Food comes in contact with metals in various stages of its processing. In order to prevent any transfer of harmful elements from metals to food, it is imperative to understand the type of interaction between the two and also select the suitable metal for handling depending on the food process. Numerous councils around the world study impact of various metals and alloys used in different stages of food processing. Measurable amounts of metallic elements in the alloy may be released into foodstuffs from food preparation and cooking, leading to human ingestion. In order to curb the impact of these elements, The Council of Europe has laid down permissible Specific Release Limit (SRLs).
A “specific release limit” (SRL) describes the maximum permitted the amount of a given metal ion or metalloid ion (in mg) when released from a material or article of a defined surface area into food (in kg) or food simulants.
Metals and alloys used in food contact materials and articles shall comply with the specific release limits (SRLs). SRL limits of most of the alloying elements are mentioned in the list. For example SRL limit for Nickel is 0.14 mg/Kg of food, for Chromium it is 0.250 mg/Kg.
Stainless Steel in Food Industry
Stainless steels are widely used in food industry due to their inherent resistance to corrosion. It can be formed into complex shapes. Stainless steel can be cleaned and sterilised without deterioration. It also does not impart any colour and flavour to food, making it ideal for the food industry.
The majority of the stainless steel used in food applications contain chromium in the range of 16 – 18 % as it is found to have optimum corrosion resistance in wide range of food and beverage media. Austenitic grades are used in manufacturing of cutlery, hollow-ware and kitchen utensils. Other applications include brewing vessels, milk tankers. High alloy austenitic grades are used for storage, transportation of corrosive food and beverage products. Grades containing Molybdenum are resistant particularly to salt containing food. 316L/1.4404 is referred to as “food grade” due to its diverse use in food industry. Similarly 304L/1.4307 is also a very popular grade for use in Food industry applications.
The specific characteristic of ferritic stainless steel is that it is magnetic. It is used in household appliances manufacturing (dishwashers, refrigerators, ovens). The grade 430/1.4016 offers sufficient corrosion resistance for a few applications where high corrosion resistance is not demanded. Another advantage of this grade is its affordability. The price of 1.4016 is significantly less than austenitic grades, thus making it a cost friendly choice. They are particularly good in highly oxidising environments (e.g. nitric acid) and offer excellent resistance to Stress Corrosion Cracking. However poor formability and weld ability limits the use.
Martensitic grades have high levels of carbon. This type of stainless steel is hardenable by heat treatment. The grade is used in applications where high wear resistance is desired. 420/1.4021 is a popular grade for the manufacturing of knives blades. For superior quality knives which retain the sharpness even after prolonged use, the grade 1.4116 can be used.
Duplex grades are preferred for handling corrosive foodstuffs as they offer high resistance to corrosion caused by saline solutions at high temperatures. Duplex grades offer superior Stress Corrosion Cracking, Crevice and Pitting resistance than austenitic grades. Applications include custard and vinegar making, manufacturing of sauces, cheese dairies and fish-canning plants.
Selection of metals and alloys used in food and beverage industry is a very important decision as it involves huge costs in terms of procuring and set-up. Right grade of stainless prolongs useful life of the equipment thus reducing the overall life cycle cost. There are over 150+ grades of stainless steel at offer. Each grade has its own characteristics that differentiates it from others. Decision for a particular grade should be based on application and suitability of grade for the particular environment.