Effect of processing conditions on some quality characteristics of vacuum-fried onion slices (Allium cepa L.) at various pre-frying treatments

Effects of pretreatments (blanching, air-drying, osmotic dehydration and freezing) and process parameters: frying temperature (100, 110 and 120°C) and frying time: (180, 300 and 420 s) at constant vacuum pressure of 70 mbar on some quality characteristics of vacuum fried onion chips (oil uptake, shrinkage, breaking force, and total color difference) were studied. The results showed that frying temperature and frying time at all pretreatments had a significant effect (p ≤ 0.05) on oil uptake and shrinkage, as well as total color difference and breaking force of vacuum fried onion chips. The results demonstrated that the oil uptake, shrinkage and total color difference of the fried onion chips increased, while breaking force decreased with increasing frying temperature and time. The results showed that the different pretreatments before vacuum frying influenced significantly (p ≤ 0.05) the quality characteristics of vacuum fried onion chips. At the same frying temperatures and frying times, the lowest oil uptakes were observed in osmotic dehydrated samples, followed by air-dried and then blanched samples. Freezing pretreated onion samples show the highest o oil uptake (p ≤ 0.05) than other pretreatments. Freezing pretreatment resulted in minimum shrinkage, compared to other pretreatments (p ≤ 0.05) followed by osmotic dehydrated samples. Shrinkage in air drying and blanching pre-treated samples did not differ significantly. Osmotic dehydration pretreated onion samples show less breaking force (p ≤ 0.05) than other pretreatments followed by air-dried pretreated onion samples while blanching and freezing pretreatment resulted in minimum crispiness. At the same temperatures and frying times, the lowest levels of total color difference were observed in blanched samples, followed by osmotic dehydrated and then the air-dried samples.  Freezing pretreated onion samples show the highest levels of total color difference (p ≤ 0.05) than other pretreatments.