Food safety is always a worrying topic, and the rich information on the Internet also gives consumers the opportunity to learn by themselves. Stainless steel is a common material for a large number of food contact equipment, including thermos cups. Out of concern for the health of themselves and their families, many friends search for professional knowledge of stainless steel on the Internet. However, stainless steel has a complex composition and numbering system. It is inevitable that there will be some confusion if you search for information by yourself. Diare, with my limited knowledge, combined with some references and national standards, I will briefly talk about the stainless steel material of food containers such as thermos cups for your reference. If there are any inappropriate points, netizens are also welcome to raise them and discuss them.
What is the difference between 304 stainless steel and 316 stainless steel? Introduction to the stainless steel numbering system
304 and 316 are two sets of numbers that often appear when discussing stainless steel materials. Using three digits to identify various standard grades of stainless steel is actually the practice of the American Iron and Steel Association. Among them, austenitic stainless steel is identified by the 200 and 300 series; ferritic and martensitic stainless steel are identified by the 400 number. The stainless steel number used in my country's national standard is marked by element content, such as 06Cr19Ni10. Although my country's marking method can intuitively reflect the elemental composition of stainless steel, three-digit numbers are easier to remember after all, and the three-digit marking method of the United States is more commonly used in the market.
At this point, professional terms such as austenite, martensite, and ferrite appear again. These terms are now appearing more and more in corporate propaganda, trying to use advanced terms to make their products appear advanced and make consumers fall into the clouds. In fact, austenite and martensite are different crystal structures of steel. Different crystal structures must be stable at room temperature, and their steel compositions will also be different. Austenite and martensite are the names of material scientists who discovered the two crystal structures. It is generally believed that austenitic stainless steel has relatively better toughness and corrosion resistance; while martensitic stainless steel has better hardness. However, corrosion resistance is a general concept. In different environments, the corrosion resistance of steel will be different, and it cannot be generalized. But I would like to emphasize that 304 stainless steel and 316 stainless steel are austenitic stainless steel. Emphasizing "austenitic 304 stainless steel" is unnecessary repetition, just like "milk produced by cows". There is no other meaning except to stun consumers with professional terms.
Having said so much professional knowledge about stainless steel grades, let's get back to the topic of the article, stainless steel for food. Let's first look at the national standards. my country formulated GB 9684-1988 "Sanitary Standards for Stainless Steel Tableware and Containers" in 1988, revised it to GB 9684-2011 "National Food Safety Standard: Stainless Steel Products" in 2011, and further revised it to GB 4806.9-2016 "National Food Safety Standard: Metal Materials and Products for Food Contact" in 2016
The current GB 4806.9-2016 "National Food Safety Standard: Metal Materials and Products for Food Contact" clearly states: "The main parts of stainless steel tableware containers and food production and operation tools and equipment should be made of austenitic stainless steel, austenitic ferritic stainless steel, ferritic stainless steel and other stainless steel materials; the main parts of stainless steel tableware and drilling and grinding tools for food production machinery and equipment can also be made of martensitic stainless steel." It can be seen that martensitic stainless steel should not be used except for some parts that have additional requirements for hardness. The standard has made clear provisions for the indicators of migration of stainless steel materials (i.e. the precipitation of heavy metal elements), ensuring the safety of stainless steel tableware. However, the standard does not specify the specific models of food contact stainless steel, which should be more considered to encourage the research and development of new stainless steel products and performance optimization. After all, it is more scientific to specify the performance of the material, rather than the composition.
Back to the common 304 stainless steel and 316 stainless steel. Compared with 304 stainless steel, 316 stainless steel has additional molybdenum. It is generally believed that 316 stainless steel will have better acid and alkali corrosion resistance, but the hardness of 316 stainless steel is relatively weak and the price is higher. However, we can also see that the superiority of 316 stainless steel in corrosion resistance compared to 304 stainless steel is generally demonstrated under more extreme conditions. Therefore, 316 stainless steel is often used in some medical devices that come into contact with body fluids or industrial products that come into contact with seawater. The various types of drinking water that thermos cups come into contact with on a daily basis are very weak in corrosion, and 304 stainless steel is sufficient to meet the requirements, so there is little need for high-end 316 stainless steel.
Is it safe to put milk or beverages in thermos cups?
We can be sure that using a thermos cup made of 304 stainless steel to hold water will not cause any harm to the body. However, in actual use, it is also possible to store beverages such as milk and juice. Is it safe to do so? Some researchers have used acetic acid and salt solutions to simulate the actual use environment and studied the corrosion resistance of different types of stainless steel [7]. I will briefly discuss this work.
This study first explored the effect of different acetic acid concentrations on the migration of heavy metal elements in stainless steel. A gradient acetic acid solution with a concentration of 0-8% was prepared and different stainless steel products were soaked at 25°C for 24 hours. The acetic acid concentration of general vinegar is below 5%.
In order to study the effect of soaking time, the researchers used a 4% acetic acid solution at 25°C and extended the soaking time to 168 hours (7 days, one week).
Since it is a thermos cup, the liquid added to it is generally not a room temperature liquid. This study further explored the effects of different temperatures, using 4% acetic acid solution at 25 ℃, 40 ℃, 50 ℃, and 70 ℃ for 24 hours, and 100 ℃ for 3 hours.
In real life, we usually encounter not pure acidic liquids, but often a mixture of acid and salt, and salt will also accelerate corrosion. This study used 4% acetic acid solution with different concentrations of salt (sodium chloride) to soak stainless steel at 25 ℃ for 24 hours.
The study found that for 304 stainless steel, when immersed in low concentration acetic acid, low chloride ion concentration and room temperature, the lead content did not exceed the standard; when immersed in high concentration acetic acid, high chloride ion concentration and high temperature, the leaching amount of lead element seriously exceeded the standard; as the immersion time increased, the leaching amount of lead element also increased and exceeded the standard. For 304 stainless steel, the four elements of chromium, cadmium, copper and zinc did not undergo obvious migration or exceed the standard under different immersion conditions and time. The amount of manganese precipitated from 304 stainless steel is greatly affected by the soaking time. As the soaking time increases, the 4% acetic acid solution exceeds the standard after one week of soaking.
The acidity and salinity of milk and juice are much lower than the solution used in the experiment, so I think it is safe to use a thermos cup to store milk and juice at room temperature. There will be no obvious safety hazards when storing milk and juice at high temperatures. However, the actual chemical composition of commercially available milk and juice is relatively complex, which may produce complex galvanic cell reactions and accelerate the corrosion of stainless steel. The reaction rate increases significantly at high temperatures, so it is not recommended to use a thermos cup to store milk and juice for a long time. More importantly, it is impossible to strictly sterilize the thermos cup. At a temperature of more than 30 degrees, the reproduction rate of microorganisms is very fast. Even if the precipitation of heavy metal elements in stainless steel is not considered, milk and juice are easy to deteriorate.
Why do rust spots appear in thermos cups?
In actual use, consumers still encounter rust spots on the inner wall of the thermos cup. What is the reason for this?
Combining the above research and basic chemical knowledge, the most likely reason is that after using stainless steel containers to hold items with a certain acidity and salinity, there is no effective cleaning. After the water evaporates, the concentration of acid and salt in the local area increases rapidly. Under the joint action of acid and salt, the stainless steel cup oxidizes and corrodes, producing rust spots. In addition, if a thermos cup is particularly prone to rust, it is necessary to doubt whether it uses steel that meets the requirements. After the steel rusts, the microscopic structure on the surface will be seriously affected. Even if the rust is scraped off with force, the corrosion resistance will be significantly reduced. Rusty thermos cups should be discarded and it is not recommended to continue using them.
What is the matter with the rainbow print on the inner wall of the thermos cup?
In actual use, some consumers will also find that the rainbow print appears on the inner wall of the thermos cup. What is the reason?
Generally speaking, this is because it is not carefully cleaned after use, and organic stains adhere to the inner wall. When white light is irradiated on the stains, refraction occurs. The refractive index of light of different colors is different, resulting in rainbow prints. Organic stains may accelerate the corrosion of stainless steel and may also breed microorganisms. They should be wiped off in time after discovery.
How to clean a stainless steel thermos?
Since stainless steel is a metal material after all, its corrosion resistance is limited, and strong acidic or alkaline or strong oxidizing (84 disinfectant) detergents should not be used when cleaning. Just use a milder detergent (dishwashing liquid, etc.) to clean the thermos and wipe it dry.
The above article mentioned many times the problem of heavy metal elements precipitating from stainless steel. Some parents often have some strict standards that are almost paranoid about their children's food safety, so I also want to eliminate unnecessary tension at the end of the article. Some researchers [8] pointed out through literature review and experimental research that: "Stainless steel is a safe food contact material. So far, there have been no reports of carcinogenicity, acute poisoning, chronic poisoning, reproductive toxicity, etc.; the Cr dissolved in stainless steel is trivalent Cr, which is beneficial to the human body, and there are no reports of hexavalent Cr detection. The amount of Ni dissolved in stainless steel is lower than that of pure Ni, but a very small number of people who are allergic to Ni should avoid using stainless steel with high Ni dissolution. Mn is an essential trace element for the human body. The daily requirement is much higher than the amount of Cr and Ni. The amount of Mn dissolved in high-Mn stainless steel tableware is much less than the amount of Mn provided by food, tea and beverages."
Therefore, even if you unfortunately buy stainless steel products that do not meet national standards, or occasionally use rusty stainless steel products for your children, there is no need for parents to worry too much and "lose sleep at night."
In summary, stainless steel is a safe metal material that can come into contact with food. Thermos cups made of food-grade stainless steel that meets national standards, such as 304 stainless steel, will not pose a health threat. High-end 316 stainless steel can further enhance the corrosion resistance of the thermos, but whether it can bring additional benefits in actual use is a matter of opinion. However, caution is still required when using a thermos to store high-temperature beverages such as milk and juice for a long time.
After storing beverages with complex ingredients in a thermos, it should be cleaned in time; mild detergent should be used when cleaning.
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