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Copper in Dairy Products and its Solution in Milk Under Various Conditions

Department of Chemistry, Cornell University, Ithaca, New York

ABSTRACT

1. The unfavorable effect of the presence of copper and other metals in milk and its products has been thoroughly demonstrated by many previous investigators.
   
2. Since the equipment used in most dairy manufacturing operations is made of copper, it is important to know something of the conditions under which the copper dissolves in the milk and to learn, if possible, how it can be reduced to a minimum.
   
3. The presence of air and oxygen was here found to increase enormously the amount of copper dissolved by milk. Therefore, open pasteurizers, coolers, and holding tanks would be expected to yield much more copper than vacuum pans, and it would be much more important to have these of non-copper material.
   
4. Copper corroded with an oxide surface yields much more of the metal to the milk than smooth bright copper. It is particularly important to keep such equipment clean and well polished. Since the chlorine disinfectants such as chlorinated soda are usually active in corroding copper surfaces, their use should be avoided on copper hot wells and vacuum pans.
   
5. Carbon dioxide does not influence the dissolving power of the milk.
   
6. The amount of copper taken up by the milk increases with the amount of copper surface exposed and with the time of exposure.
   
7. About the same amount of copper is dissolved in the milk at room temperature as at boiling; at 145°F. considerably more copper is dissolved than at either. Therefore, in the manufacture of condensed and evaporated milk where copper hot wells are employed it is important to bring the temperature of the milk to boiling as rapidly as possible.
   
8. Inasmuch as milk once boiled dissolves about the same amount of copper at 145°F. as at boiling temperature, some transformation or loss of the agency which attacks the copper is indicated.
   At the present time no conclusions can be drawn as to the cause of this phenomenon. It has only been shown that replacing the air in boiled and cooled milk does not reëstablish the same solvent power exhibited at 145°F. in unboiled milk.
   
9. The presence of 18 per cent sugar does not increase more than a little the amount of copper dissolved by milk. The amount of copper dissolved from pans and hot wells in the manufacture of sweetened condensed milk is not, therefore, much greater than in other manufacturing operations.
   
10. Milk slightly sour dissolves but little more copper than normal milk.
    
11. On passing milk containing copper through a separator, the metal distributes itself between the cream and skim milk fractions approximately in proportion to the water. It would seem, therefore, that it is dissolved in the water. The results show conclusively that the copper does not go with the fat, and is, therefore, not dissolved in the fat.
    
12. Analyses of commercial samples of condensed and evaporated milk indicate that copper is taken up in considerable quantities in the manufacture of these products.
    
13. From theoretical considerations and from experimental results it was shown that copper may be lost from milk to tin when it comes in contact with this metal, and also that less copper is dissolved when the milk is exposed to surfaces of copper and tin together. Consequently, it would be expected that condensed and evaporated milk would lose some of its copper to the tin of the container and consequently a can purchased on the market would show somewhat less copper in solution than was present at the time of manufacture. Also a copper cooler or pasteurizer imperfectly tinned would yield less copper to solution than if the milk were exposed to the same surface of copper without the tin.
    On the basis of the same theoretical considerations the tin should dissolve in the milk much more rapidly when in contact with the copper, which would in part account for the rapid wearing of tin from such equipment. The latter point however, is not supported by any experimental evidence in this paper, no determinations for tin having been made in any samples.