an enzyme activity assay, but stopped, thinking that the readings would be a reflection of procedural errors.