Chemical, physical and microbiological

Chemical, physical and microbiological studies have been carried out on three onion cultivars Giza 6, Giza 20 and Beheri which are widely cultivated locally. These included different pre-drying treatment (1.0% citric acid + 0.5% NaCL; 0.2% CaCL2; 1.0 % ethyl alcohol; 2.0% NaCL) and dehydration methods using freeze-drying, conventional drying and solar drying. Studies were also extended to investigate the effect of suitable packages for the dehydrated products through their storage for 9 months. Results indicated that, the moisture content of fresh onion was found to be 81.1, 82.8 and 84.9% for onion cvs. Giza 6, Giza 20 and Beheri, respectively, while it decreased to be only 1.45 to 1.92% by using freeze drying, 1.89 to 5.90% by using conventional drying and 2.70 to 5.71% by using solar drying. Fresh onion had higher amount of total pungency (10.5 and 13.3 umol pyruvate/g fresh onion), while it decreased as a result of volatilization during dehydration to be 5.2-9.8 umol pyruvate/g. Beheri onion samples contained the highest amount of total pungency as compared with Giza 6 or Giza 20.. The laminates of “PAPEP” were considered to be the best packaging material as compared with (PE) or (PP). The stored dried onion samples of all cultivars had less amounts of moisture at the end of 9 months storage. The dried samples packaged under vacuum had slightly higher amounts of sugars acid content than those packed under atmospheric pressure. The pungency of packed onion into PAPEP either under vacuum or under atmospheric pressure recorded the highest values throughout storage periods, however, it was also slightly decreased with the increasing of storage period. The estimated Hunter lab color differences (L,a and b) of fresh and the corresponding dried onion cultivars could reflect the variations between fresh and dried samples. The least color index (O.D) was found with freeze dried onion samples. Freeze dried onion samples showed the highest values of rehydration ratio followed by those dried by solar technique and conventional oven. The changes into Hunter lab color difference during storage of dried samples for 9 months in different packaging materials were slightly increased as the storage period extended. Beheri gave the darkest dried onion sample, hence it recorded the highest value of O.D (color index). Onion samples pretreated with 1.0% ethyl alcohol were lighter than the others. Results showed a slight decrease in re-hydration ratio of onion cultivars by increasing storage period which packaged into different packaging materials. Effect of dehydration techniques indicated that solar drying had the highest amounts of total count, followed by the conventional drying method and then freeze dried method. Onion samples treated with 2.0% table salt contained the lowest microbial load, followed by those treated with 1.0% ethyl alcohol and citric acid 1.0% + Nacl 0.5%, as compared with the control samples. The total count was nearly stable during 9 months storage in the different packaging materials.