Effect of growing conditions on the corrosivity and ascorbic acid retention in canned tomato juice

Tomatoes (Lycopersicon esculentum) (M82-1-8) grown hydroponically in a greenhouse experiment at 16 combinations of nitrate/chloride nutrient solutions were processed as juice and the canned product was stored at 22 degrees C and 36 degrees C for 27 months to study corrosion of the packs from the point of view of detinning or delamination of the internal lacquer and comparative retention of ascorbic acid content. Detinning in plain tinplate cans was greater with juices from tomatoes grown in high nitrate, low chloride solution as a result of the high nitrate content of tomatoes. Addition of chloride decreased the nitrate uptake of the tomatoes, thereby reducing detinning in plain cans significantly. A relevant positive correlation between detinning and high nitrate, high chloride nutrient regime after 27 months' storage was attributed to an increase in total acid (as anhydrous citric acid) content of the respective juices and hence increased corrosivity. Delamination, with underfilm corrosion, in lacquered cans was correlated with tomatoes grown under a higher nitrate, medium chloride regime, giving medium levels of nitrate (0.30 0.65 mmol litre-1) and chloride (20-25 mmol litre-1) but higher levels of acidity (24-37 mmol litre-1 citric acid) in the juice. Tomatoes grown under a higher nitrate, low chloride regime exhibited nitrate concentrations as high as 1.0 mmol litre-1 and high corrosivity. Tomatoes grown under low nitrate and increasing levels of chloride regimes gave juices containing as low as 0.18 mmol litre-1 of nitrate but higher chloride concentrations up to 42 mmol-litre-1; no delamination was observed in lacquered cans of these juices but some pitting corrosion and perforations occurred at the end of 27 months at 36 degrees C. Ascorbic acid retention was mainly correlated with the type of can as well as with the initial ascorbic acid level of the raw tomatoes, itself the result of the interaction of Cl and NO3- in the nutrient solution. During 27 months' storage, the ascorbic acid retention was consistently higher in tomato juice canned in plain tinplate cans (88% at 22 degrees C or 70% at 36 degrees C) than in lacquered cans (24% at 22 degrees C or 18% at 36 degrees C).