The Care & Cleaning of Stainless Steels

It is of primary importance that all food and beverage contact surfaces be clean and hygienically sanitized initially and on a regular basis thereafter.

Fortunately, austenitic stainless steels, usually S30400 and S31600 or their low-carbon counterparts, are easily cleaned and decontaminated by standard techniques. These cleaning techniques and procedures are not intended for fabrication damage or soiling, but in-process requirements. Subjects such as heat tint and scale, tool marks and passivating treatments are reserved as a separate matter.

  1. Definition of Terms
  2. Basics of Cleaning
  3. Types of Cleaners and Their Functions
  4. Factors Affecting Cleaning Efficiency
  5. Cleaning Methods
  6. Sanitizing
  7. Factors Affecting the Efficiency of Sanitizers
  8. A Typical Sanitizing Method

1. Definition of Terms

The following are brief descriptions of standard cleaning and sanitizing terms:

Cleaning -- A process which removes soil and accumulated food and beverage residues. These substances may support the growth of disease-causing organisms or the production of toxins.

Sanitizing -- A process which destroys (kills) disease-causing organisms on food and beverage contact surfaces.

Soil and Dirt -- have been defined as "matter-out-of-place". For example, petroleum oils and greases on mechanical joints are lubricants, but the same substances on processing equipment contact surfaces is soil and dirt.

Water -- The primary constituent of food processing cleaners is water, but not just any water. It should be free of disease-causing organisms, toxic ions, objectionable odors and tastes and have a minimum amount of suspended matter.

2. Basics of Cleaning

The essential functions of cleaning are to bring the cleaning agent into intimate contact with the soil, to displace the soil from the surface, to disperse the soil in the cleaning solution and to prevent redepositing the soil back onto the surface.

The various functions performed in the cleaning activity with the aid of cleaning agents are as follows:

  1. Dispersion -- the action in which groups of particles are suspended in the cleaning solution.
  2. Dissolving -- certain types of soil become water soluble through detergent action. They are then washed away during subsequent rinsing steps.
  3. Emusification -- fats are broken up and suspended in the solution.
  4. Peptization -- formation of colloidal solutions from partially soluble materials.
  5. Saponification -- action of alkalis in cleaners resulting in water soluble soaps.
  6. Suspension -- through the action of surface active agents, soil particles are suspended in detergent solutions and removed during rinsing.
  7. Rinsability -- removal of soils and cleaning agents so that surfaces will drain dry and clean.

3. Types of Cleaners and Their Functions

Depending on the type of soil, there are various kinds of cleaning agents; although most of them fall into two rather broad categories: substances and degreasers.

Detergents are usually water soluble, alkaline substances, or soaps of proven cleansing properties.

Degreasers are commonly organic solvents with the ability to readily dissolve oils, greases and waxes. Each must dry readily without residue and not interfere with subsequent activities.

Within groups there are often synergistic effects which produce solutions that have improved cleaning or degreasing effects over individual substances.

Detergents and degreasers are seldom used together, but consecutively. Many of the substances are complicated proprietary mixtures of a number of chemicals, so care should be taken to obtain them from reputable suppliers.

4. Factors Affecting Cleaning Efficiency

A number of factors affect the efficiency of cleaning agents. Among these are the specific cleaning agent, its concentration, the temperature of the cleaning and rinsing solution, the velocity or force with which it is applied and the time the cleaning solution is in contact with the surface.
The Care and Cleaning of Stainless

5. Cleaning Methods

There are many methods for exposing equipment surfaces to cleaning solutions. Which method to be used, solely or in combination, usually depends on the effectiveness and economy of application. Some of these are as follows:

  1. Flushing with warm or cold water to remove loose particles.
  2. Soaking in the cleaning solution for 15-30 minutes, with or without ultrasonic scrubbing.
  3. Spraying the cleaning solution using fixed or portable spraying units.
  4. Using automatic clean-in-place (CIP) systems.
  5. Abrasive cleaning using abrasive powders and pastes to remove difficult soil.
  6. Rinsing with clean, potable water to remove all traces of the cleaning solution.

6. Sanitizing

Sanitizing is a distinct process separate from cleaning. While some sanitizing certainly occurs during cleaning, it usually is not effective in destroying all of the micro-organisms that cause disease and food spoilage. Therefore, these have to be dealt with using special substances that have requisite toxic properties to the organisms, but do not cause corrosion of the surfaces or are too toxic for use.

Common sanitizers fall into four main categories (these materials are all water soluble and are either surface-active agents or have them added to them). Following, are their characteristics and limitations:

1.    Inorganic Chlorine Products -- usually calcium or sodium hypochlorite.

        a)    Effective, if high enough concentration is used;
        b)    Organic matter may cause a reduction in bactericidal action;
        c)    Temperature and pH may influence bactericidal effectiveness;
        d)    Relatively unaffected by water hardness;
        e)    No film left on surface but may leave odor or taste if not rinsed completely;
        f)    Bactericidal activity good against a number of micro-organisms;
        g)    Can lead to corrosion of stainless steel if not rinsed completely; and
        h)    Minimum concentration required -- 50 ppm.

2.    Organic Chlorine Products -- Chloramine-T, di- or tri-chlorocyanuric acids.

        a)    Slower bactericidal action than hypoclorites;
        b)    Similar limitations and characteristics as hypochlorites;
        c)    Relatively non-irritating to the skin;
        d)    Relatively non-corrosive to stainless steel; and
        e)    Minimum concentration required -- 200 ppm.

3)    Iodophors -- Aqueous solutions of elemental iodine, sodium or potassium iodide and various organic substances.

        a)    Rapid bactericidal action in acid pH range in cold and/or hard water;
        b)    Less affected by organic matter than hypochlorites;
        c)    Nontoxic in ordinary concentrations;
        d)    Non-irritating to the skin;
        e)    Deepness of yellow or amber colour of solutions is proportional to concentration;
        f)    Non-staining;
        g)    Minimal taste and odor;
h) Relatively non-corrosive to stainless steel; and
i) Minimum concentration required -- 12.5 ppm.

4) Quaternary Ammonium Compounds -- Complicated di- or tri- methyl- or butyl- organic compounds containing ammonium chloride.

a) Degree of dilution usually determines germicidal action;
b) Very selective in destruction of various types of organisms;
c) Can form unwanted bacteriostatic films on surfaces after contact;
d) Non-irritating to the skin;
e) No taste or odor in usual dilutions;
f) Incompatible with soap and some detergents;
g) Residual quantities often undetermined;
h) Non-corrosive to stainless steel; and
i) Minimum concentration required -- 200 ppm.

7. Factors Affecting the Efficiency of Sanitizers

The most important factors affecting the efficiency of sanitizers are:

  1. Intimate contact with the surface being sanitized.
  2. Concentration of sanitizer -- higher concentrations of sanitizer are usually more effective up to a certain point where a drop-off in effectiveness may be seen.
  3. Temperatures that are too high may limit the effectiveness of some sanitizers.
  4. pH has various effects, depending on the sanitizer.
  5. Time of exposure depends primarily on type of micro-organism to be destroyed and its population.

8. A Typical Sanitizing Method

There are many possible sequences in cleaning and sanitizing. Usually, the cleaning is done first, followed by sanitizing. Although there are many similar ones, a typical sanitizing sequence is as follows:

  1. Thoroughly rinse free of all cleaning solutions;
  2. Immerse all surfaces for at least one minute in clean, hot water at a temperature of at least 80 degrees Celsius;
  3. Immerse all for at least one minute in a solution containing at least 50 ppm of available chlorine as a hypochlorite at a minimum temperature of 25 degrees Celsius;
  4. Rinse in clean, hot water;
  5. Immerse for at least one minute in a solution containing at least 12.5 ppm of available iodine at a pH of 5.0 or below and a minimum temperature of 25 degrees Celsius;
  6. Rinse as in 4) above;
  7. Immerse in a third sanitizing solution of equivalent strength to the one used in 3) above;
  8. Rinse again with clean hot water (greater than 80 degrees Celsius) or clean steam; and
  9. Drain dry.