I agree with TDOL that "one and the same" is the correct phrase, but it really doesn't fit here, unless there is supporting context in a previous paragraph. I'll leave it for now.
"Cow milk from one and then same region, obtained during the summertime, was processed at ultra high temperature in a newest technology plant, indirectly hetated, with tubular heat exchangers, at two different temperatures, keeping in both cases a 3,3 s holding time.
This part is fine.
Lactulose and acid-soluble beta-lactoglobulin contents were determined before and after each heat treatment. According to the statistical analysis of the acquired data, an increase in temperature of only 1 degree centigrade produces further beta-lactoglobulin degradation and a higher lactulose content."