Built to Endure

Several desirable properties of stainless steel (SS) make it a material of choice for the preparation, manufacture, storage, presentation, and transportation of products in the food and beverage industry [1].

These very characteristics are also why Supercritical Fluid Extraction (SCFE) equipment is made from SS [2]. The SCFE process isolates required molecules from edible / agro raw materials for the cannabis, tea-coffee, medicinal herbs, spices, beer and blended alcohol, aromatherapy and other industries.

Now, the FDA does not publish a list of permitted materials. It only mentions what metals are permissible and how to utilize them. These guidelines require the metals to withstand corrosion, scratches, pitting, scoring, crazing, chipping, and repeated washing with strong chemicals, while also being strong, easily cleanable, smooth, impervious, and durable [3].

Similarly, food-industry Good Manufacturing Practices (GMPs) lay down principles for the maintenance and cleanliness of the utilized utensils and equipment [4]. SS is best suited to comply with all these guidelines.

Chief among its coveted features is SS’ inert or non-reactive nature which prohibits it from reacting with foods and beverages. The same characteristic also normally shields it from corrosion [5]. Importantly, SS remains corrosion-resistant despite repeated sanitization by strong chemicals [6].

AISI 304, 316, and 316L are most popularly utilized by the food-beverage industry. Use of duplex stainless steels viz. AISI 2304 and 2205, as well as high-chromium AISI 430 is restricted to specific applications only [1].

Be as it may, good material makes good equipment only with excellent workmanship, proper maintenance, and the provision of correct operational conditions. In the absence of these, even SS loses its nerves of steel and gets exposed to various forms of corrosion.

Why Food-Beverage Industry Prefers Stainless Steel?

SS contains minimum 10.5% chromium by weight [5]. It is this chromium that forms a passive, self-repairing, and continuous surface layer and makes SS non-reactive and non-corrosive. Such inert surfaces do not react with food products. These are the two major reasons for the food industry’s preference for SS.

However, if the local chromium content falls below 12%, the passive layer cannot repair itself [5]. The design, construction, operation, maintenance, and cleaning of SS equipment / parts is geared towards avoiding such perilous fall of chromium concentration.

Materials employed for making food contact equipment and parts must be [7]:

  • Inert: Not contaminate the food products it is in contact with.
  • Hygienic: Simple enough to decontaminate and clean, immune to fouling, and smooth.
  • Mechanically Stable & Physically Durable: Hard, strong, and tough while not being vulnerable to abrasion, cracks, wear-tear, and impact.
  • Chemical Resistant: Not react with the food, disinfectants, and cleansing agents that it normally comes in contact with.
  • Compatible with Other Materials & Functional Substances: Such as refrigerants, lubricants, and other materials in the assembly.
  • Simple to Join: Via welding or other continuous bonding processes.
  • Easy to Fabricate: In particular shapes and sizes.
  • Resistant to Temperature: Must not deform at high or low ends of the operational temperature.

Popular Stainless Steel Variants in the Food-Beverage Industry

AISI 304 (EN 1.4301) and AISI 316 / 316 L (EN 1.4401 / 1.4404) are most popular materials in the food industry. Duplex stainless steels viz. AISI 2304 (EN 1.4362) and AISI 2205 (EN 1.4462) as well as AISI 430 (EN 1.4016) are utilized for certain limited applications only [1].

EN number is the European system of designation while AISI stands for American Iron and Steel Institute (AISI). SUS is the Japanese Industrial Standard (JIS) nomenclature and UNS is the Unified Numbering System.

Various designations for stainless steels in the food industry are [8] [9]:

 

AISI EN UNS JIS
304 1.4301 S 30400 SUS 304
316 1.4401 S 31600 SUS 316
316L 1.4404 S 31603 SUS 316 L
2304 1.4362 S 32304
2205 1.4462 S 32205
430 1.4016 S 43000 SUS 430

Table 1. Designations for Stainless Steel in the Food Industry

 

  • AISI 304: Exhibits great corrosion resistance and is, as a result, compatible with a majority of food materials. It contains 16-24% chromium and other elements such as carbon, nickel, and manganese. Chlorides can however eliminate the passivized layer and erode its surface via pitting [10].
  • AISI 316 / 316L: Overcomes the chloride-related limitation of AISI 304 through the inclusion of 2-3% molybdenum. That apart, its composition is similar to that of AISI 304. Resistance to chlorides makes it a better material to handling meat products and somewhat salty foods [10].
  • AISI 2304 & 2205 Duplex Stainless Steels: Make a good material when processing salty food that is at high temperature and moving slowly [10].
  • AISI 430 with 17% Chromium: Is good for building housings, splash backs, and enclosures where corrosion resistance is less important [1].

Design, Installation & Maintenance Conditions

Choosing the right material takes care of one requirement only. Others aspects are equally important considering that SS is prone to [1]:

  • Stress Corrosion Cracking (SCC): Produces localized cracks in areas exposed to stress, chloride ambience, and over 500C temperatures.
  • Pitting & Crevice Corrosion: Results from a combination of chloride solutions and higher temperatures. Pitting produces tiny and deep surface depressions. Crevices are narrow and allow accumulation of solids and liquids.
  • Intergranular Corrosion: Occurs in the narrow zone around the weld.

As a result, SS equipment and parts in the food-beverage industry must adhere to the following guidelines during design, installation, cleaning, and maintenance:

  • Design Smooth Surfaces free of cracks, dents, scratches, and crevices which do not allow food-beverage residues to pile up and serve as the breeding ground for bacteria and microbes [10]. These surfaces also prevent cleaning solutions from accumulating and contaminating food-beverage material.

Such surfaces deny a hiding spot for microbes and residues from the cleaning and disinfection action of scrubbers and chemicals. Moreover, cleaning smooth surfaces is simpler [10].

 

  • Avoid Sharp Corners and Hard Bends that allow residues to accumulate and expose the part / equipment to stress corrosion cracking [10].

 

  • Minimize / Eliminate Contact between SS and Non-SS Parts which leads to galvanic and spot corrosion [10]. Galvanic corrosion results from the physical connection between two dissimilar metals or alloys through a conducting liquid. The more active metal corrodes [11].

 

Machining operations on carbon steel and iron releases their particles. If these accumulate on nearby SS parts, they destroy the passive chromium layer causing localized spot corrosion [12]. Remember, the passive layer on SS ceases to be self-repairing if the local chromium concentration falls below 12% [5].

 

For the same reason, nylon brushes make better cleaning tools than abrasive, steel-wool scrubbing pads [5]. The latter also leave behind scratches, ruining the smooth surface finish.

 

  • Refrain from Using Chloride and Acid Cleaners both of which damage the all-important surface chromium layer [5]. Mild detergents offer less destructive cleaning [5]. Ammonium and iodine solutions as well as nitric acid are similarly less damaging to the passive layer than hypochlorites, bleach for example [10].

 

Typical cleaning techniques for SS are [1]:

  • Water-Steam
  • Alkaline Solutions
  • Mechanical Scrubbing
  • Organic Solvents
  • Nitric Acid
  • Scouring Detergents-Powders

 

  • Appropriate Heat Treatment particularly along welded joints where chromium combines with carbon to form carbides, leaving the zone chromium deficient. The passive surface layer loses continuity and is open to corrosion [5].

Finally

By its very nature, stainless steel is compatible for use in the food-beverage industry. With proper design, construction, operation, and maintenance, it becomes the best available material for the industry.

Buffalo Extraction Systems (BES) teams up with reliable partners in the Carbon dioxide (CO2) Supercritical Fluid Extraction (SCFE) arena and offers turnkey solutions for extraction of Cannabis, Spices, Flavors and Fragrances, Essential Oils, Aromatherapy, Herbal Medicines, Algae, Phytochemicals, and Neutraceuticals.

Contact Buffalo Extraction Systems at info@buffaloextracts.com for state-of-the-art CO2 extraction technology.

 

 

 


 

References

  1. British Stainless Steel Association – Selection Of Stainless Steels For The Food Processing Industries
  2. Analytica Chimica Acta – Volume 417, Issue 2, 24 July 2000
  3. The Best Food-Safe Metals for Food Manufacturing Applications – Marlin Steel, November 19, 2019
  4. HACCP Mentor – GMP in the Food industry
  5. Reliance Foundry – Cleaning Stainless Steel
  6. Team Stainless – Disinfection Of Stainless Steel In Hospitals
  7. Journal of Hygienic Engineering and Design – Materials Of Construction For Food Processing Equipment And Services: Requirements, Strengths And Weaknesses
  8. Columbus Stainless [Pty] Ltd – Stainless Steel Products
  9. Weisz – Austenitic Stainless Steel
  10. net – Stainless Steel in Food Manufacturing: Grade Selection and Care
  11. WebCorr Corrosion Consulting Services – Different Types of Corrosion
  12. Penn Stainless Products – Corrosion Prevention for Stainless Steel