If your jars of home preserved food go mouldy, should you just scrape off the mould and eat the rest?
There are at least three concerns with mould.
- Mould can create conditions in which botulism toxin can form;
- Mould releases its own toxins;
- Mould can be indicative of underprocessing which allowed other nasties to also survive.
We also realize that half the people who read this are going to say “poppycock and nonsense” and go on eating mouldy preserves. We can only try to inform people; we don’t get paid to go into their kitchens and try to save them.
Here is a review of the facts (yes, facts), for those who haven’t yet decided that a bit of bluster and bravado will overcome any health issues.
Note: to be clear, mould on the outside of jars is fine. It just means you didn’t wash the outsides of the jars thoroughly enough after processing.
- 1 The short read…
- 2 Moulds are a new health concern
- 3 Scraping mould off jam and other preserves
- 4 Mould in tomato jars
- 5 Does boiling the preserve make it safe again?
- 6 The physical nature of moulds
- 7 What moulds are beneficial
- 8 Potential health impact of moulds
- 9 Are bottled or refrigerated foods immune to mould
- 10 What foods with mould can and can’t be saved
- 11 Moulds are not bothered by acidity
- 12 Moulds can work to lower the acidity
- 13 Improper canning methods lead to moulds in jars
- 14 Reduced or sugar jams are vulnerable to mould once opened
- 15 Further reading
The short read…
The overwhelming majority of researchers and public health professionals recommend that when mould affects any soft, liquidy or porous food item, the food item in its entirely should be tossed. Mould affects a much wider area of such foods than is apparent. That means that scraping mould out of jam, jelly, chutney, relish, salsa, pickles, etc, and eating the rest of the jar, should be off the menu. Boiling a food item will not destroy the type of toxins released by moulds. The concern is the health impacts of the toxins, which take a long time to manifest themselves.
You can skip down for a recommended list of what and what not to try to salvage from food mould.
Moulds are a new health concern
Health concerns about moulds are rather new in human history. Before, moulds were just regarded as a nuisance, aesthetically spoiling food.
Although poisonous mushrooms are carefully avoided, moulds growing on foods have generally been considered to cause unaesthetic spoilage, without being dangerous to health. Between 1960 and 1970 it was established that some fungal metabolises, now called mycotoxins, were responsible for animal disease and death. In the decade following 1970 it became clear that mycotoxins have been the cause of human illness and death as well, and are still causing it.” J.l. Pitt. An introduction to mycotoxins. In: R.L. Semple, A.S. Frio, P.A. Hicks and J.V. Lozare, eds. Mycotoxin prevention and control in foodgrains. UNDP/FAO. 1989.
In fact, some researchers suggest that we may soon realize we should have been paying more attention to toxins from moulds than we have been to man-made ones.
Natural toxins have been termed “nature’s pesticides” because they often confer a protective or competitive advantage to the organism or plant that produces them. Because the diet contains at least 10,000 times more natural toxins exist compared with [man-made] toxins… natural toxins probably pose the greater threat to human and animal health. One large group of natural toxins that are nearly universal contaminants of food and feed are the mycotoxins, the toxic secondary metabolites produced by [mould].” Coulombe, R.A., Jr. (1993). Biological Action of Mycotoxins. J. Dairy Sci. 76:880-891. Accessed August 2017 at https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1034&context=advs_facpub
Scraping mould off jam and other preserves
The answer you get depends on who you listen to.
All health professionals and home food preservation professionals in Canada and the US give a resounding no. In the UK, you are going to hear a “yes.”
We tend to support the North American answer because it is based on evidence-based research. The UK has just simply not invested the kind of time, money and resources into the kind of pro-active, forensic and ongoing studies into foodborne illnesses that North American governments have. Perhaps they have simply lacked the capacity to do so.
That being said, there is no denying the conflicting answers you will find, so we do present them.
As the University of Hertfordshire found in a (rare for England) investigative field study on household food safety, advice and practice in England in general relies a great deal on interpreted knowledge from domestic kitchen practice, rather than letting evidence-based research lead the recommendations or practices. Wills, Wendy et al. Domestic Kitchen Practices: Findings from the ‘Kitchen Life’ Study. Report for the Food Standards Agency. University of Hertfordshire. July 2013. Page 56, 59
In many aspects of nutrition and food safety, a lot of advice (informed and uninformed ) is passed on by celebrity chefs (though this of course is not unique to England.)
Nigel Slater gave this advice about mouldy jam in his Ask Nigel column,
Q: I have been making jam, on and off, for the past 30 years or so. This year I made two batches of blackberry and apple jelly. One batch is developing mould on top of the waxed seal. Help! I have been scraping off the mould and discarding the first couple of inches of jam. Is the rest of the jar safe to eat? Fiona
A: Fiona, I can’t suggest you eat the jam underneath because as well as the mould on the surface there will probably be spores penetrating down through the jam and they can sometimes be dangerous. So my ‘official’ answer is no, don’t eat it. However, I can tell you that I, and many other jam eaters, have been scraping the mould off homemade jam for years and come to no harm at all. I just can’t actually recommend you do.” Slater, Nigel. Ask Nigel. Manchester: The Guardian. 18 October 2009. Accessed August 2017 at https://www.theguardian.com/lifeandstyle/2009/oct/18/ask-nigel-recipes-chutney-jam
Delia Smith gives this advice in her “Complete Illustrated Cookery Course” book:
“If a mould develops on the surface, remove it plus about half an inch (1 cm) of the jam underneath. The rest of the jam will not be affected”. Smith, Delia. Complete Illustrated Cookery Course. London: BBC Books. 2000. Page 552.
Turning to professionals, the advice is similar. We emailed Certo UK and got this response from a Dr Colin May there.
With full sugar jam, I have often done this [Ed: scrape mould off jam], and am still alive! I would only suggest doing it with small spots of mould, and take a generous spoonful off around the spot. Can’t make this a formal recommendation, though.” Dr Colin May, Certo UK, to Healthy Canning, 23 May 2017. Email on file.
BBC quotes a Dr Patrick Hickey in saying,
The moulds you find on jam, are fine – just scrape them off.” Michael Mosley. How safe is mouldy food to eat? 22 October 2014. Accessed August 2017 at https://www.bbc.com/news/magazine-29701768
The Guardian quotes a Peter Wareing, food safety expert at Leatherhead Food Research, England, saying:
According to Wareing, most jam mould can safely be spooned away with no further threat to life and limb” Daoust, Phil. Spoilt rotten: good and bad mould. Manchester: The Guardian. 26 October 2011.
Looking for some or even any UK advice against eating mouldy jam was very hard to find, but we did manage to find the following few bits after a few days of searching.
Nigella Lawson is a celebrity cook. Unlike Delia Smith or Nigel Slater, however, she gives more cautious advice. The “Ask Nigella” section of her website says,
We would suggest discarding any jars of jam that have mould growing on top.” Accessed August 2017 at https://www.nigella.com/ask/storing-jam-and-marmalade
University researchers from the University of Hertfordshire did a field-study of food safety practices in various real-world homes. They recorded their findings in a report for the UK Food Standards Agency. Part of the findings touched on mouldy preserves:
Claire Thorpe reported being cautioned against using mouldy home-made jam when she was a student. Her mother told her that the ‘toxins go deep’… Julia showed the researcher a jar of home-made chutney, dated 2008, which, she believed, would be safe to eat. She reported that although she was a little wary of eating things that had gone mouldy, she would scoop the mould off preserves and ‘put it in the microwave for a couple of minutes and that would kill anything that was there’. Here, doubt is ameliorated by a practice which, possibly, had some foundation in a belief that microwaves will kill anything…..Another example of someone pushing the boundaries of safety in the spirit of avoiding food waste…… ” Wills, Wendy et al. Domestic Kitchen Practices: Findings from the ‘Kitchen Life’ Study. Report for the Food Standards Agency. University of Hertfordshire. July 2013. Page 56, 59
We found a report from a public health officer in Scotland, inspecting home-made jams for sale. The inspector took a dim view of the vendor’s presumably lackadaisical concern about mouldy jam:
The leaflet with the jams suggests the possible formation of mould growth from the film cover after opening. This Section would not consider this to be an accurate enough description without further supporting evidence that such growth is not harmful to health. It is therefore not considered appropriate to include this information as it could be construed as false and misleading in terms of Section 15 of the Food Safety Act 1990. In addition, if jam was sold in a mouldy condition, this would invariably be construed by the purchaser as not being of the quality demanded which would constitute an offence in terms of Section 14 of the Food Safety Act 1990.” Environmental Health Officer, Moray Council, Scotland. Reference 13/00030/COMM. 14 October 2013.
As a side note, note that commercially and industrially, food producers in England are allowed to scrape mouldy bits off big blocks of cheese, and process those cut-off mouldy bits into other food products for human consumption:
Cheese contaminated with visible mould which is not present as part of the production process or integral to the final product will normally need to be disposed of in accordance with animal by-product legislation. . An exception to this is large blocks of hard cheese where the mould is hygienically removed …. The mouldy off-cuts removed from the hard cheese may be released for further processing for human consumption.” Cheese Recovery Guidance. Food Standards Agency May 2007. Page 2, para 5, 6, and 7. Accessed August 2017 at https://www.food.gov.uk/sites/default/files/multimedia/pdfs/cheeserecovery.pdf
At the time of our search, almost all popular advice appears to overwhelmingly support scraping mould off preserves and carrying on eating them. There was very little professional or public health guidance to be found.
We managed to find only one statement from a public health person, and that was against the practice:
The Brisbane times quoted Lydia Buchtmann, of the Food Standards Australia New Zealand (FSANZ), as saying,
A little bit of fur never hurt anyone. Or did it? It all depends on the type of mould… moulds can penetrate more deeply than the eye can see, so what looks like a small patch on your chunky raspberry jam or vintage tasty cheese might be a lot larger. The best idea is to chuck it out….. ” Top five food safety myths. Brisbane Times. 21 April 2008. https://www.brisbanetimes.com.au/news/health/top-five-food-safety-myths/2008/04/21/1208742837735.html
The USDA Food Safety and Inspection Service says,
Jams and jellies. Discard. The mold could be producing a mycotoxin. Microbiologists recommend against scooping out the mold and using the remaining condiment.” United States Department of Agriculture, Food Safety and Inspection Service. Molds On Food: Are They Dangerous? 2013. Accessed August 2017 at https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/safe-food-handling/molds-on-food-are-they-dangerous_/ct_index
Here’s more information from the USDA:
Soft fruits, lunch meats and jams also must be tossed once moldy, says Marianne Gravely, a senior technical information specialist for the United States Department of Agriculture ‘With soft food, it’s very easy for the roots [of the mold], or the tentacles, or whatever creepy word you want to use, to penetrate” deeper into the food.’ What’s more, by the time mold has moved in, other harmful kinds of bacteria associated with food spoiling may also have infiltrated the food.”  Jacewicz, Natalie. Is It Safe To Eat Moldy Bread? 21 April 2017. Accessed August 2017 at https://www.npr.org/sections/thesalt/2017/04/21/523647669/is-it-safe-to-eat-moldy-bread
Clemson University in South Carolina says,
Even though sugar helps preserve jellies and jams, molds can grow on the surface of these products. Research now indicates that the mold people usually scrape off the surface of jellies may not be as harmless as it seems. Mycotoxins have been found in some jars of jelly having surface mold growth. Mycotoxins are known to cause cancer in animals; their effects on humans are still being researched.” P.H. Schmutz and E.H. Hoyle. Basics of Jelly Making. Clemson Cooperative Extension. Revised September 2007. HGIC 3180. Accessed August 2017 at https://www.clemson.edu/extension/hgic/food/food_safety/preservation/hgic3180.html
The University of Missouri Extension says,
Even though sugar helps preserve sweet spreads, molds can still grow on the surface of these products unless they are heat-processed. It is not a safe practice to scrape the mold off the surface of sweet spreads and use what’s left in the jar. Poisons called mycotoxins, which are known to cause cancer in animals, have been found in some jars of jelly with surface mold growth. The effects of mycotoxins on humans are still being researched, so you should discard any sweet spread containing mold.
Paraffin or wax seals don’t prevent mold growth and are no longer recommended for sealing any sweet spread, including jelly. To prevent mold growth and to keep good flavor and color, pour hot sweet spreads into sterilized jars; leave ¼ inch of headspace. Seal with two-piece lids, and process as directed in Table 1. Be sure to use the processing time recommended for your altitude.” Mills-Gray, Susan. Quality for Keeps: Sweet Spreads and Syrups. Publication GH1461. Reviewed April 2017. Accessed August 2017 at https://extension.missouri.edu/p/GH1461
There’s more information on one of its myth buster pages:
Myth: If I find mold growing inside a jar of home canned food, I just scrape it off and eat the food.
Mold growth in foods can raise the pH of the food. In home canned products, this could mean that the high acid products could become low acid and therefore run the risk of botulism or other bacterial spoilage. Thus, any home canned product that shows signs of mold growth should be discarded. USDA and microbiologists now recommend against even scooping out the mold on jams and jelly products and using the remaining jam or jelly, even though that used to be suggested.
It is best to follow guidelines set out by So Easy to Preserve, University of Georgia Cooperative Extension Service, and other reliable sources such as Missouri Families or the National Center for Home Food Preservation.” Maude Harris, EdD, Nutrition and Health Education Specialist, University of Missouri Extention. Friday, June 4, 2010. Accessed August 2017 at https://nutritionmythbusters.blogspot.ca/2010/06/myth-if-i-find-mold-growing-inside-jar.html
Oregon State says,
Moldy jams and jellies are not safe to eat and should be discarded.” Jams and Jellies. Oregon State University Extension Service. SP 50-746, 2002. Reprinted March 2013. Page 2. Accessed August 2017 at https://extension.oregonstate.edu/fch/sites/default/files/documents/sp_50_746_problemsandsolutionsjamsandjellies.pdf
The University of Tennessee says,
Remember the main spoilers of jams, jellies and preserves are molds. Molds can send ‘root’-appearing growths down into the jelly, which can produce toxins. Therefore, moldy jelly should be discarded.” Morris, William C. Low or No Sugar in Jams, Jellies and Preserves. University of Tennessee Agricultural Extension Service. SP325F(Rev.)-1.5M-5/04 . 2004. Page 1. Accessed August 2017 at https://extension.tennessee.edu/publications/Documents/SP325-F.pdf
Washington State University Extension (food safety and nutrition faculty) has one of its agents, Zena Edwards, being quoted as saying
… jams, jellies or any canned food with mold on it should be tossed (despite what grandma used to say).” Mapes, Diane. Homemade holiday food baskets may give gift of botulism. NBC News. 13 December 2010. Accessed March 2015
Even back in 1944, the Virginia Polytechnic Institute and State University knew of the danger:
It is not safe to remove the mould and eat the food, because certain types of mould reduce the acidity to a point where botulinus spores may develop.”  Cameron, Janet L. and Mary L. Thompson. Canning for the Home. Bulletin No. 128. Virginia Agricultural and Mechanical College and Polytechnic Institute and the United States Department of Agriculture, Cooperating Extension Division. Revised June 1944. Page 5.
All public health officials at all levels in Canada, from local to provincial to federal, actively advise against scraping mould off preserves and eating the preserves.
Health Canada advises,
Moulds have branches and roots that are very thin and thread-like, making them difficult to see when they grow on food. They are able to survive in both sugar and salty conditions – in opened cans of jam, jelly, and on cured salty meats such as ham, bacon, salami, and bologna as well as on foods left uncovered for long periods. The removal of mould on food does not remove the toxin and food with mould growth should be discarded (USFSIS, 2010).” Food Safety for First Nations People of Canada: A Manual for Healthy Practices. Health Canada. Ottawa, Canada. 2011. Page 15.
A food-handling guide from the province of Ontario’s Ministry of Health explains:
Some moulds make toxins called mycotoxins that can cause serious illness or infections. You can’t tell by looking whether the mould you see is one of the poison-producing types. Examples of toxins produced by moulds include:
• Aflatoxin often found in nuts, peanuts and peanut butter
• Ochratoxin A often found in grain, coffee and wine
Mould can grow on almost any food at any storage temperature and under any conditions. Freezing prevents the growth of mould but won’t kill any mould cells already in the food. The mould that you see on food isn’t the only mould that’s there. If it creates poisons, they’re generally under the surface of the food. Mould can be thought of like a plant. The part you can see is like the flower. Underneath that are roots inside the food that can make it unsafe. The softer the food, the further into the food the mould is likely to spread.”  Food Safety. A Guide for Ontario’s Food Handlers. Ministry of Health and Long-Term Care – Public Health Division, updated January, 2017. Page 30.
A resource binder from Ontario’s Ministry of Agriculture says
Mould develops from spores which are frequently found on food. The spores develop into silk-like threads that can look like streaks through a food or a mat of fuzz over the surface. Some cheeses, such as Stilton or Roquefort, have mould deliberately added to create their flavour.
Some moulds produce mycotoxins that are very harmful. Mycotoxins are chemicals that seep under and around the mould. They remain in a food even after the mould is removed. Liquid and semi-solid foods, such as jam and maple syrup, should be discarded if mould is found. Cheese that has mould confined to one area can be salvaged by cutting away the mould to a depth of 2.5 cm (1 inch). When in doubt, discard any food with mould. Discard any wrapping that has been in contact with mouldy food to prevent cross-contamination with other food.” Bartle, Barbara, et al. Community Food Advisor, Resource Binder. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). Section 6 : Food Storage and Preservation –Page 1. April 2013.
A high-school educational guide from the province of Manitoba teaches youth:
Discard mouldy jams, syrups, nuts, or grains. Mould spreads in these products and can be dangerous.” Grade 12 active healthy lifestyles. Manitoba Education, Citizenship and Youth. Winnipeg, Manitoba, Canada. 2009. RM 10–NU: Safe Food Guidelines, page 311.
Mould in tomato jars
Tomatoes that are just “bottled” (sealed in a jar, with no processing) are prone to developing mould. Besides the disappointing food loss, this renders the entire jar extremely unsafe.
Attempting to use mouldy jars of tomato products is particularly risky.
The growth of mold on the surface of improperly canned tomatoes can result in a potentially dangerous situation. Metabolites produced by the mold can lower the acidity of the tomatoes. The pH may be raised to a point where C. botulinum spores, if present, can germinate, multiply and produce toxin. Therefore, any moldy tomato product should be discarded in its entirety.” Penfield, Marjorie P. and Ada Marie Campbell. Experimental Food Science. San Diego, California: Academic Press. 1990. Page 273.
The authors of Putting Food By explain what happened to one family when mould spores in a jar of tomato weren’t killed off by heat processing of the jar:
…early in 1974 there were two deaths from botulism poisoning traced directly to home-canned tomatoes and tomato juice…. Meanwhile the public health officers discovered what actually allowed the spores of C. botulinum to make the toxin that killed the victims. Common bacteria or molds grew in the food in the jars and thereby reduced the acidity because the natural acid in the tomatoes was metabolized by the micro-organisms as they grew and developed. It was established after compassionate, but thorough, investigation that these bacteria or molds survived either because the tomatoes were canned by the discredited open-kettle method, or entered under the lid of a jar that wasn’t adequately sealed. Inadequate processing is virtually always the cause of food poisoning that develops during shelf life.”  Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 119-120). Penguin Publishing Group. Kindle Edition.
Does boiling the preserve make it safe again?
Most moulds are heat-sensitive and will be destroyed by being held at a temperature of 100 C (212 F), which is boiling water at sea level, for a given period of time.
As for the toxins (‘mycotoxins’) they produced and left behind by the time that happens, the boiling water temperatures won’t even come close to inactivating them.
Most mycotoxins are relatively heat-stable within the conventional food processing temperature range (80–121°C), therefore little or no destruction occurs under normal cooking conditions, such as boiling and frying, or even following pasteurization….In general, the application of a food process reduces mycotoxin concentrations significantly, but does not eliminate them completely.” Francesca Bosco and Chiara Mollea (2012). Mycotoxins in Food, Food Industrial Processes – Methods and Equipment, Dr. Benjamin Valdez (Ed.), ISBN: 978-953-307-905-9, InTech, Available from: https://www.intechopen.com/books/food-industrial-processes-methods-and-equipment/mycotoxins-in-food
So, concentrations of mycotoxins can be reduced somewhat, and particularly, with the right level of heat. What kind of heat?
… processing at temperatures greater than 150 degrees C is needed to give good reduction….. The greatest reductions …. occur at extrusion temperatures of 160 degrees C.” Bullerman LB and Bianchini A. Stability of mycotoxins during food processing. Int J Food Microbiol. 2007 Oct 20;119(1-2):140-6. Epub 2007 Jul 31. Abstract accessed August 2017 at https://www.ncbi.nlm.nih.gov/pubmed/17804104
So, 150 to 160 C for a “good reduction”. That’s 300 to 320 F. That’s about 35 lbs pressure at sea level in your home pressure canner — which of course can’t go anywhere near half that even. Autoclave Temperature and Time Pressure Chart. Accessed August 2017 at https://www.sterilizers.com/autoclave-time-temperature-pressure-chart.html
And even so, note that we are still looking at “good reduction” but “not completely.”
So no, opening your mouldy jars of jam, boiling them and rebottling them doesn’t make them safe.
The physical nature of moulds
Moulds can be more visible when they are in their spore producing stage.
Molds are made of many cells and can sometimes be seen with the naked eye…. In many molds, the body consists of:
- root threads that invade the food it lives on,
- a stalk rising above the food, and
- spores that form at the ends of the stalks.
The spores give mold the color you see… Molds have branches and roots that are like very thin threads. The roots may be difficult to see when the mold is growing on food and may be very deep in the food. Foods that are moldy may also have invisible bacteria growing along with the mold.” United States Department of Agriculture, Food Safety and Inspection Service. Molds On Food: Are They Dangerous? 2013. Accessed August 2017 at https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/safe-food-handling/molds-on-food-are-they-dangerous_/ct_index
What moulds are beneficial
There’s always going to be the people who say, “A little mould never hurt anyone. In fact, it’s free penicillin!”
So let’s address that point.
A small handful of moulds are indeed beneficial. Such moulds include the following applications.
- Aspergillus oryzae – soy sauce
- Botrytis cinerea – desert wines such as Sauternes and Trockenbeerenauslese
- Fusarium venenatum – quorn (vegetarian meat substitute)
- Geotrichum candidum – Reblochon cheese
- Neurospora sitophila – oncom (like tempeh)
- Penicillium camemberti – used in making Camembert and Brie cheeses
- Penicillium chrysogenum – used in making penicillin
- Penicillium glaucum – used in making Gorgonzola cheese
- Penicillium roqueforti – used in making Roquefort and Stilton Cheeses
- Penicillium nalgiovense – used in some cured salamis to reduce spoilage and improve flavour. Appears as a powdery white dust on the surface of the sausage
- Rhizomucor miehei – rennet for vegetarian cheeses
- Rhizopus oligosporus – tempeh
There’s a few more that you can add to that list.
Note, however, that there are at least 100,000 different species of mould — some people estimate up to 300,000.
Suffice it to say the list of moulds that are known to be “tried and true good for us” is very short compared to the list of those that are out there. That’s a lot of ones that are potentially not good for us.
Outside a lab, you really can’t guess just by looking whether a particular mould staring at you is good or bad.
The Guardian newspaper gets a food safety lecturer at Bournemouth University in England to explain:
Philippa Hudson, senior lecturer in food safety at Bournemouth University, [says] it’s important to say that not all of the Penicillium moulds are safe…. Some of them do produce toxins and you can’t necessarily tell which are the bad ones by looking at them. It’s not as if all green moulds are good, all white moulds are good and all brown ones are to be avoided.’….. How dangerous can moulds be? ‘Seriously dangerous,’ she says. ‘The genus Aspergillus, which grows on peanuts and peanut products, produces a group of toxins called aflatoxins. They can cause liver cancer – and cooking won’t destroy them.’ Daoust, Phil. Spoilt rotten: good and bad mould. Manchester: The Guardian. 26 October 2011.
Potential health impact of moulds
Moulds can be such bad health news that they register on the “Richter scale” of human history.
There is no doubt about the importance of mycotoxins in the human history. The first recognised acute intoxication was described in France in 945, when a large number of persons were ill of ergotism (2). This potentially fatal disease, caused by metabolites of ergot, has now almost disappeared due to the use of barley resistant to various Claviceps strains which produce the ergot alkaloids. The importance of application of hygienic measures in prevention of human exposure to mycotoxins was demonstrated in the eradication of so called »yellow rice disease« (shosin-kake in Japanese). This fatal cardiomiopathy similar to the lesions seen in beri-beri, was caused by citreoviridin, the metabolic product of Penicillium citreonigrum (3). The disease, common in lower social strata in Japan, disappeared immediately after the introduction of rigorous measures which excluded mouldy (or »yellow«) rice from the market. Another disease, that has not been seen in a severe form for decades and which involved a large number of persons, was the »alimentary toxic aleukia« common in the USSR (4). The population was exposed to trichothecenes from unharvested wheat contaminated by Fusarium moulds.” Peraica M, Domijan A-M. MYCOTOXINS IN FOOD AND HUMAN HEALTH Arh Hig Rada Toksikol 2001; 52:pp. 23–35. Accessed August 2017. https://core.ac.uk/download/pdf/14375433.pdf
Some people have an allergic sensitivity to mould spores.
While research on the overall impact of moulds on health is still ongoing, it’s pretty settled that moulds which produce something called “mycotoxins” do pose serious health threats to both people and animals. (Some people believe that pet dogs have been killed by mould toxins in dog kibble.)
Some molds are dangerous because they cause allergic reactions and respiratory problems in susceptible people. Some molds also produce mycotoxins that are poisonous substances that can make people sick. When a food looks moldy, the mold spores have already invaded deeply into the product. Mycotoxins are most often contained in and around these spores but may also spread throughout the food.” Burtness, Carol Ann. Are molds on foods dangerous? University of Minnesota Extension. Accessed August 2017 at https://extension.umn.edu/preserving-and-preparing/are-molds-foods-dangerous
When they see mouldy food, some people instinctively sniff it. Don’t. Inhaling the spores can be asking for trouble.
The impact of mycotoxins is long-term.
Mycotoxins are ubiquitous, mold-produced toxins that contaminate a wide variety of foods and feeds. Ingestion of mycotoxins cause a range of toxic responses, from acute toxicity to long-term or chronic health disorders.” Coulombe, R.A., Jr. (1993). Biological Action of Mycotoxins. J. Dairy Sci. 76:880-891. Accessed August 2017 at https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1034&context=advs_facpub
An FAO report says it’s probable that the effects don’t happen until a long time after you consume them.
Fungal toxins… are almost all low molecular weight chemical compounds which are not detected by antigens, and hence produce no obvious symptoms. Mycotoxins are insidious poisons… Mycotoxins can be acutely or chronically toxic, or both, depending on the kind of toxin and the dose. In animals, acute diseases include liver and kidney damage, attack on the central nervous system, skin disorders and hormonal effects… Toxins which act on the liver and kidney are especially difficult to detect and levels much lower than those producing acute effects are often carcinogenic. When eaten in minute quantities in the daily diet, they can cause cancers in experimental animals long after the time of eating. It is probable that humans can be affected the same way.” J.l. Pitt. An introduction to mycotoxins. In: R.L. Semple, A.S. Frio, P.A. Hicks and J.V. Lozare, eds. Mycotoxin prevention and control in foodgrains. UNDP/FAO. 1989.
Are bottled or refrigerated foods immune to mould
The University of Minnesota says,
Moulds grow best in warm, humid conditions, but can also grow at refrigerator temperatures. Molds tolerate salt and sugar and can survive on high-acid foods like jams, pickles, fruit, tomatoes and cured salty meats such as bacon, ham and bologna.” Burtness, Carol Ann. Are molds on foods dangerous? University of Minnesota Extension. Accessed August 2017 at https://extension.umn.edu/preserving-and-preparing/are-molds-foods-dangerous
The USDA Food Safety and Inspection Service says that even sealed, acidic bottled foods are susceptible to moulds unless the filled jars are heat processed:
Must Homemade Shelf-Stable Preserves be Water-Bath Processed? Yes, molds can thrive in high-acid foods like jams, jellies, pickles, fruit, and tomatoes. But these microscopic fungi are easily destroyed by heat processing high-acid foods at a temperature of 212 °F (100 C) in a boiling water canner for the recommended length of time.” United States Department of Agriculture, Food Safety and Inspection Service. Molds On Food: Are They Dangerous? 2013. Accessed August 2017 at https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/safe-food-handling/molds-on-food-are-they-dangerous_/ct_index
What foods with mould can and can’t be saved
Michigan State University Extension gives us the following list of salvageable versus not.
There are some foods than can be used if mold is discovered, however the list is short. Foods that can be consumed after finding mold include:
- Hard salami
- Dry-cured country hams
- Hard cheese
- Firm fruits and vegetables such as cabbage, bell peppers and carrots
The hard salami and dry-cured ham should have the mold scrubbed off the surface. The mold found on hard cheese and firm fruits and vegetables should be cut off at least one inch around, and below the mold. Be careful to keep the knife away from the mold itself so it does not cross-contaminate other parts of the food.
Many foods must not be used in the presence of mold. Michigan State University Extension advises not using:
- Luncheon meats
- Bacon or hotdogs
- Cooked leftover meat and poultry
- Cooked casseroles
- Cooked grain and pasta
- Soft cheese such as cottage, cream or chevre
- Yogurt and sour cream
- Jams and jellies
- Soft fruits and vegetables
- Bread and baked goods
- Peanut butter, legumes and nuts
These are foods with high, moisture content and are therefore more likely to have mold and/or bacteria growing below the surface.” Hart, Jane. Moldy food – what should I do? Michigan State University Extension. 5 December 2014. Accessed August 2017 at https://msue.anr.msu.edu/news/moldy_food_what_should_i_do
How safe is scooping away just a mouldy part of a soft food, such as jam or tomato juice? Researchers found that though botulism spore growth was greatest near the surface, where the mould had lowered the acidity, the toxin the spores released had permeated throughout the entire product:
Both Huhtanen et al. and Odlaug were able to demonstrate a pH gradient in tomato juice inoculated with molds when a heavy mold mat formed on the surface. The pH values near the mat were near neutrality, and the lower portions below the mat were more acid. Odlaug showed that C. botulinum growth was greatest at the surface and that the counts decreased with distance from the mycelial mat. Toxin was present through the tomato juice.” Odlaug,T.E. and Pflug,I.J. Clostridium botulinum and acid foods. Journal of Food Protection 41, 566-573. 1978. Page 570.
The University of Minnesota Extension reminds us to use fresh wrap on the foods we have salvaged:
In general, it’s best to throw out any food that has become moldy, with the exception of hard cheese, hard salami, dry cured ham and firm produce like carrots and bell pepper. Because it’s difficult for molds to deeply penetrate these products, they can be saved if they are not heavily affected by mold. To save the product, cut off at least 1 inch (3 cm) around and below the mold spot, keeping the knife out of the mold to prevent cross contaminating other parts. After trimming off the mold, re-cover in fresh wrap.” Burtness, Carol Ann. Are molds on foods dangerous? University of Minnesota Extension. Accessed August 2017 at https://extension.umn.edu/preserving-and-preparing/are-molds-foods-dangerous
Moulds are not bothered by acidity
A high acid level, such as can be found in a jar of jam, or pickles, or chutney, is not going to deter some moulds from taking hold.
Yeasts and molds are generally more acid tolerant than bacteria and can grow at lower pH values. Foods with pH values below 4.5 are usually not easily spoiled by bacteria but are more susceptible to spoilage by yeasts and molds.” Department of Food, Nutrition, and Packaging Science, Clemson University. Explain what pH is and how it relates to bacterial growth. Accessed August 2017 at https://www.foodsafetysite.com/educators/competencies/general/bacteria/bac3.html . 2012
The USDA points out, “molds can thrive in high-acid foods like jams, jellies, pickles, fruit, and tomatoes.” United States Department of Agriculture, Food Safety and Inspection Service. Molds On Food: Are They Dangerous? 2013. Accessed August 2017 at https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/safe-food-handling/molds-on-food-are-they-dangerous_/ct_index
Incidentally, there are some other nasties that also don’t mind acidity. Some strains of e-coli, listeria and salmonella have been noted in labs to be resistant at times to acidity.
“While these pathogens [Escherichia coli O157:H7, Salmonella enterica, Listeria monocytogenes and other pathogenic bacteria] do not grow in acidified vegetables, they may survive long enough to cause disease. The infectious dose for E. coli O157:H7 may be as low as one to ten cells. For this reason, acidified vegetables must be processed to assure a five log reduction in acid resistant pathogenic bacteria….E. coli O157:H7 has been found to be the most acid resistant pathogen of concern for these products….The research done here documents how innocuous items such as pickled onions could hold E. coli that aren’t immediately killed by the vinegar, and what must be done to ensure its destruction.”  Bredit, Frederick, et al. Use of Linear Models for Thermal Processing of Acidified Foods. In: Food Protection Trends, Vol. 30, No. 5, 2010. Pages 268–272. Accessed March 2015.
All those above nasties, though, including mould, are eliminated simply by proper boiling-water canner heat processing of the filled jar of preserves.
Moulds can work to lower the acidity
Not only do many moulds not mind acidity, they actively work to lower it.
Moulds grow seen and unseen (some are really only noticeable when they produce spores), and when they do grow, many will lower the acidity of the medium they grow in. When the acidity level gets to be low enough, any botulism spores present that had previously been held neutralized by the acidity can then spring to life and produce the botulism toxin. That is how people have in fact got botulism from improperly home canned jams.
An English report from the Institute of Food Research in Norwich in 2000 summed up incidents of botulism in home-canned fruits caused by moulds which had raised the pH (i.e. lowered the acidity) enough to allow botulism spores present to germinate.
There are several reports of botulism associated with heat-processed acid fruits with a low pH (Odlaug and Pflug 1978); in each of these cases there was evidence that growth of yeasts or moulds in the product had raised the pH sufficiently to allow growth of C.botulinum. Two deaths from botulism were attributed to home-canned pears, which were stated usually to have a pH lower than 4.0. Yeasts were found to have grown in the pears and the pH of the toxic samples was ³6.0 (Meyer and Gunnison, 1929 cited by Sperber 1982). In the 76 years prior to 1978, 34 outbreaks of botulism were reported in the US that were associated with home-canned, high-acid foods and in every case there was evidence of concomitant growth of yeast or mould that would raise the pH (Odlaug and Pflug, 1978). Thirteen of these outbreaks were associated with fruit or fruit juice, 18 with tomatoes or tomato preparations and three with pickles.” Barbara M. Lund and Michael W. Peck. EVALUATION OF THE RISK OF GROWTH AND TOXIN PRODUCTION BY CLOSTRIDIUM BOTULINUM IN SELECTED NEW PRODUCTS OF CONCERN. Food Safety Science Division, Institute of Food Research, Norwich Research Park, Colney, Norwich. 2000. Page 21.
Here’s a brief review of a few examples of how mould transformed normally safe home-canned products (raspberries, cherries, etc) into environments were botulism was able to form.
Other investigators have studied the interaction of microorganisms with C. botulinum in acid media. Tanner et al. detected botulinum toxin in low acid fruits inoculated with C. botulinum spores where Penicillium sp. or Mycoderma sp. had grown in the food and shifted the pH from 3.25 from 4.9 to 5.4 in raspberries, and from 3.75 to 5.1 to 5.3 in cherries. Lagarde and Beerens observed that a contaminant such as Trichosporon was able to raise the pH of pear syrup from 3.8 to 4.8 where C. botulinum produced toxin. Huhtanen et al. were able to demonstrate toxin production in tomato juice (pH 4.2) where a Penicillium sp. or Cladosporium sp. was present to raise the pH at the surface above 4.6. Odlaug was able to demonstrate C. botulinum toxin production in pH 4.2 tomato juice if Aspergillus sp. was present to raise the pH at the surface above 4.6.” Odlaug,T.E. and Pflug,I.J. Clostridium botulinum and acid foods. Journal of Food Protection 41, 566-573. 1978. Page 570.
Improper canning methods lead to moulds in jars
If you are following a modern, tested home canning recipe, including its processing directions, you should never see a jar of mouldy preserved food on your shelf unless the seal on the jar fails.
Using the old methods of just whacking hot food in a “sterilized” jar, putting a lid on and calling it job done (this was called “open-kettle” or “over-flow”), mould is very common and leads to a load of food wastage.
A New Zealand home canner noted,
Jam and marmalade almost always needed to have the mould scraped off before we could eat it when a new jar was opened…but it was a thing that everyone expected to happen, so no-one ever worried about it. We used to joke about getting our measure of penicillin, but it also meant we wasted a lot of jam.” Blog posting by user Ansy Pansy from Opotiki, New Zealand. “Reminisce Board” on https://www.grownups.co.nz/ . 9 December 2009. Accessed March 2015 at https://www.grownups.co.nz/discuss/show/id/3408/page/32
Dr Elizabeth Andress, PhD, head of the National Center for Home Food Preservation, warns:
An old out-dated method of canning — the open-kettle method — is now considered unsafe… This method results in a very real danger of botulism with low-acid foods or acid foods that experience mould growth.”  Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 18
The authors of Putting Food By say,
Mold is one of the leaders in the air-borne danger brigade, and it can settle on the underside of a canning lid and grow. In the process of growing it can metabolize the safe margin of acid just enough to allow surviving C. botulinum spores to develop and throw off their wicked toxin. So your jar of supposedly “safe” open-kettle-canned tomatoes—or dill pickles or jams or condiments or pears or peaches, all of which traditionally have been regarded as strong-acid enough to be protected—may contain a deadly threat. And aside from botulism, there could be mycotoxins from mold itself.”  Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (pp. 60-61). Penguin Publishing Group. Kindle Edition.
The Ball Blue Book said,
Our grandmothers, and even our mothers, were content to lose entirely many quarts of fruit each year; and they were never surprised to find a layer of mold on top of each jar.”  Ball Blue Book, Edition E. 1920s
Some people who don’t process their jars of jam rely on reduced “water activity” to inhibit mould. The theory is that the jam contains just so much sugar binding up so much water in the product that not enough is left for moulds to use to grow.
The water activity of pure water is 1.0 and goes down to zero when absolutely no water is available. Plants tend to wilt when water activity in soil reaches 0.98, maple syrup has a water activity of 0.90, jam 0.80 and dried fruit 0.7. A few fungi can survive water activity as low as 0.65 so if any of these find their way into the pot [Ed: jar] after opening they may be able to grow.” Isaac, Susan. Why does jam go mouldy, even in the fridge? Department of Genetics & Microbiology. University of Liverpool. Mycologist Magazine. Volume 7, Issue 4, November 1993. Page 198.
So some moulds can handily flourish in the low water activity of jam, if the spores are left alive by the jar just being sealed and not heat processed. Without the heat processing to kill them, you’re actually doing them a favour by just sealing the jar and giving them a nice, protected playground with lots of yummy food inside.
Remember that the air itself is full of unseen mould spores, the same ones that can turn a loaf of bread mouldy in short order in humid weather. Even as you work quickly to fill your “sterilized” jars of jam, these invisible mould spores in the air are settling on the surface of the jam newly-ladled into the jar, onto your waiting lids, onto your wax or cellophane discs, onto the spoon you are using to fill the jars, onto the cloth you are using to wipe the jar rims — you hands will never be fast enough to “outrun” them in the kitchen.
Consequently, modern canning always uses heat processing of filled and closed jars as part of the procedure to ensure a safe, quality product. The processing ensures that no mould spores can play a helping hand to botulism spores or produce toxins of their own.
Reduced or sugar jams are vulnerable to mould once opened
Even when properly processed, and refrigerated after opening, low-sugar jams are susceptible to mould in the refrigerator. This is why most such recipes recommend they be canned in smaller jars, so that they can be used up more quickly (generally, such home-made jams, with no other preservatives, should have a shelf-life in the fridge of 4 to 6 weeks.)
William Morris, Extension Agent at the University of Tennessee, writes:
Because these contain less sugar than regular jams and jellies, they are more susceptible to mold growth. Reduced-sugar jams and jellies may require longer processing in the water bath canner to kill these micro-organisms that might cause spoilage.” Morris, William C. Low or No Sugar in Jams, Jellies and Preserves. The University of Tennessee Agricultural Extension Service. SP325-F. https://ag.tennessee.edu/foodscience/Documents/Low%20or%20no%20sugar%20in%20jams,%20jellies%20and%20preserves.pdf
But as Morris points out, and as we saw in the section above on processing times, this issue is dealt with by longer processing times in the water bath, which your tested recipe from a trusted source will already have provided you with. As well, no-sugar needed pectins from Kraft® (Sure-Jell® ), Mrs Wages® (Lite Fruit Pectin) and Jarden (Ball® and Bernardin®) also may have preservatives added to them to help out.
These ‘lite’ or nosugar pectins usually have mold inhibitors added in the form of potassium sorbate, potassium benzoate or sodium benzoate.”  Morris, William C. Low or No Sugar in Jams, Jellies and Preserves. The University of Tennessee Agricultural Extension Service. SP325-F. https://ag.tennessee.edu/foodscience/Documents/Low%20or%20no%20sugar%20in%20jams,%20jellies%20and%20preserves.pdf [Ed: Pomona pectin does not.]
In consolation, though, note that sugar-laden jams aren’t safe from mould concerns, either: “Despite the high-acid content of fruit and the large quantities of sugar that are used to make jellied products, mold growth often occurs on the top.”  Boyer, Renee R. and Julie McKinney. Boiling Water Bath Canning: Including Jams, Jellies, and Pickled Products. Virginia Cooperative Extension. Publication 348-594. 2013. Page 12. Accessed March 2015 at https://pubs.ext.vt.edu/348/348-594/348-594_pdf.pdf Water bath processing minimizes the risk. Ibid. Page 12.
Burtness, Carol Ann. Are molds on foods dangerous? University of Minnesota Extension.
Hart, Jane. Moldy food – what should I do? Michigan State University Extension. 5 December 2014.
Jacewicz, Natalie. Is It Safe To Eat Moldy Bread? National Public Radio. 21 April 2017.
J. W. Bennett and M. Klich. Mycotoxins. Clin Microbiol Rev. 2003 Jul; 16(3): 497–516.
United States Department of Agriculture, Food Safety and Inspection Service. Molds On Food: Are They Dangerous? 2013.