10 CHANGES DURING THE SPOILAGE OF TOMATOES. 



of sulphuric acid, filter, wash the precipitate with a small amount of water, and 

 evaporate the filtrate on the steam bath to a convenient volume for extraction in a con- 

 tinuous liquid extractor. (In this laboratory this volume is 100 cc.) Extract for from 

 18 to 20 hours in a liquid extractor with washed ether. In case the quantity of lactic 

 acid present is greater than 0.5 gram it is usually necessary to extract for a longer period. 

 In any case it is best to reextract for from 8 to 10 hours to make sure that the extrac- 

 tion is complete. (Ether sufficiently pure for this purpose may be prepared by shak- 

 ing out ordinary ether once with a sodium hydrate solution and then 10 times with 

 small quantities of water.) The ether is just evaporated on the steam bath, and the 

 residue at once taken up in water and filtered. This procedure is made to get rid o^ 

 a small amount of coloring matter and substances other than lactic acid which may be 

 extracted from ketchup by ether but which are insoluble in water. The nitrate is 

 heated on the steam bath for some time to remove all traces of ether or alcohol. 

 Approximately 3 grams of sodium hydrate are then added to the water solution and 50 

 cc of a 1.5 per cent solution of potassium permanganate are added from a pipette. 

 This mixture is heated on a water bath at 100 C. for one-half hour. At the end of 

 that time, or before, if the color is not a decided blue-black or purple, but is green 

 or colorless above the layer of brown precipitate, more standard permanganate in meas- 

 ured portions is added until, after heating one-half hour on a boiling water bath, the 

 color is a blue-black or purple. The oxidation is then complete. The hot solution is 

 strongly acidified with dilute sulphuric acid, about 50 cc of 10 per cent sulphuric acid, 

 and standard oxalic acid run in from a burette until the solution is decolorized. (In 

 this laboratory a 5 per cent solution of oxalic acid is used for this purpose.) Any 

 slight excess of oxalic acid is titrated back with the same standard permanganate 

 solution. It should be understood that any standard permanganate and oxalic acid 

 solution may be used within reasonable limits of strength. In alkaline solution the 

 permanganate oxidizes the lactic acid quantitatively to oxalic acid according to the 

 equation: 



2C 3 H 6 O 3 +10KMn0 4 =2(COOH) 2 +4H 2 0+2C0 2 +5MnO 2 +5K 2 Mn0 4 . 



Then in acid solution, the oxalic acid is further oxidized by the permanganate to 

 carbon dioxid and water according to the equation: 



5(COOH) 2 +2KMnO 4 +3H 2 SO 4 =10C0 2 +8H 2 O+K 2 S0 4 +2MnSO 4 . 



Calculation: The total weight of permanganate used in the oxidation of the lactic 

 acid is determined by subtracting the permanganate equivalent of the oxalic acid 

 used from the total amount used. The weight of permanganate times 0.237 equals 

 the weight of lactic acid. 



That the substance under oxidation in the proposed method is 

 actually lactic acid was proved in several spoiled ketchups and in 

 spoiled tomatoes by preparing the zinc salt, which has a rather char- 

 acteristic form. Zinc was determined in several of these salts and the 

 percentage present found to agree with that calculated for zinc 

 lactate. 



A large number of experiments were made on the determination 

 of lactic acid which seem to be important in this connection. Sev- 

 eral experiments were carried out with solutions containing from 0.1 

 to 1 per cent lactic acid. In such solutions lead or barium acetates 

 give no precipitate either in aqueous solutions or in 80 per cent 

 alcohol. Stannous chlorid gives no precipitate in dilute lactic acid 

 or sodium lactate solutions. The zinc lactate is not easily or 

 completely precipitated from weak lactic acid solutions (0.1 to 0.2 



