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Every morning after breakfast, I drink a large mug of nespresso coffee with a large and a small dose from the same capsule, around 300 ml in total I think, with 2 lumps of cane sugar and 4 pellets of stevia (I want my coffee REALLY, REALLY sweet). I usually drink a barrista creation, like caramel, vanilla or (when they're in season) pumpkin spice cake. I also drink a large glass of water alongside this, one of those large beer glasses with a separate foot that are used for European specialty bears.

Sometimes, about once or twice per week, I want more coffee the same day, somewhere between 2-5 hours later, so I get a refill. And every day I also drink a large glass of decaf coke zero after lunch and another after dinner, and usually a small can of lipton ice tea of around 15-25cl once per day.

I use the same mug and glass for this because I only have 3 mugs and 3 glasses of this size and the timing of other dirty kitchenware usually works out such that I have a full enough dishwasher every 3 days, so I always have a clean mug and glass every morning.

I usually don't rinse the mug first because I always assumed that the heat of the coffee would be enough to kill any pathogens that might have cropped up. I also generally don't rinse the glass because they're hard to dry by hand, and rinsing it but not drying it affects the carbonation in the soda. However, lately I'm wondering if this might bring any food safety risks.

Are there any food safety risks related to reusing the same mug and glasses between 2 and 5 hours later without a rinse first?

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    The sugar might make a difference, but plenty of people routinely do similar things - I properly wash my mug in work once a week if I remember, but that's used for unsweetened black coffee, and drained every time, while hot enough that it dries after drinking. Otherwise it gets a rinse at most. I reckoni t would be nicer to rinse the glass anyway, because the flavour combinations would be pretty horrible.
    – Chris H
    Jan 26 at 16:42
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    Not going to answer this, because the answer is according to food safety standards you shouldn't do that, because you wouldn't do it in a restaurant setting, but that's not really applicable to you.
    – FuzzyChef
    Jan 26 at 19:22
  • So the coke/iced tea goes in the glass used for water?
    – bob1
    Jan 26 at 20:44
  • @bob1 Yes. AFAIK the prior presence of water doesn't affect the taste, but I'm not sure if it might be negative for other reasons.
    – Nzall
    Jan 26 at 21:44

1 Answer 1

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As mentioned in the comments, you would never do this in a commercial setting, though there are plenty of commercial settings where "unlimited top-ups" or "bottomless cups" occur. In these cases I think the expectation is that the customer is unlikely to be sitting around all day, rather eating and drinking within a relatively short time-frame (1-2 h) and dilution as well as other factors such as time, inoculum volume, pH, temperature will have some effect here.

This answer is going to assume that you drink the drink relatively fast (say within 30 minutes of pouring) and that it mostly doesn't sit around for the 2 hours or so between drinks - so you have some residue, that will be contaminated with microorganisms from your skin/mouth that sits at the bottom of the glass/cup and that this may or may not dry out in the interval between drinks.

There are very few studies that I could find that looked at the effect of bacterial growth in drinks. You can look for scientific papers on many topics on the PubMed database administered by the National Library of Medicine and the National Institute of Health. The vast majority of studies I could find related to drinks were for spoilage of the various drinks at the production/shipping/storage stages.

In a drink with a short duration, such as in your scenario, you don't need to worry about fungi apart from yeasts, as these all grow very slowly, generally needing days before they show up at a concerning level and are usually easily visible. Bacteria make up the vast majority of food poisoning cases, but yeasts can cause food spoilage too.

If the glass/cup dries out between drinks, then it is very likely safe, though there are a few relatively uncommon pathogenic bacteria that can survive this sort of dehydration and start growing again once rehydrated.

In general, the best (for the microorganism that is) you can expect of bacterial or yeast growth is one replication cycle per 20 minutes, generally much slower for yeast than bacteria. This happens under ideal conditions of moisture, growth nutrients, pH and temperature.

For a common food spoilage bacterium such as Escherichia coli (also part of our natural and normal microflora; not always pathogenic), the conditions will be lots of available water, pH around 7.0, lots of carbohydrates and temperature around 37 C (98.6 F). Variation outside these conditions will slow growth. Most pathogenic bacteria will not grow or grow very slowly below 4 C (39 F), which is why food safety rules state to cool food and put it in the fridge for storage. Almost all will not tolerate pH below 3.0, which is one common preservation method - pickling (gherkins, tomato sauce etc all have low pH). All bacterial, fungal and yeast (which are actually a special sort of fungus) will not grow below certain levels of moisture, called water activity.

I had a look around the literature (see the searche I did here and here) and found that the drinks you should be most concerned with are things like fruit juices and teas (see review by Kregiel here 1, as these typically have a relatively high pH (i.e. more than 3) and more ideal conditions for growth - better range of nutrients than just sugar (or sweetner), water and acids as in carbonated soft-drinks. However, the few studies I found looking at the drinking scenario (see search link above), particularly Sheth et al., (1988)2, found that known pathogens were inactivated/dead within 48 hours of contamination, and that diet colas were sterile within 48 hours. Watanabe et al (2014)3 also looked at beverages being consumed, and came to the conclusion that they were best consumed more or less immediately (i.e. short time-frame, not leaving them for 48 hours as in their study), as there were pathogens that could grow in them over time. However, note that they were growing the bacteria contaminated drinks at 35 C (95 F), which is not generally room temperature for most of us, so growth would be slower.

No one will have studied your particular combination of coffee, sweeteners, etc., because there are so many possible combinations. However, in general coffee is hot. Black coffee is also fairly acidic, so lower pH (4.8-5.1 apparently) From a food safety perspective, temperatures over 65 C (149 F) for 5 min are considered safe for consumption. Low pH will also inhibit bacterial growth to some extent, but the addition of milk and sugars might well abrogate any effect from the pH.

Water itself doesn't normally support much in the way of bacterial or yeast growth, but this may not always be the case, depending on your water source and how much back-wash there is from your mouth and what is in the back-wash.

Long story short: There is some risk associated with using the same drink container over time. Your best practice is to not be lazy and wash it out between each drink, preferably with hot soapy water, or a hot rinse at minimum.

References:

  1. Kregiel D. Health safety of soft drinks: contents, containers, and microorganisms. Biomed Res Int. 2015;2015:128697. doi: 10.1155/2015/128697. Epub 2015 Jan 28. PMID: 25695045; PMCID: PMC4324883.

  2. Sheth NK, Wisniewski TR, Franson TR. Survival of enteric pathogens in common beverages: an in vitro study. Am J Gastroenterol. 1988 Jun;83(6):658-60. PMID: 3287903.

  3. Watanabe M, Ohnishi T, Araki E, Kanda T, Tomita A, Ozawa K, Goto K, Sugiyama K, Konuma H, Hara-Kudo Y. Characteristics of bacterial and fungal growth in plastic bottled beverages under a consuming condition model. J Environ Sci Health A Tox Hazard Subst Environ Eng. 2014;49(7):819-26. doi: 10.1080/10934529.2014.882644. PMID: 24679089.

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