Pasteurisation of orange juice and Vitamin C
Hodgins AM, Mittal GS, Griffiths MW. 2002. Pasteurization of Fresh Orange Juice Using Low-Energy Pulsed Electrical Field. J Food Sci 67(6):2294-2299

This is a study of non-thermal food preservation, but it cites two studies that measured ascorbate loss from pasteurisation. I can't cite the studies directly because I can't find them online. According to these authors, they reported losses of 7.0-15.0% and 18.0% after 95°C for 15s. This is high compared to the pulsed electrical field method. 

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Yeom HW, Streaker CB, Zhang QW, Min DB. 2000. Effects of Pulsed Electric Fields on the Quality of Orange Juice and Comparison with Heat Pasteurization. J Agric Food Chem 48(10):4597-4605

This study compared ascorbate loss during storage after PEF or thermal treatment. While no difference was observed when stored at 22°C, PEF treated orange juice maintained significantly higher ascorbate concentration than thermally treated orange juice when stored at 4°C. The authors attribute this to the "higher processing temperature".

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They also say (and reference other studies):


  Ascorbic acid is a typically heat sensitive nutrient. High temperatures during processing and storage cause loss of ascorbic acid.