Module 8. Milk and metals
Lesson 32

32.1 Introduction
Action of Milk on certain metals would dissolve small amount of the metal and form metallic salts which would give rise to a ‘metallic’ taste to the milk. Some of the salts in milk have a catalytic action on the oxidation of milk fat resulting in the development of an oxidized flavour. These metals are said to taint milk.
32.2 Factors Influencing the Action of Milk on Metals
The factors which influence the degree of action by milk on the metal are:
  • Temperature of milk.
  • Period of contact.
  • Cleanliness and polish of metal.
  • Amount of free air in milk.
  • Acidity of milk etc.
32.3 Selection of Metals for Dairy Equipment
During the processing of milk the metallic surface will definitely come in contact milk as such the interaction between the two cannot be avoided. As such it is necessary to take in to consideration this aspect while choosing the metals for designing various equipments with which milk will be in direct contact. The following aspects need to be considered in such situations.
  • Non-tainting
  • Non-toxic
  • Insoluble (in milk or its products)
  • Highly resistant to corrosion (by milk, cleaning and sanitizing agents, etc.)
  • Easy to clean and keep bright
  • Light yet strong
  • Good agents of heat transfer
  • Good in appearance throughout its use
  • Low In cost
  • Non-absorbent
  • Durable
Since it is not possible to get any single metal which can meet the entire above requirement,alloys of different metals like stainless steel is recommended for use in the dairy industry. The most satisfactory combination of metal that could be used is 18:8 stainless steel and aluminum alloy presently.
32.4 Characteristics of Different Metals and Their Alloys Used for Dairy Equipment
32.4.1 Copper and its alloys
This metal is mostly used for the preparation of Milk pails, coolers, vats, strainers,pipe fittings, milk pumps, pasteurizer coils, etc. (used for tinning only). Due to the action of milk it will cause taints in the milk or the dairy products. For preventing such effect the surface is coated with tin. Due to the action of milk on this metal a Green corrosion product is formed which is toxic for human consumption. The greatest advantage of this metal is that it is a Soft metal and it is easy to work with this metal. In order to increase the usefulness of this metal it could be used by having tin coating on the surface and this makes the metal more durable. Retinning of the metal is cheap and it will increase the makes it reusable for a considerable period. It high conductivity promotes rapid heat transfer and its cost is also reasonable
32.4.2 Aluminum and its alloys
This metal and its alloys are often use in the preparation of milk cans, milk pails, linings for tanks and tankers, etc. Aluminum will not taint the milk and being a Soft metal it is very easy to work with this metal. If the metal is impure its durability is reduced. However if aluminum alloyed or anodized it is much more durable. The greatest advantage of this metal is its light weight. The disadvantage of this metal is difficulty in cleaning due to its porosity, corrosion when alkaline dairy detergents and sanitizers are used. It is difficult to solder this metal.
32.4.3 Low-carbon steel
This low carbon steel is widely used for bodies of tanks, vats, bottle-washers, conveyors, etc.
32.4.4 Stainless steel (18:8)
This alloy of iron is most widely used for all milk/ dairy product contact surfaces. The pure iron taints milk/dairy product while stainless steel does not taint. The ‘Rust’ caused to this metal in its pure state is slightly toxic. As this is a tough metal it will presents problems in fabrication.
  • Stainless steel is highly durable in the preparation of various dairy equipment.
  • The advantages in the use of stainless steel are highly resistant to corrosion by common acids and alkalis
  • Takes high polish and therefore easy to clean
  • Corroded by brine and chlorine solution
  • Welding has to be done to repair cracks etc
32.4.5 Tin
This metal is used mainly as a ’coating’ for milk/dairy product contact surfaces of cans, vats, etc. It will not taint milk although quite soluble in it. This metal is too soft to be used for any kind of equipment. Tin coating is not durable as it wears off readily by corrosion, abrasion, etc. However re-tinning process not at all difficult
32.4.6 Nickel and its alloys
This metal is used as a coating for milk/dairy product contact surfaces of pasteurizing vats coolers etc. Ni-alloy used in freezing chamber of ice-cream freezers, cylinders and plungers of homogenizers, etc. It has very slight effect on milk flavour although the most soluble in milk among dairy metals. It is mildly toxic. An alloy with iron makes it very tough and is quite difficult to handle during fabrication. However this metal is much more durable than tin coating, but it is more expensive. Lactic acid causes corrosion of this metal however it is not corroded by the alkaline washing powders. The highest disadvantage in the use of this metal is that it is more costly when compared with the chromium and tin
32.4.7 Chromium and its alloys
This metal is more useful for the coating on various types of equipment. It is mostly used for the milk/dairy product contact surfaces. This metal is non tainting and is resistant to corrosive action by acid and alkaline cleaners. The most important characteristic of this metal is that it is very expensive
32.5 Corrosion Control
From the practical point of view corrosion cannot be entirely prevented in dairy equipment; however its rate can be controlled to a large extent. The following measures will help in preventing corrosion of stainless steel surface:
  • Keep the surface clean
  • Permits surface to air dry, whenever possible.
  • Use cleaners and sanitizers in the lowest concentration and for the shortest duration that will do the desired cleaning job.
It is the common observation in dairy industry that the stainless steel which is used extensively in the dairy industry also will form an invisible film of chromiumoxide forms on its surface when the stainless steel is dry and exposed to the atmosphere. This film protects the surface forms corrosion. But when the film breaks or wears away, and the active metal gets exposed and corrode more easily. As the chlorine and its compounds are very corrosive it is advisable that equipment should be sanitized with chlorine solutions, preferably just before it is to be used, so as to avoid pro-longed contacts, and thus corrosion (pitting).
Milk contact surface and Metallic contamination: it is an established fact that milk normally contains metallic elements like aluminum, copper, iron, manganese, silicon, zinc and others. Although concentration of these metals is very low in normal milk they may be added to milk during processing due to the action of milk with the contact surfaces on these metals.
Milk utensils in our villages are commonly made of earthenware, zinc plated or galvanized iron, brass or copper. The product contact surfaces and other portions of modern processing equipment are increasingly being made of stainless steel. In the normal milk copper is bound with the fat as such this metal is not active chemically. Milk when comes in contact with copper it will dissolve in milk and would exist in active form as copper ions. In the presence of direct sunlight and /or air the copper ions catalyze the hydrolysis of milk fat which an irreversible reaction resulting in off flavor development. This flavor defect is more in high fat products. Copper salts formed due to the reaction of itsions with milk slats or lactic acid are greenish in colour, quite bitter to taste and is toxic.
In normal milk iron is present in a chemically bound form. It is largely related to the fat globule membrane and the enzyme proteins. Similar to the copper, milk also dissolves iron when it comes in contact with it and will be forming free ions resulting in the development of off flavors and toxic compounds. Iron will also stain the product contact surface. A common practice prevailing when utensils, cans or buckets made of galvanized iron are to be used for handling milk and milk products, to coat the contact surface with zinc (Zinc plating) but upon long usage these vessels become defective due to wearing off the plating exposing the iron to be in contact with the milk directly. Special consideration is to be given to the reaction of these metals with milk when they are used for preparing the equipment and containers for handling milk and milk products. Development of off flavours, discoloration of the products, corrosion of the vessels, formation of toxic compounds, are some of the consequences when milk contact surface is made with these metals. Stainless steel is an alloys containing chromium, managanese silicon and nickel. The most satisfactory materials for the product surfaces is the stainless steel containing 18% chromium, 8% nickel in a low carbon steel. The processing equipment made with surfaces with this alloy are easily cleaned.
Last modified: Friday, 26 October 2012, 6:26 AM