The practice of acidifying the tomato (Lycopersicum esculentum) by additives in canning was investigated from the standpoint of adjusting and maintaining a pH desirable to prevent microbial growth without adversely effecting quality. Approximately one-third or more of tomato acids were in their salt form and, consequently, the most effective acid groups are neutral. In 75°F storage for 52 weeks the pH of non-acidified tomatoes packed in #303x406 cans rose 0.1 units. This rise was believed to be due to corrosion points through pinholes in the enamel films of all the cans by the reaction: H⁺ + Sn⁰ → H⁰ + Sn⁺. Ten tomato varieties from four geographical regions were canned and evaluated for effects of additives on pH and composition. No distinguishing characteristics were noted in packs from the different regions. Salts with citric and malic acids added at less than 1.0 meq/100 g tomato equally depressed the pH by about 0.2 units. Fumaric and phosphoric acids were more effective. None of the acids used as acidulants showed any significant effects on drained weight, refractive index, can vacuum, can corrosion, or chemical composition. The effects of neutral salts for adjusting canned tomato pH were evaluated. Calcium and magnesium chloride depressed the pH by 0.06 units for each meq added up to two meq/100 g tomato. These salts were approximately seven times more effective than sodium chloride for depressing pH. Calcium chloride was more effective than magnesium, sodium, or potassium chlorides in preventing drained weight loss when up to 2.0 meq of each of these salts were added per 100 g tomato (2 meq CaCl₂ = 0.11% CaCl₂). Evidence shown by alcohol soluble acidity indicates that sodium ions were removed from added sodium chloride into the alcohol insoluble fraction. Processing times of 15 and 45 minutes at 210℉ were compared as to their effect on tomato compositional changes. Heat penetration curves 0 showed that internal fruit temperature reached 150℉ in the 15 minute 0 heat process, while over 190℉ was reached inside tomatoes processed 45 minutes. Five tomato varieties showed no significant pH change related to length of heat process. Titratable acidity was slightly higher by 0.1 meq/100 g in tomato samples processed 45 minutes. Other variables for the sample sets of the two processing times were compared by Duncan's multiple range tests and by partial correlations among them. Variables also included alcohol soluble and insoluble acidities, water insoluble solids, reducing sugars, amino nitrogen, phosphoric acid, refractive index, and lactic, oxalic, fumaric, succinic, malic, citric and pyrrolidone carboxylic (PCA) acid determined by gas chromatography. Although differences between the two processes were small, correlations among variables were higher in the sample set processed for 45 minutes. These relationships imply that thermal, non-enzymatic effects were slightly more pronounced in the set processed for 45 minutes. The rate of formation of PCA appears to be higher during the early stages of heat processing. Small within-can variations were found to exist. Centrifuged tomato solids were 0.028 pH lower than that of the serum. / Ph. D.