On Rot: Does Mustard Go Bad?

Food Writer Casey Corn reached out to me for an expert opinion on the perennial question screamed at spouses the world over from across the house while standing in front of a cracked fridge door… “HEY BABE? DOES MUSTARD GO BAD?” The article and my quotes were published by Epicurious, but my full correspondence went a bit overboard, and kind of ended up being the most thorough deep dive into rot I’ve ever had to put onto paper. So for posterity, here it is in it’s entirety.

CC: What are some factors that lead to food products going bad? How does refrigeration affect this?

DZ: The spoilage of food can come about in a few ways. First, there’s rot, like what you’d come home to if went on vacation and forgot to empty your fruit bowl. This is typified by any and all opportunistic organisms in the environment consuming that food source instead of, well, you. The exterior of food itself not to mention the air, is full microorganismic life in the form of tiny bacteria light enough to travel on motes of dust and settle wherever they may, and fungal spores as well. The longer food sits, the more time these microorganisms must make their way inside that food, multiply and consume all that they can touch. This microbial degradation goes part and parcel with the foods own deterioration. Even if you took a banana, and had the ability to somehow render it sterile, even within an inert bubble, it would still begin to brown and decay, as enzymes within the fruit’s flesh start to undo all they built, indiscriminately acting on cell structure once normal life processes (aka homeostasis) cease. Cells undergo programmed cell death, and “lyse” or burst open en mass. These processes alone make us turn up our nose. But bubble boy like bananas are indeed pretty rare. Outside of strict sets of circumstances that I’ll get into, it’s incredibly difficult to exclude microbes from the picture while leaving any other feature of your food “untouched” as such, the aforementioned decay also helps to make it much easier for microbes to dig in, effectively opening the door for them. As they feed, enzymes released by the microbes tend to further degrade the food’s structure, liquefying what was once solid, turning it mushy. And then there’s the smell, as the microbes grow and divide, the by-products of their metabolism build up in the food, some of which we recognize as foul smelling, like putrescine, (as is the case with animal flesh) which is a byproduct of protein degradation. The totality of these actions generally makes food pretty unappetizing, leading our food to a state most of us would consider “spoiled”, “off”, or “bad”. And that before we even get into the really bad parts of food going bad.

What must be understood is that in rot, everyone’s invited to the party, and from the moment a living food stops living (in the case of an animal’s slaughter or a plant’s harvest) microorganisms do all they can to feast, grow, and divide; increasing their total numbers, but importantly, do so while trying to increase their numbers relative to each other. To do this, many microbes have evolved a chemical arsenal that sees end products of their metabolisms include exclusionary chemicals, that is, chemicals that inhibit or deter the growth of other microbes, but incidentally also, to other, much larger, forms of life. To a bacterium gorging at the banquet table, you’re going hungry if another microbes edge you out of your dinner seat, but then again, it’s straight up game over if the dining hall itself caves in, as it might in the mouths of animals like us. There’s a whole panoply of chemical weapons within that cache, that facilitate deterrence, some of which are harmless to humans, while others carry varying degrees of toxicity. In organisms like filamentous fungi (say, what you’d find growing on a week old loaf of bread in your ktichen), mycotoxins are produced primarily to deter other microbes, you won’t get immediately sick from taking a bite of a sandwich and seeing whitish blue mold on the crust one bite too late, but that’s not that’s not to say it’s healthy. Consistent, repeated exposure can lead to increased risk of diseases like esophageal, or stomach cancer. But then again, ergot, a parasitic crop fungus that attacks rye in the field and in storage is responsible for Holy Fire, which can cause a variety of symptoms, such as burning sensations in ones, arms and legs, gangrene — as when whole limbs swell and fall off — convulsions, muscle spasms, severe diarrhea, and hallucinations (this last symptom is attributable to lysergic acid diethylamide, or LSD, and it is indeed from ergot that it was first isolated, and thankfully, its effects separated from the fungus’ other symptoms).

But if it’s not the molecules left behind by microbes that have grown through your food, the ingestion of large numbers of microbes themselves can lead to illness. For example, in what’s known as “fried rice syndrome” the bacteria Bacillus cereus, a normal member of the soil microbiome in many places, can come to contaminate grain supplies. Now this normally isn’t an issue as rice is cooked in 100°C water, which kills off most everything. The problem is that B. cereus is a spore forming microbe. When times get tough, say, like when rice grains are drying out for and harvest and packaging, the bacteria can sense diminishing quantities of available food in their surroundings, and as a last-ditch attempt to fight another day, will produce spores in their final moments in lieu of dividing into two daughter cells. Spores are like the time capsules of the microbial world. They’re much smaller than cells themselves, come with a hard case, and contain the minimum amount of genetic material within them to begin life anew once conditions become more favourable. Spores are also far more resilient then living or even dormant microbes. The internal machinery and cell walls of all but the most extreme and robust microbes will not be functioning properly above 50°C, and break completely above 70°C, ending their life. But bacterial spores can hold out in temperatures above boiling for several minutes. So even if, in the cooking of rice, live B. cereus bacteria may die, their spores can germinate and resurrect the population if conditions either rise or dip into the “danger zone” for extended periods of time. The danger zone is the temperature range between 10°C and 60°C, but more specifically between 18°C and 45°C, where bacteria can and will thrive, and in the latter case, will do so with a marked vigor, in the most extreme cases, dividing exponentially, forming a new generation every 20 minutes. Since many consider the best rice for fried rice to be day old, and slightly air dried, rice used explicitly for fired rice tends to spend a fair bit of time in the danger zone, where steamed rice kept on “keep warm” before being consumed never does — doubly so if the fried rice is prepare in advance and then reheated improperly before serving. With enough time, B. cereus populations can reach pathologically significant levels, with numbers strong enough to have a sizable number of individuals survive the journey through the acidic conditions of the stomach and make their way into the more basic small intestine. Once there, havoc is wreaked as the surviving bacteria actively produce enterotoxins that lead to nausea, diarrhea, vomiting and abdominal pain, symptoms that can last up to 24 hours and take hold just 30 minutes after consumption of the food in question.

So, in consideration of all the above, when we talk about food going “bad”, its usually the foods inevitable transformation at the hands of microbes in the environment, without restriction. As I mentioned before, it’s pretty difficult to exclude life from food, and that’s because to heterotrophic omnivores like us, food is after all, life itself. Life grows on life, but also upon and inside of it. The science of the human microbiome may be trending today but its nothing new, nor even special. It’s not just us that host microbiomes; all plants and animals have zoos of their own living on and in them, including cows, corn, apples, grains, and yes, even mustards plants. A living organism, through the maintenance of the cells that make up its boundary with the world (be that skin or bark) and the action of its immune systems, can keep those beings at bay, at the expense of work preformed (and energy spent) by its metabolism. This baseline status quo is homeostasis (staying the same, or still). But once metabolism ceases, all bets are off, the microbiome transforms into the necrobiome as the invited guests tear down the house. That’s rot for you.

The last thing to consider in the “bad” question is perhaps the most contentious. The best before date. Most food you buy in the grocery store comes with one. For things like milk in the refrigerated aisle, this potentially counts as the timeline where foods that can’t be made sterile, when kept at low temperature, will have whatever endemic bacteria are in the neighbour divide and multiply their numbers above some upper limit. There may be changes in taste that come along with that, say like a product becoming increasingly sour, but crossing that threshold by no means implies that the food is unsafe to eat. Products of fermentation like yogurt, that have inherent preservatives naturally baked into their creation don’t simply turn bad from one day to the next. As a butcher, I’ve wet aged steak for 45 days in the fridge to the benefit of the cut of meat, yet at the grocer, they’ll tell you to cook it within 2 weeks or throw it out. Food manufactures do this primarily as a way of managing liability. As they say in the biz, CYA. The downside to all this though, is the enormous amount of food waste wrought by huge amounts of perfectly edible food being thrown out “just in case”. The even worse side of best before dates are those attached to shelf stable foods, things you’d find in the dry goods aisle at ambient temperature. Barring an accident where water enters a package or a container gets cracked, when you see an expiry date labeled for a year or two down the road, there’s no real way to say when or even if the product would actually go bad. What manufactures in those situations are most concerned about is the perception of “staleness”. In the same way that wine can change and mature just by sitting around in the bottle for a couple of years, so too can tomato sauce. But no one’s throwing out 5 year old wine because it’s undrinkable and dangerous to your health upon consumption. Well, the same is true for Chef Boyardee. There are slow moving but marked chemical reactions that will take place at room temperature over the course of months and years. But that doesn’t mean that the products of those reactions can harm you. It’s just that most trained sensory panels would agree that only a certain amount of deviation from a taste profile on day 1 is permissible to a consumer packaged good, and at some point, its better to clear the shelves — of a grocery store, or one’s home — than have the brand image suffer from something that wasn’t off, but wasn’t perfect.

So how do you keep rot at bay? Well, there are a few ways to do this. First and foremost, its important to note that all living processes require water. DNA can’t be read and translated into protein without it, nor can most cells keep their most basic machinery running when it’s supply gets low. So starving life of water is one of the best ways to preserve food, and make sure microbes don’t have the water they need to wet their palates. Drying food out through dehydration was one of the first means humans had to preserve the foods they foraged and hunted, think jerky and dried seeds. Once desiccated, they can keep for a long, long time indeed. Using any means at your disposal to take water off the table for life processes is is the act of “lowering the water activity level”. A bowl of fresh jello effectively has a water activity of 1. Sun bleached fish bones, a water activity of 0. All organisms have their own respective crucial level of water activity they can’t dip below, lest they shrivel up and die. Various factors within an organisms environment will dictate what they water activity in their surroundings is like.

Some of those factors are chemical. Even without physically removing water, water can be made unavailable to life processes through osmotic pressure. Water is the universal solvent. Via the addition of ionic compounds like salts or hygroscopic ones like sugar, the water in an aqueous, hetergenous medium will become electrochemically drawn away from regions of low concentration of such additives to regions of high concentration, making it unavailable for life processes. To a microorganism, this osmotic pressure produces the same effect as straight up dehydration, and all sees cells shrivel up and die.

Next, there’s pH. That’s the relative concentration of hydrogen cations aka protons (H+) in an aqueous solution. Water molecules (H2O) have a tendency to freely dissociate a little bit. Maybe a molecule gets jostled too strongly one way or the other, and shakes a hydrogen nucleus loose, leaving a hydroxide ion (OH-) behind in its wake. But so long as those products appear in equal measure, there’s no net change in the charge of the solution, as the dancing partners will eventually pair back up again. But certain chemical processes, like oxidation, can alter that ratio. The point is that whether its an acid or a base, a lone electrically charged ion wants to find a partner to dance with, desperately. And if enough cations (+) or anions (-) build up in solution, they’ll start ripping apart other inert molecules in their surroundings in order to do so. That includes the cell walls of a microbe and the enzymes and proteins it uses to run its machinery and build its structure. Now, life can soak a little bit of damage, but the pH scale is logarithmic. So while you start at 7 (which is neutral, distilled water) a jump or drop of just 3 numbers is in fact a 1000 fold increase or decrease in the relative ionic concentration. Now, due to quirks of planetary evolution and biology, the living world tends to be more acidic than alkaline, so even though both ends the of the pH scale are antibiotic, acids are what tend to be employed more regularly to keep food safe. Below a pH of 4.5, more microbes can’t grow, or degrade your food, and it will last, pickled on your shelf just fine.

Lastly for the sake of the your question, there is also the question of temperature. All organic molecules, as in the most basic building blocks of life that make up cells and organelles and intracellular structures, have a temperature range in which they operate ideally. When you burn your hand on a hot pan handle, its because the metal of the pan contained more thermal energy than the flimsy proteins in your skin were able to conduct without being knocked out of shape and torn apart. That thermal damage, the literal destruction of structure is the upper limit of temperature for many organisms. But well before proteins are broken up and “denatured” with the energetic molecules in their surroudnings, normal life processes can also stop functioning well. Think of it this way, your body sits at a cozy 36°C, and your completely comfortable in a 21°C room. That’s because your body has to shed heat to operate ideally. You can’t survive long in a setting that’s hotter than you without doing additional work to stay cool. If you didn’t, many of the chemical processes that your cells rely on to fuel their metabolism couldn’t take place of their own accord. Many of those reactions, like the burning of sugar inside your cells, are what’s called exothermic, meaning they release heat when they take place. But to release heat, the surroundings have to be cooler than the temperature of the reaction taking place. One person can’t get out of a room by pushing open a swinging door that’s being pushed on by 3 people on the other side trying to get in. If an organism’s surrounding get too hot, bit by bit, individual pieces of metabolic chain reactions will begin to fail, and life will cease.

Likewise for the cold. If there’s not enough background heat energy around to get a chemical reaction close to its reaction point, it doesn’t matter how much you dump in, it aint gonna happen. For example, if you had to fill a thimble with water and all you had to so was one match and one ice cube, you’d obviously reach for an ice cube that was already at -1°C, not the cube fresh out of liquid nitrogen at -196°C; if you did, you’d spend all your time fuel heating up a brick that never cross the crucial threshold of interest. Well the same is true for reactions inside cells. When the surroundings get too cold, it begins to take more and more energy to keep systems running optimally, as many of the enzymes that operate the machinery of life have very narrow windows for their reactivity. In mammals, we work very hard to keep our body temperature stable. Cold blooded animals like lizards, have to actively seek out warmth to operate well, lest they slip into a “cold-stunned” state. Microbes act a bit like cold-blooded animals, even if they don’t have blood, per se. South of their optimum temperatures, their metabolism slows to a crawl as they selectively choose what processes are the most essential to keep running until things warm back up again. Importantly, cell division is one that takes a back seat, as many bacteria will prioritize keeping themselves alive in that instant over producing progeny, and dividing. At fridge temperatures, 4°C-8°C, life slows to a crawl, which in turn, slows the decay. If you were paying attention, you’ll notice there’s a subtle irony there. Decay is life.

Lastly, and as a quick one, we need to talk about freezing. Below 0°C, water molecules lock themselves into a lattice and stop being able to slide past each other, act as a solvent, or be handled by enzymes for hydrolytic processes (cleaving by holding onto a water molecule and using it like a magnetic hammer). The solidification of water also reduces water activity levels, effectively driving them down to 0. Because if water’s locked up in ice, then you can’t get at it. And even though water expands as it freezes and can damage cell walls in the analogous way to extreme heat (the reason why frostbite hurts like a burn once your back inside) many microbes can survive being frozen and thawed without any damage to their body. That’s why if you freeze food (with normal measures) only presses pause on the reel of rot, as soon as its thawed, life starts back up again. But so long as it stays frozen it won’t rot (though it can suffer freezer burn via the sublimation of ice in a freezer over long timescales if the food is improperly stored, but like best before dates, that’s more about taste profiles than actually spoilage of the food itself).

In keeping rot at bay, its important to note that these buffs stack. A low acid food, with a high salt content that’s also kept in the fridge after being sterilized is more resistant to microbial encroachment than a food that’s seen only one method of preservation applied. And indeed, there are many foods that do just that to ensure their safety and edibility through time, both historically, and contemporaneously. Mustard included.

Speaking of keeping rot at bay, we’ve yet to talk about the single most important discovery in the history of microbiology, that kind of launched the field of microbiology in many ways. Pasteurization. It’s technically linked to the heat treatment described above, but encompasses much more than that, basically a paradigm shift in human understanding.

You see, the desire to make food last is an ancient one, and one so strong that often shaped the fates of nations at war. The Phoenicians and Romans used concentrated garums as fuel for their armies, and battled over the factories used to produce it. As Napoleon would say: “An army marches on its stomach.” In fact, Napolean believed that so ardently that he created a national award of 12,000 francs to improve upon the prevailing food preservation methods of the time. The purpose, of course, was to better feed his army “when an invaded country was not able or inclined to sell or provide food”. Over a decade after the prize was first announced, it was claimed by one Nicolas François Appert, a candymaker who’s winning innovation was inspired by the process of bottling wine. Appert placed food in large jars, close off the opening with a wide cork, and seal it with hot wax. Afterwards he’d wrap the jar in canvas, then boil the whole thing in water. Genius, yes, but blind. he was never able to explain the science behind why his method worked. In the book he published on his findings, Appert could only reason that there had to be an “absolute deprivation from contact with the exterior air” and “application of the heat in the water-bath.” In fact, no one in his lifetime would have learned the true mode of action. Fast forward fifty years to the northern French town of Lille, an industrial town peppered with factories, breweries, and wine bottling plants. The town in which Louis Pasteur was hired to be the dean and professor of chemistry at the Faculty of Sciences at the local university. Once hired, due directly to his proximity to the region’s high concentration of French food industry, he wound up setting to task to understand and alleviate many of those industries’ problems through science. The wineries had to constantly fight to prevent their vintages from souring to vinegar. Breweries and bakeries had to deal with skunky yeast. Much of Pasteur’s work on organic chemistry was rooted in these studies of fermentation. Under the mechanical eye of his microscope, Pasteur categorized and distinguished between the zoo of microbes populating these fermented foods. He isolated varieties and typified those responsible for the crafting of fine wine, then spoiling of that wine to vinegar, or unwelcome invaders that had the potential to turn the whole thing afoul. It wasn’t long before Pasteur clued into the fact those very lessons in fermentation stretched beyond the barrel, and spilled out into the human world. As evidenced by a 1959 paper published by Pasteur on microbial fermentation in which he notes that “everything indicates that contagious diseases owe their existence to similar causes.”

Pasteur is heralded as the Father of Microbiology for his disproval of spontaneous generation. Through cleverly constructed experiments he demonstrated that the established understanding that food would spontaneously spoil if left alone to it’s own devices (as in, completely of its own accord), was erroneous. For millennia our best guess, spontaneous generation, was the answer for the unseen and a placeholder for our ignorance, harking back to the time of Aristotle. Back then, in the world of the ancient Greeks, it held that from dirty rags left alone in a cellar, mice would inevitably spring forth, and that a log left at the bottom of a river would invariably become a crocodile. Through the most basic and careful observations in the two thousand years hence, the concept of spontaneous generation of the macroscopic was ushered aside, but humanity was still left at odds with how to explain the curdling of milk, the transformation of juice into wine, or wort into beer, or good food into bad. The kernel of spontaneous generation held fast well into the 19th century, until Pasteur devised a simple yet ingenious experiment to do just that using swan necked glass flasks filled with nutrient rich broths. Pasteur had them boiled in order to sterilize them. The shape of the vessel, with its thin, long, winding neck, made it impossible for (what we now know of as) wild bacteria floating in the air to reach the main chamber of the flask, and consequently, the broths did not spoil. Once the neck of one of the flasks was broken off however, bacteria floating on particles of dust could fall into the broth and propagate, leading to spoilage. It was a simple but illuminating experiment that fixed the idea of biogenesis (Omne vivum ex vivo or “all life from life”) as the underpinning of all modern biology. If you’d ever wondered where the term pasteurization comes from, it’s here. The practical ramifications of these experiments led to the widely adopted industrial practice of pasteurizing wine, milk, and beer, simple procedures that had huge effects on global public health.

It was known before Pasteur’s time as folk knowledge that the heating of products like milk could help to “keep it sweet” for longer than unboiled milk. But the link between microbial life, sterilization, and hermetic isolation was not made until Pasteur rigorously proved that germ theory lay at the core of all transformations of food, good or bad.

It’s hard to understate the effects the advent of pasteurization had not just on food, but humanity. If you consider any typical grocery store by shelf space ANYWHERE on earth, most of it is pasteurized. That is to say, inert, canned, and unlikely to spoil like fruits in the green aisles are. Even traditional means of preservation, like lactic or acetic fermentation, find their modern mass produced incarnations pasteurized anyways, simply to eschew any liability that could come about, lest anything go a tiny bit wrong; biology, after all, is the science of exceptions. Pasteurization removes any guess work from the equation. To get milk to last weeks in the fridge, heat it up to 90°C for 10 minutes. To get it to last years on the shelf, heat it up to 120°C under pressure and seal it in a sterilized Tetra Pak carton devoid of oxygen. To kill E. coli in your canned tomatoes, steam treat the can they’re sat in for 30 minutes. To kill botulism spores in your canned mushrooms, pressure can them until they reach 128°C and hold them there for 6 minutes. Its development changed what we could eat, and when, allowing us to nibble on asparagus in the depths of winter, and importantly, not die trying.

CC: How would this relate to a product like mustard? Are there any characteristics of mustard that give them a longer or shorter shelf life?

DZ: Well, mustard is made of just a handful of ingredients. But the ingredients it is made from are by themselves, all pretty resistant to spoilage. A typical recipe for mustard will call for mustard seeds, white wine, white wine vinegar, salt, and water. Individually, none of those things are going bad. Mustard seeds are dried in their harvest, so off the bat, they have a water activity level well below the threshold for survivability of any microbe. But the potent chemicals within mustard itself are the part of the plant’s antimicrobial defense mechanisms. So many of the flavours of herbs and spices we love don’t simply exist to thrill our human senses, they exist to ward off pathogens. Then there’s the wine, which is protected by alcohol, and the vinegar by an extremely low pH. Traditional Dijon mustard recipes call for verjus, the juice of unripe green grapes usually harvested at 8 weeks of fruit growth. It too has an extremely low pH and not much sugar there to serve as a food source. And any salt in the recipe is only helping to preserve the final concoction. The added water is the lynchpin. Mustard seeds do contain enough soluble carbohydrates in the form of sugars to fuel microbial metabolisms should they get wet enough (around 9% by mass). Any seed is after all, a free lunch for a baby plant packed by its mum. Many seeds are as perfect a food as eggs are. So get them wet, and you’ll find a whole slew of organisms wanting in on that free lunch too. That said, in the final recipe, there are so many other inherently preservative ingredients, even if the water activity level goes up during grinding, blending, and liquefaction, that doesn’t mean the environment is still one conducive to microbial growth. And like I said before, for most industrial food producers, just because a product that comes with a low pH, or high salt content doesn’t mean they’ll forgo a pasteurization step. It doesn’t have to be full fledged irradiated sterilization, but heat treatment steps similar to that employed in milk production still goes a long way in improving the viability of many condiment-like, shelf stable staples. Any big brand of mustard you buy in a grocery store, be it French’s or Maille, is heat treated in a clean facility, and they all come with instructions to refrigerate after opening.

CC: Would you eat mustard that’s been opened and kept in the fridge for a year? Two? Ten?

DZ: A year? Yes. Two? I’d smell it first. Most mustard on the shelf comes with a best before date inside of 12 months after manufacture. But as I stated before, its not like at 12:01 on New Year’s day that mustard spontaneously transforms into rat poison. My guess is, it looses potency above anything else. And we can all imagine why mustard companies, maybe more than other canned food producers, would want to ensure that their product delivers the indented punch. Ten years? I think if it had already been opened, it’d probably be real crusty inside and also smell like everything else that had been in that fridge over that ten year span, so no.

CC: Ultimately, would you say that mustard goes bad?

DZ: I would say mustard CAN go bad, but it’s a question of what sort of bad you’re talking about. Can it dry up and get nasty and crusty and taste stale after a time? Ya. Is the pH low enough to let it age like balsamic vinegar? It can be (most commercial mustard sits in a pH range between 3.9 and 6.0, meaning the former boundary is plenty safe, while the latter could legitimately leave you sick if it was sitting in the hot sun for a whole summer. But most squeeze bottles sitting out on hot dog vendor stands a) definitely have a pH much lower than 6.0 and b) just get used up too fast to cause problems. French’s yellow mustard has a pH of 3.9, well below the 4.5 that would qualify something as a low acid food capable of staving off pathogens like botulism. But then again, there are plenty of different types of mustard. Honey mustard adds more sugar to the occasion, but honey’s so full of sugar so hygroscopic that it doesn’t go bad on its own, it has to get pretty dilute to spoil. I think its when you get into other flavours of mustard that see ingredients like violet puree or fruit mixed in where you could potentially open yourself up to more opportunities for microbial degradation. That said, if you adjust your recipe accordingly, condiments like mostarda, can be considered just as safe.

In conclusion!!!

Basically, keep it in the fridge after opening. But no, you don’t have to throw it out on the day of its best before date. It’s by no means a guarantee its much worse after. Having said all of the above, I realize I didn’t even open into the wikipedia article until just now, and found a much more succinct answer to your question…

“Because of its antibacterial properties and acidity, mustard does not require refrigeration for safety; it will not grow mold, mildew, or harmful bacteria. Mustard can last indefinitely without becoming inedible or harmful, though it may dry out, lose flavour, or brown from oxidation.”

And that’s my final answer. Stay curious!

DZ