It was June 2020 when frozen Norwegian salmon allegedly caused an outbreak of SARS-CoV-2 at a Chinese market. State media stated that the corona virus was discovered on chopping boards where imported salmon was cut at a Beijing market. Beijing supermarkets hastily removed the salmon from their shelves, and European salmon imports were halted. Earlier this month (January 2021), the Chinese authorities reported that the corona virus was found in ice cream, made with ingredients from New Zealand (milk powder) and Ukraine (whey powder).

The virus is commonly passed on:

• directly, through contact with an infected person’s body fluids (for example, droplets from coughing or sneezing – aerosols)
• indirectly, through contact with surfaces that an infected person has contaminated by coughing or sneezing on.

Also, we are seeing a number of publications in peer reviewed journals, almost exclusively from Chinese research teams, that report findings of the virus in the cold chain.

It was found that the virus persists on many dry surfaces. Consequently, indirect transmission of SARS-CoV-2 is in possible. Notably, the virus will persist on high-touch surfaces. This is especially true in cold, dry climates as SARS-COV-2 persistence is inversely related to temperature and humidity.

This deserves a closer look, and a number of questions need to be asked.

For once, it is noticeable that only a single country reports findings, both, at government level and research level that the virus on a cold surface has actually caused an infection of one or several persons. Between the first report of finding the virus in mid-2020 in Beijing and the most recent report from an ice cream manufacturer close to Beijing, more than six months passed. A search in PubMed [1] resulted only in research articles from China, reporting about corona in the cold chain.

That begs some questions:

1. are no research teams from other countries performing (and publishing) research into the corona virus in the food supply cold chain?
2. Have the competent authorities of other countries not discovered SARS-CoV-2 in the food supply cold chain?
3. Is there a chance that the test results are “false-positive”, i.e. the test erroneously reported the presence of the virus although it was not there?
4. If the primary transmission route of the virus are aerosols [2], [3], [4], how significant are findings of the SARS-CoV-2 virus in the food supply cold chain?

Let’s take look at the most recent case, the finding of the virus in ice cream at a Beijing manufacturer. They used reportedly two imported ingredients: dried milk powder and whey powder. Here the question needs to be asked: can the corona virus “survive” spray tower drying? Dried milk powder is typically produced by spraying milk into the top part of a heated tower, and the water evaporates from the droplets on the way down, leaving dried milk powder to arrive at the bottom on the tower. The pressure with which the milk is sprayed, according to the manuscript of Nikolova et al. is somewhere between 100 and 200 bar. This likely already causes deformation of the virus protein hull structure. And the structure is needed for the virus to be infectious. The temperatures of the spray tower range typically from 70°C to 100°C. This is the temperature where denaturation is observed.

Here, the question has to be asked: even if the corona virus was originally in the milk, how likely is it to survive the spray-drying process with the pressure and temperature used (and still be infectious)?

Let´s look at another question: is the virus likely to survive in the cold chain.

The simple answer is: viruses are generally more likely to survive at lower temperatures. But does this present a risk for consumers or cold chain workers?  Here it is important to put things into perspective: every time we go food shopping, there is a probability that we cross path with an infected (and potentially asymptomatic) person. Also, there is a chance that when we use the shopping cart, it has previously been used by an infected individual.

We have not seen conclusive evidence that there is a significant risk to food shopping, provided the hygiene concept is in place, distance is maintained and masks are worn.

Similarly, especially in the cold chain, the probability of being infected by a co-worker is higher – and we have seen the examples of several abattoirs in Germany and UK last year – than being infected by frozen products that we handle. We also need to bear in mind that most frozen foods are not consumed frozen, but are heated, mostly to temperatures significantly above 70°C where proteins start to denature and viruses get deactivated.

Overall, to date, there is no unequivocal evidence that the corona virus in the food supply cold chain is present in forms or quantities that present a significant risk to consumers or workers if common-sense hygiene rules are observed.

Aerosol transmission from person-to-person, not from food to person, still presents the primary path of infection [2], [3], [4].

[1] PubMed advanced queries on 2021-01-19: https://pubmed.ncbi.nlm.nih.gov/?term=(“SARS”+or+”covid”+or+”corona”)+and+(“cold+chain”+or+”coldchain”) and  https://pubmed.ncbi.nlm.nih.gov/?term=(“SARS”+or+”covid”+or+”corona”)+and+(“cold+chain”+or+”coldchain”)+and+Food

[2] Jayaweera et al. Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy. Environmental Research 2020, 188, 109819

[3] WHO. Coronavirus disease (COVID-19): How is it transmitted?

[4] Zuo et al. Airborne Transmission of COVID-19: Aerosol Dispersion, Lung Deposition, and Virus-Receptor Interactions. ACS Nano 2020, 14, 12, 16502–16524