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Metals in Dairy Equipment. Metallic Corrosion in Milk Products and its Effect on Flavor

Blue Valley Creamery Company, Research Laboratories, Chicago, Illinois

ABSTRACT

This experiment involves a study of the various metals used in dairy equipment. The action on the metals of weak organic and mineral acids and of sweet and sour milks and creams, and the effect of the metals on the flavor of the milk products were observed.

The metals used included aluminum and aluminum alloys, copper, nickel and copper alloys, iron, zinc, and galvanized iron, tin, tinned copper and tinned iron, two ordinary chromium steels and one chromium-nickel steel. The liquids used consisted of 1 per cent solutions of the following acids: hydrochloric, sulphuric, acetic, butyric, and lactic, and 0.2 per cent solutions of citric; and of the following milk products: sweet milk, acidophilus milk, butter culture, and buttermilk, sweet and sour cream, and sour neutralized cream. The metal strips were immersed half way in the liquids and the periods and temperatures of exposure were five days at 70°F. and five hours at 145°F.

In general, the corrosive effect on the metal was greater in the acid solutions than in the milk products. This was evident even where the acidity of the milk product was equal to or greater than that of the acid solution. This suggests that the acidity is the chief factor in metallic corrosion and that some of the non-acid milk constituents apparently exert a corrosion-protective action.

With minor exceptions, the corrosion was more intense in the high-acid products than in the low acid products and at high temperatures than at room temperature. But even in the sweet milk products and at room temperature corrosion was by no means absent.

Generally those metals that showed definite corrosion also had the most damaging effect on the flavor of the milk product. This refers particularly to iron, galvanized iron, copper, zinc, tinned iron, and nickel silver, the first three of which produced a marked metallic flavor in all milk products and the last three in the majority of cases.

Allegheny Metal, tin, and properly tinned copper produced no damaging flavor effect whatever. These were closely followed by the aluminum products, nickel, Monel Metal, and the ordinary chromium steels. The latter group of metals, however, were not entirely negative and did produce a metallic flavor in some instances.

The accurate placement of these metals on the basis of the results here recorded according to their suitability for use in dairy equipment is difficult. Some of the metals, while harmless in dairy products of low or moderate acidity, showed a marked tendency to impair the flavor in the high-acid products. Others, while practically harmless in most of the dairy products showed lack of resistance to corrosion under certain severe conditions and, therefore, are not sufficiently dependable to be recommended for general use.

While the effect on the flavor of the milk product must ever be considered of paramount importance, a consistent classification of these metals must, in addition, take into consideration their dependability to stand up under a great variety of conditions, such as are encountered in commercial plant operation, which includes also resistance to steam, washing solutions, and cooling brines, etc. With these numerous aspects in mind the following alignment according to relative merit appears justifiable:

1. Allegheny Metal, tin, and heavily tinned copper.
   These metals have no effect on flavor and show maximum resistance to corrosion and tarnishing. The tin coating on copper must be heavy and intact.
   
2. Nickel, aluminum, and manganese aluminum alloy.
   These metals are not entirely satisfactory in high acid milk products. Furthermore, the nickel tarnishes readily and the aluminum is sensitive to alkali washing powders.
   
3. Monel Metal, Enduro, Ascoloy, and nickel silver.
   Monel Metal tarnishes appreciably in milk products. In these tests it had only slight effect on flavor. Enduro and Ascoloy are sensitive to high-acid products and their performance in dairy equipment is uncertain. Nickel silver tarnishes severely and was injurious to flavor in the majority of milk products.
   
4. Tinned iron, copper, galvanized iron, iron, and zinc.
   With the exception of properly tinned iron, this group is unfit for use in contact with milk products. Tinned iron is equally so whenever iron is exposed.
   

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