Determination of ice crystal damage of high pressure frozen foods by light microscopy and image analysis

Two different methods were developed for studying ice crystal damage. Wheat starch gels and wheat dough were high pressure frozen and conventionally frozen in plastic, cylindrical tubes (inner diameter 15 mm). Cryosections (diameter the same as the plastic tube) with a thickness of 15 µm were cut from two different positions of the frozen samples. Sections were stained with iodine and examined in brightfield. Image analysis was used to determine the amount and area of pores. Total analysed area was about 85 mm2/ section. The damage caused by freezing was very much dependent on freezing method. Pressure-shift freezing at 200 MPa caused less damage than conventional freezing. Another method based on embedding was developed for frozen and thawed potato and carrot samples. Freezing caused separation of the cells in the studied vegetables. This was also demonstrated by confocal laser scanning microscopy. The area of the spaces between the cells was determined by image analysis.