A new comprehensive study conducted by scientists from the Virginia Polytechnic Institute & State University-USA has found that the SARS-CoV-2 coronavirus can in fact remain infectious on a variety of refrigerated deli food, meats and fresh produce for up to 21 days.
The survival and high recovery of SARS-CoV-2 on certain foods support the possibility that food contaminated with SARS-CoV-2 could potentially be a source of infection, highlighting the importance of proper food handling and cooking to inactivate any contaminating virus prior to consumption.
The study findings were published in the peer reviewed journal: Foods.
https://www.mdpi.com/2304-8158/11/3/286
The World Health Organization (WHO), US Food and Drug Administration (FDA), and European Food Safety Authority (EFSA) have issued statements that food is unlikely to be associated with transmission of SARS-CoV-2 [23,24,25]. However, reports of SARS-CoV-2-contaminated foods and food packaging raise concerns about potential health risks associated with food, food packaging, and food contact surfaces that may become contaminated during processing, transport, or preparation [26,27,28,29,30].
On common food contact surfaces, infectious SARS-CoV-2 is stable on stainless steel and plastic for up to 72 h [32]. Taken together, these studies suggest that contamination of food and environmental surfaces by SARS-CoV-2 may present a potential risk of infection for susceptible individuals.
A complete understanding of transmission routes for any pathogen is essential to guide the development of appropriate public health policies and control measures to prevent, or at least reduce, the risk of infection and spread within the population. Given the mounting evidence that SARS-CoV-2 can infect cells of the oral cavity and GI tract, one must consider the possibility that infection can potentially occur through ingestion of virus-contaminated foods.
Since some types of food may present a greater risk of infection than others, we sought to determine the relative risks of infection from various types of foods. We previously developed and validated methods to effectively recover infectious enveloped viruses from the surface of different types of food and investigate the survival of SARS-CoV-2 on a limited number of foods over a 24 h period of time (chicken, salmon, shrimp, spinach, apple skin, and mushroom) [29].
Others have focused on assessing the survival of SARS-CoV-2 on cold-chain seafoods and meats [26,27,28]. The objective of the current study was to identify additional types of foods that may present a potential risk of infection, by assessing the time of survival of SARS-CoV-2 on foods grouped into three broad categories, including ready-to-eat deli foods, fresh produce, and meats (which includes seafood), held at refrigeration temperature (4 °C) up to 21 days.
Since ground beef burgers are commonly cooked rare or medium, instead of well done, we also assessed survival of SARS-CoV-2 in ground beef burgers cooked to various internal temperatures. Furthermore, since PCR-based assays are commonly used to detect pathogens on food and food packaging, we compared viral RNA quantities with infectious virus titers to determine if PCR-based assays could accurately assess the risk of infection presented by the presence of infectious virus. This study addresses knowledge gaps on understanding the potential of food products as carriers of SARS-CoV-2, particularly ready-to-eat foods and those that are consumed raw or intentionally under-cooked, such as ground beef burgers.
Discussion
Although SARS-CoV-2 is known to be transmitted person-to-person by airborne transmission and respiratory droplets, other potential routes of infection have not been adequately investigated [4,11,12,30,36]. Although it is unlikely to be a major route of transmission, foodborne SARS-CoV-2 has the potential for entry into a host through ingestion, as the oral cavity and GI tract express the receptor and accessory protease by which the virus enters cells [18,19].
Here, we determined that SARS-CoV-2 can survive in an infectious state on several different types of foods, many of which are ready-to-eat or consumed raw, while other types of foods inactivate the virus and present a much lower risk of infection.
The angiotensin-converting enzyme (ACE2) receptor is abundantly expressed in the oronasal mucosa and esophagus, as well as in stomach and intestinal tissues [40,41], providing potential entry points for SARS-CoV-2 besides the respiratory tract. Additionally, several other molecules have been suggested as alternative receptors for SARS-CoV-2, including C-type lectins, TIM1 (T cell transmembrane, immunoglobulin, and mucin), AXL (AXL receptor tyrosine kinase), and NRP1 (neuropilin 1), suggesting that the virus may enter cells that do not express ACE2, as well [42].
Although no specific foodborne cases have been reported, outbreaks have been traced to imported foods or food packaging [26,27,43]. The mechanism of GI tract infection is not fully understood, but SARS-CoV-2 has been reported to infect the oral cavity, salivary glands, enterocytes and intestinal organoids, resulting in inflammatory cell recruitment and GI tissue damage [13,19,20].
In addition, intragastric inoculation in nonhuman primates led to the detection of the virus and inflammation in multiple tissues, including digestive tissues, lung, liver, and pancreatic tissues [20]. Although the acidity of the stomach (less than pH 3.5) typically inactivates most enveloped viruses [44], gastric pH can elevate to near neutral with a meal [45], which would allow viruses to survive transit through the stomach and cause infection in the intestines.
MERS-CoV and HCoV-229E, two coronaviruses similar to SARS-CoV-2, were completely inactivated within 30 min in simulated fasting-state gastric fluid in vitro. However, they remained unaffected after 2 h of exposure in simulated fed-state gastric fluid, demonstrating that coronaviruses can likely survive the acidic environment of the stomach when food is also present [46].
Although SARS-CoV-2 was rapidly inactivated in simulated fasting-state gastric fluid in vitro [47], the stomach environment varies with the presence of a meal, which may allow SARS-CoV-2 to transit through the stomach and infect intestinal cells. Since SARS-CoV-2 can infect cells of the alimentary tract, consumption of virus-contaminated raw, undercooked, and ready-to-eat foods may be an alternative route for SARS-CoV-2 infection.
Whether some types of foods pose a greater risk of infection is a question we sought to investigate.
In general, high-protein unprocessed and minimally processed foods (raw meat and seafood, roasted and chilled turkey) and foods high in both protein and fats (cheese and plant-based meat alternative) supported viable SARS-CoV-2 for at least 14 days at refrigeration temperature.
In addition, foods with higher moisture content prolonged infectivity of SARS-CoV-2, likely by preventing their desiccation and inactivation. These findings concur with other studies reporting similar results [28,29]. SARS-CoV-2 on pork, beef, and salmon were reported to remain infectious for at least 9 days at 4 °C and 20 days at −20 °C [28]. Foods cannot support the replication of viruses, yet most foods cannot inactivate the virus.
Ready-to-eat deli foods are often consumed without further cooking and oysters are commonly consumed raw. Since viruses may survive acidic gastric environments with a meal, consuming foods heavily contaminated with SARS-CoV-2 may produce a risk of transmission via GI tract infection. Beef, pork, and plant-based meat alternatives are usually cooked before consumption, effectively negating any risk of infection through ingestion.
Previous studies have shown that SARS-CoV-2 is susceptible to heat inactivation in virus transport medium when heated to 70 °C (158 °F) for 5 min or 98 °C (208 °F) for 2 min [48,49]. SARS-CoV-2 spiked into human milk (7 log PFU) could also be inactivated by pasteurization at 62.5 °C (145 °F), but not at 56 °C (132 °F), for 30 min [50].
To ensure food safety, the USDA recommends steaks and roasts be cooked to the internal temperature of 62.5 °C (145 °F, medium), and ground beef and pork should be cooked to a minimum internal temperature of 71 °C (160 °F, well-done). Given that SARS-CoV-2 would be attached on the surface of steaks and roasts, cooking to an internal temperature of 145 °F should be sufficient to elevate the temperature of food surfaces higher than 158 °F, which would inactivate the virus on the food surface within a short time.
However, ground beef, ground pork and plant-based meat alternatives could be contaminated by the virus within the matrix of the meat during grinding or preparation of burgers. Although ground pork and plant-based alternative burgers are typically cooked well-done, ground beef burgers are commonly cooked rare to medium (125 °F/51.2 °C to 145 °F/62.5 °C internal temperature), which requires a short period of cooking time and provides an environment rich in protein and fats conducive to survival of SARS-CoV-2.
Our results demonstrated that SARS-CoV-2 survived in ground beef burgers cooked rare and medium, while no viable virus was detected in well-done ground beef burgers. Therefore, consumption of undercooked ground beef that has been contaminated with a high concentration of SARS-CoV-2 could be a potential risk for SARS-CoV-2 infection of the GI tract. Our findings highlight the importance of appropriate cooking, in addition to proper handling and infection control measures, to ensure food safety.
Although SARS-CoV-2 remained infectious on deli turkey and cheese for 21 days, virus concentration on hard salami was significantly reduced within 24 h post-inoculation. Hard salami is a dry-fermented sausage with a final pH of 4.8–5.3 (0.5–1% lactic acid) and with a moisture: protein ratio less than 2.3:1 in the end product [51]. Although the relatively dry environment on salami could contribute to the inactivation of a viable virus, the acidity may not, since a previous study reported that SARS-CoV-2 is not susceptible to inactivation in a wide range of pH values (pH 3–10) at room temperature [48].
In addition, food preservatives and flavor enhancers are usually added to processed meats to extend freshness and protect flavor. Butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are added to salami to serve as antioxidants and antimicrobials to enhance food safety.
BHT is reported to inactivate some enveloped viruses, including herpes simplex virus (HSV) and Newcastle disease virus [52]. Furthermore, citric acid is also added to salami to enhance flavor, and lactic acid is produced during the fermentation process. Organic acids, such as citric and lactic acid, are listed as active ingredients of SARS-CoV-2 disinfectants authorized by the Environmental Protection Agency in the United States [53].
Therefore, those organic acids present in salami could have antiviral effects against SARS-CoV-2. Collectively, inactivation of SARS-CoV-2 on salami could be caused by the comprehensive effects of low moisture, antioxidant additives, and organic acids produced during fermentation.
As we reported previously, fresh produce has variable effects on SARS-CoV-2 [29]. In our previous studies, infectious SARS-CoV-2 recovered from spinach and apple skin remained constant over 24 h post-inoculation, but mushrooms exhibited significant antiviral effects within one hour, destroying both infectivity and viral RNA.
In this study, we determined that SARS-CoV-2 can survive with no substantial reduction for 7 days when inoculated onto the surface of tomatoes or grapes. After this time point, we observed a significant reduction in virus titer but viable SARS-CoV-2 was still detectable on tomato and grape outer skin 21 days after inoculation.
Grape skin contains multiple phenolic compounds, such as resveratrol and gallic acid [54,55], which have previously been shown to inactivate viruses by binding of phenolics to the protein coat of the virus, interfering with the ability of the virus to bind to host cells [56].
In addition, grape extracts (skin and whole grapes) were reported to inactivate various enteric viruses and HSV-1 [57], and grape pomace exhibited antiviral effects against adenovirus [58] and influenza [59]. However, our results demonstrated that virus concentration on the surface of intact grapes was not significantly reduced within 7 days, indicating that the outer grape skin did not have substantial antiviral effects against SARS-CoV-2.
As cherry tomatoes and grapes are typically eaten raw, these results highlight the importance of washing fresh produce prior to consumption. Anecdotally, people who are concerned about SARS-CoV-2 contamination of foods have been known to soak their produce in bleach, which can introduce dangerous levels of sodium hypochlorite into fruits with porous skins, such as tomatoes and grapes.
Our method of virus recovery from foods, in which rinsing with media recovers nearly 100% of inoculated virus from the surface of the foods immediately after inoculation, suggests that simply rinsing produce with water is sufficient to remove nearly all infectious virus.
Compared to tomatoes and grapes, we recovered a less viable virus from both the outside (shell) and inside (pulp) of avocados.
Additionally, initial virus recovery (0 h) from the avocado shell was substantially less than the other produce. The avocados used in these studies were purchased from the local grocery store and avocados are often coated by commercial antimicrobial sprays, potentially contributing to the lower initial virus recovery.
In contrast to tomatoes and grapes, avocado pulp exhibited a strong antiviral effect against SARS-CoV-2, reducing infectious virus to less than 1 log PFU/mL within 24 h. Extracts of avocado pulp exhibit antiviral activity against dengue virus by inhibiting viral replication, causing an increased survival rate among mice infected with the virus [60].
Other studies have reported that phenols and unsaturated fatty acids have antiviral effects against SARS-CoV-2. Rutin, one of the main flavanols in ripe avocado pulp [61], was found to have high antiviral activity as a SARS-CoV-2 protease inhibitor, preventing viral replication [62].
However, the mechanism of antiviral activity must be different with SARS-CoV-2 placed directly on avocado pulp, as the virus does not replicate on the foods. The flavonoids quercetin and luteolin, as well as the polyphenol gallic acid, in avocado pulp show high binding affinity towards the ACE2 receptor in human cells, potentially inhibiting SARS-CoV-2 attachment to ACE2 [63,64]. In addition, the free fatty acid linoleic acid, which comprises 19.3% of pulp oil from the Barker cultivar [65], tightly binds to the receptor binding domain of SARS-CoV-2, resulting in the reduction in interaction between the ACE2 receptor of human cells and the spike protein of SARS-CoV-2 [66].
Thus, the avocado pulp contains a multitude of phytochemicals that may inhibit the binding of SARS-CoV-2 to its receptor ACE2, effectively reducing the infectivity of the virus.
In the food industry, the gold standard for foodborne pathogen detection is based on rapid PCR screening [67]. For bacteria, PCR-based detection is confirmed by growth in appropriate media [67]. Confirmation of foodborne viruses is challenging, however, due to a lack of necessary facilities, expertise, and time associated with propagating a virus in an appropriate cell culture, particularly for viruses that may require higher biocontainment environments.
Although nucleic acid extraction and PCR-based assays are standardized for the common foodborne viruses hepatitis A and norovirus, confirmatory assays to demonstrate a viable virus are rarely performed [68]. The detected presence of the viral genome typically initiates disinfection procedures and disposal of presumably contaminated foods, which can become quite costly. To determine if a PCR-based assay for SARS-CoV-2 would accurately correlate with the quantity of infectious virus, we investigated the relationship between viral RNA genome copy number, using a PCR-based assay, and infectious viral titers, determined by plaque assay, in several foods from our three categories (deli, produce, and meats).
At all time points tested, SARS-CoV-2 viral RNA copies were 2–3 times higher than infectious viral titers at the same time point. Regardless of food type, infectious SARS-CoV-2 was inactivated over time, while viral RNA was not degraded in similar trends. Therefore, the presence of viral RNA identified by PCR-based tests is not necessarily equivalent to the presence of infectious virus in foods.
Without a confirmatory test to demonstrate the presence of infectious virus, costly disinfection procedures and disposal of potentially contaminated foods may be conducted based solely on a PCR-based assay that cannot differentiate between the presence of non-infectious viral genome fragments and a viable virus that may pose a risk of infection. SARS-CoV-2 RNA has previously been detected on food and food packages, based on PCR-based assays [26,27,69].
However, previous studies, including ours, have also reported that SARS-CoV-2 RNA can be present on food, while infectious SARS-CoV-2 is undetectable by plaque assay [29,70]. Recent studies have reported on the development of amplification-free CRISPR-Cas assays for direct detection of SARS-CoV-2 RNA from patients as alternatives to PCR-based assays [71,72,73].
Although CRISPR-based assays produce results more rapidly than PCR amplification-based assays, they still detect viral RNA but not viral infectivity. PCR-based tests or CRISPR-based assays may be used for an initial screen, but the presence of an infectious virus must be confirmed by other methods, since the infectious virus is the form that presents the risk to public health.
Rapid and inexpensive technologies are thus needed in the food industry to reliably confirm the presence of infectious foodborne viruses to avoid economic losses from disposal of food thought to be contaminated, as well as from costly disinfection procedures that may not be warranted.
