NITRATE, NITRITE AND UREA LEVELS IN SOME FRESH, COOKED AND REFRIGERATOR STORED VEGETABLES: CHEMICAL AND MICROBIOLOGICAL STUDIES

Nitrate, nitrite and urea were determined in 11 fresh vegetables (eggplant, white
potatoes, squash, okra, spinach, green French bean, Jew's mallow, yellow carrot,
onion, gariic and tomato) collected from five Alexandrian markets. The highest nitrate
level was recorded in spinach and squash, and the lowest was that of tomato. The
highest nitrite content was recorded in Jew's mallow and okra while the lowest nitrite
value was recorded in tomato. Gariic was found to contain the highest urea level while
spinach showed the lowest. Cooking significantly reduced nitrate levels in the studied
vegetables. The overall mean nitrate content showed a n.66 % deCrease on cooking.
The mean nitrite content of the studied items showed increases on cooking of
eggplant, white potatoes and squash. While okra, spinach and Jew's mallow nitrite
levels decreased with cooking. The mean urea level of six of the studied vegetables
showed increases after cooking. The highest rise in urea took place in eggplant
(250.5%) while the lowest increase was recorded in squash (12.8%). Jew's mallow
showed 16.3% reduction in the mean urea level after cooking. Nitrate level
Significantly increased after two days of storage. Then a significant drop off occurred
after 4 and 6 days of storage. Nitrite concentration increased in cooked vegetables on
cold storage. It was also recognized that there was a significant variation in the effect
of refrigerator storage on different vegetables under investigation. Cooked Jew's
Mallow and white potatoes showed the highest Increases in concentrations of nitrite
(622 % and 556 %) after six days of refrigerator storage. On the other hand, cooked
squash showed the least rise in nitrite contents (129.6%) after the same period of
storage under the same conditions. Urea level decreased through storage period.
This reduction in urea level was directly proportional to the storage time. The total
bacterial counts of the studied items showed a highly significant increase with storage
under refrigerator temperature. Cooked white potato and Jew's Mallow were highly
susceptible to contain higher microbial counts during storage at refrigerator
temperature than other kinds of cooked vegetables. A highly significant decrease was
recorded in spore counts of the studied items with storage under refrigerator
temperature. Coliform bacteria counts increased significantly with the storage duration
in Coliform- positive samples, okra, spinach and jew' s mallow. The other studied
types were negative for Coliform. All cooked items were positive for Staphyfococcus
aureuos and Citrobacrer sp < /em>. and negative for. Salmonella sp < /em>. along the storage
period. Shigella sp < /em>. was detected in cooked eggplant, French bean and vegetable
soup, while others were negative for it. Enterobacter sp < /em>. was detected in eggplant
and vegetable soup. A signifteant negative correlation between nitrite and urea levels
and a highly significant positive correlation between nitrite and bacterial total counts
were found in cooked vegetables throughout the experiment. Nitrite also showed a
highly Significant positive correlation with Coliform count. Urea levels showed
inSignificant positive correlations with nitrate and nitrite and insignificant negative ones
with bacterial, spores and Coliform counts during the storage period of the studied
cooked vegetables. These findings indicated that storage of cooked vegetables in the refrigerator did not sufficiently inhibit bacterial growth in the studied foods. Certain species of
food-born bacteria assimilate proteins and nitrate of the cooked stuffs for the ultimate synthesis of their own proteins resulting in increasing nitrite level. Such Increase may represent a health hazard for the consumer.