One of the most long standing myths or undeniable truths that my mother has always reinforced at home was to avoid putting cooking pans, metal pots and the like, in the fridge. From her 'limited' toxicology knowledge metals would leak out of the pot to the food stored in it, hence contaminating the food and consequently the organism of the person eating it/from it. I was a little child when I started hearing her go on about that, but where most of the times it's a hear-say stigma, dragged on through generations and multiplied by the satisfaction of empirical knowledge without consubstantiation by any kind of proven science, other times life has shown me that there is something there to learn from and apply, for the sake of our health and our safety.
Because of all these myths and empirical structuralism of undoubted 'truths' The Toxicologist Today blog was born back in 2010 - 9 years ago!! Today I will once again attempt to unveil the real toxicology facts that lie dormant in this idea that metal pans in fridges is definitely A BIG NO NO!
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- Danger of Corrosion,
- Reactivity between food and container,
- Temperature exposure.
Pardon me for what can be interpreted as a blunt assumption, but from where my common sense stands I can see no other immediate possibilities, but I am well open to contribution (use the comments section down below). However, bear in mind I am not discussing food contamination by bacteria. This is purely related to the putative linking to metal contamination of food when stored in the fridge in a metal pan/container!
I guess the best way to address the question is:
Can food be contaminated by metals present in food-storing containers when at a temperature equal or below 4 degrees Celsius?
The first thing we can assume is that if the food-storing containers (let's call them FSC for the sake of simplicity) are without lid, exposure to air will (allegedly) dissolute and more rapidly encourage transferring of metals to food that is in contact with [1] - simple oxireduction chemistry. The recommended maximum permissible levels of tin in food are described in Blunden and Wallace (2003)'s review [1] as of typically 250 mg/kg (200 mg/kg UK) for solid foods and 150 mg/kg for beverages. The authors of this review additionally state that based on published data there is only small and almost unintentionally cherry-picked evidence suggesting that the consumption of food or beverages containing tin at concentrations at or below 200 ppm has allegedly caused adverse gastrointestinal side effects in a small fraction of those exposed to it. They say allegedly because this is based on evidence from reports of adverse side effects anterior to follow-up studying of these cases, therefore with "limited", "incomplete" data of "uncertain veracity". Moreover, they have provided numbers on clinical studies reflecting on the same matter, and these refer 700 ppm or above as the concentration relates to adverse gastrointestinal side effects, but they also imply that the observations at higher concentrations lacked uniformity and linearity. Thus, concluding that there is little evidence for a link between "consumption of food containing tin at concentrations up to 200 ppm and significant acute adverse gastrointestinal effects" based on the available studies.
Moreover, and as a matter of additional protection when there are concerns that the metal might corrode, food tins are usually coated internally with lacquer. Lacquer is "a thermosetting resin polymerised on the surface of the tin" . There is a French study showing that such layer provides an approximately 24-fold increased protection in comparison to other tins without said coating (check [2] for references). Over the years this protection might be insufficient, especially in cases where acidic food is stored within, however for the sake of this post I am not considering food stored in fridges for longer than a couple of months.
Conor Reilly (2002) [2] considers that many factors can be involved and must be studied in relation to metal corrosion and metal transferring from containers to food, such as, the "presence of nitrate" in food that can increase the solubility of metals, "storage temperatures, length of storage time, presence of oxidising and reducing agents in the food". But he also states that after lead has been abandoned, the new lead-free containers have reduced tremendously the number of reported cases of metal-contaminated food in developed countries.
Aluminium appears to be a great food/beverage-storing option for safety/quality reasons as [2] considers that the internal coating of aluminium cans (usually with resins such as vinyl epoxy) and the thermal resistance of the metal, inhibit, to an extent, the dissolution of the metal (check [2] for references).
In what concerns cooking utensils, there are many safe and valid options these days (e.g., stainless steel and ceramics, etc...) that not only allow rapid conductivity of heat, but represent an adequate barrier to metal degradation even in the presence of acidic foods. Classic tools based on tinned copper are made with 'impure' led-tin mixtures and some studies (check [2] for references) have shown it. On the other hand traditional cast iron pots can contribute to higher levels of iron in food, especially if cooked in low intensity combustion. Both aluminium pots and foils have been linked to aluminium re-uptake levels, and associated, directly or indirectly, to a range of neurodegenerative diseases like Alzheimer's [3]. Aluminium is a well-established, well-studied neurotoxic agent (can trigger oxidative stress, promote inflammation and cellular apoptosis, etc).
From this very quick investigation of the vastly available literature on this topic, I have understood that the real issue is not so much linked to whether one stores food in metal pots put in the fridge, but the exposure of food to such metal containers, that due to intrinsic properties of both food and container, associated to the different cooking processes (temperature levels, length of heat exposure, etc), can contribute to the degradation of the metal itself and their inner coating layers.
The idea, I genuinely believe, is to:
1) always follow the instructions provided by each particular company on their specific product (read food-containing pot)
and
2) minimise exposure and contact between food and metal container, regardless of how safe the storing temperature might appear to be.
There is an 'infinite' list of studies out there that show many external participants involved in the metal degradation process, and some of these are fairly and unequivocally invisible to our naked eyes, nevertheless determinant in terms of effects to our general health.
[1] Blunden, S. and Wallace, T. (2003). "Tin in canned food: a review and understanding of occurrence and effect - Review". Food and Chemical Toxicology, 41(12), pp. 1651-1662.
[2] "Metal Contamination of Food: Its Significance for Food Quality and Human Health". Book from 2002 by Conor Reilly- Chapter 4.2.2 Metal pick up during canning, page 51.
[3] Maya, S., Prakash. T., Madhu, K. D., Goli, D. (2016). "Multifaceted effects of aluminium in neurodegenerative diseases: A review". Biomedicine & Pharmacotherapy, 83, pp. 746-754.
First post photo by nrd on Unsplash
Second post photo by Scott Umstattd on Unsplash
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