The Beekeeper’s Companion Since 1861
icon of list

The Classroom

The Classroom – October 2022

- October 1, 2022 - Jamie Ellis - (excerpt)

The Classroom - ABJ - Jamie Ellis
HMF and honey

 Jamie’s note: I received two questions about HMF and decided to list them both, before taking a stab at answering them.

I am wondering if it is chemically safe to feed very old honey back to bees or to use for human consumption. I know the warnings about feeding honey to bees because of the possibility of it containing American foulbrood, but just assume this is not an issue for the purpose of my question. I am specifically concerned about the presence of HMF (5 hydroxymethylfurfural) that may have continued to form and accumulate slowly over that time period when the honey has been stored at room temperature (at the higher end of that — closer to 80°F (26.7°C) in the summertime here in the south).

I have honey that has been stored this way for over 5-7 years. At some point a couple of years ago, it crystallized so I heated it gently in a water bath to around 110°F (43.3°C) for a short period to reliquefy it (it has not recrystallized since then). I know that heating honey above 104°F (40°C) degrades invertase [an enzyme that degrades — hydrolyzes — sucrose into fructose and glucose] but a temperature below that would not liquify mine. Otherwise, it has just been sitting in glass jars. It still tastes good but there may be a slight bitter note now in some — may be my imagination thinking so hard about HMF. Can a person taste HMF in honey? I know from “The Hive and the Honey Bee” (2015, page 692), and from personal experience, that heat and long-term storage of honey at or above room temperature negatively alter the flavor of honey and darken its color, but I do not know why.

HMF levels in high fructose corn syrup (HFCS) of 150-200 ppm (parts per million) are apparently toxic to bees. They show that storing HFCS at elevated temperatures (104°F) can increase the HMF starting levels of 10-30 ppm to twice that in just a month. At lower temperatures of 89°F (31.7°C), the increase was not as obvious and in some cases it decreased. HFCS is not honey so I know we need to be careful about extrapolating. But in “The Hive and the Honey Bee,” it says that at 86°F (30°C), it takes 150-250 days for honey to reach 30 ppm HMF although it does not say what happens after that. Honey with >40 ppm HMF from temperate regions cannot be sold for human consumption (I am not sure if that is due to a health or quality issue however).

My bottom line questions are: Would you expect HMF to continue to increase over an extended time in honey stored at around 80°F or does it peak at a certain level that is not toxic to bees and humans? Also, do you know if the 40 ppm level limit for human consumption is just a quality concern or actually also a health issue?

Suzy Spencer
April, North Carolina


I am wondering about old honey. I am helping the widow of an old beekeeping friend who has 10 years of honey still in comb. After about three weeks in a warming room at 105°F (40.6°F), it mostly liquefies and can be extracted, but it is very dark, of course. If I understand correctly, this is due to the formation of HMF over time in honey. Furfurals are bad for bees; so, are there enough furfurals in this recently extracted honey to be harmful? Or, can we sell the frames with the comb in it to beekeepers who will let their bees clean up the residual honey?

Tina Sebestyen
Colorado, July



To make answering Suzy’s and Tina’s questions easier, I copied/pasted below the exact questions they wanted answered, and then followed those questions with my answers.


1) Is it chemically safe to feed very old honey back to bees or to use for human consumption?

It depends. Generally speaking, old honey is safe for human consumption. The color and flavor may have changed, but the flavor is a matter of taste anyway. Of course, you do not want the honey to be fermented, which can happen to stored honey when the moisture content is too high. You also do not want the honey to have too much HMF. You can only know the levels of HMF if you test the honey.

2) What is HMF?

HMF is a compound produced when sugars degrade through something called the Maillard reaction, when the reducing sugars dehydrate. This is a non-enzymatic browning reaction, and it happens during the processing or long-term storage of honey. HMF is mostly absent in fresh honey and its concentrations increase during processing or aging. Heat, low pH, time, moisture content, presence of acids, and even container storage type can affect the development of HMF. As an example of the latter, metallic containers facilitate the production of HMF.

Interestingly, acids (including those in vinegar, lemon juice, etc.) can support the production of HMF (which is ironic since beekeepers often put these into sugar syrup to “stabilize” it). The Codex Alimentarius Standard for honey established the maximum limited for HMF at 40 mg/kg (or 40 ppm). Apparently, they allow 80 mg/kg in honey coming from tropical regions, probably because it is difficult to achieve low levels of HMF in honey in these warm climates. This helps ensure the product has not been overheated and is safe for humans to consume. Incidentally, most foods that contain sugar and are heated during processing contain HMF. It is a very common byproduct in sugar-containing foods.

3) At what temperatures does HMF form?

It begins to form naturally at low temperatures even around 68°F (20°C), but forms quicker at increasingly warming temperatures. This process is exacerbated in metal containers. This is especially important during the transport and storage of honey (or, for that matter, sucrose syrup or high fructose corn syrup feed for bees). Kept at room temperature, honey will only form very small amounts of HMF, usually levels that are not toxic to bees or humans. I saw an interesting table that shows how long it takes 30 mg/kg HMF to form at 30°C (86°F), 40°C (104°F), 50°C (122°F), 60°C (140°F), 70°C (158°F), and 80°C (176°F). It was 100-300 days, 20-50 days, 4-10 days, 1-2.5 days, 3.5 hours and <2 hours, respectively (White, J. W., I., Kushnir and M. H. Subers, 1964. Effect of storage and processing temperatures on honey quality. Food Technology, 18 (4): 153-156). You can clearly see the impact of heat on the formation of HMF. This same table is reproduced in the 2015 edition of “The Hive and the Honey Bee.” Traynor, K. 2015. Honey. In (J. Graham, ed) The Hive and the Honey Bee. Dadant and Sons, Hamilton, IL. 673 – 703.

4) What are the effects of HMF on bees?

The answer to this was not particularly clear to me. The general consensus is that HMF is not good for bees. It was shown to increase mortality in caged bees fed the product for 20 days. It is believed to cause bee dysentery and ulcer formation in the bees’ digestive tracts. This can lead to their death.

5) Can a person taste HMF in honey?

Apparently, it smells “buttery, caramel, musty, waxy. ” It tastes “herbal, hay, tobacco,” which is not really a helpful description to me since I am not an expert taster. I could only find the taste reference at one location. That is a little odd given that many authors noted the flavor of honey changes as it ages, this due to increasing HMF concentrations over time. Yet, they all stopped short of saying what that change in taste was! See

6) Would you expect HMF to continue to increase over an extended time in honey stored at around 80°F?


7) Does it peak at a certain level that is not toxic to bees and humans?

It does not appear to peak. The research I saw suggests that it continues to form as honey ages.

8) Do you know if the 40 ppm level limit for human consumption is just a quality concern or actually also a health issue?

This might be the most difficult question given I am not a medical doctor or toxicologist. I did read some research articles on this topic and the answers seem to be what I consider all over the place. Authors of one paper referenced a tolerable daily intake (TDI) of 132 mg/day, this with a built-in 40-fold safety margin. Based on this, my guess (and it is very much just a guess) is that the 40 ppm is below what would be a problem for humans. I did see a long list of suspected impacts of HMF on humans. They include irritant, carcinogenic, toxicant, enzyme inhibitor, etc. However, I also read about likely positive impacts on human health as well. These included antioxidant, anti-allergen, etc. Finally, I read that some risk assessment agencies suggest that HMF does not pose a health risk to humans. Given this is not really my field, I hesitate to speculate too much beyond what I have shared here.

9) I heated 10-year-old honey in the comb in a warming room at 105°F (40.6°C). The honey liquefied and is dark. Is this due to the formation of HMF over time?

It is certainly one possible explanation for the honey darkening. It is really impossible to know with certainty without having the HMF levels tested in the honey.

10) Is there enough HMF in this recently extracted honey to be harmful?

As noted above, it really is impossible to tell without knowing the level of HMF in the honey.

11) Is it safe for bees to rob this honey?

If the honey built up sizable levels of HMF over the time you mention, then it would not be safe for bees. If it did not build up sizable amount of HMF, it would be okay. At this point, the only way to answer the question is to have the HMF level determined. I am sorry that this seems to be my answer over and over. However, my motto here: When in doubt, throw it out.

There is a good recent review on HMF. You can find it here:

Shapla, UM, Solayman, M, Alam, N, Khalil, MI, Gan, SH. 2018. 5-Hydroxymethylfurfural (HMF) levels in honey and other food products: effects on bees and human health. Chemistry Central Journal, 12, 35:

Honeydew honey

I found this quote online: “Dark honeys, which are high in minerals, are harder on bees than light honeys. When bees face long periods with no cleaning flights, very dark honeys, which contain many indigestible solids, can nearly be a death sentence.”

If you could address/expound a little on this please, it would be very helpful. This kills the theory of leaving the fall flow of spotted lanternfly poo for the bees to overwinter on. This may become a future problem if spotted lanternflies become too numerous and that becomes the predominant foraging nectar source.



It is generally true that dark honey contains higher levels of minerals than do light honeys. Furthermore, honey can be dark for a few reasons. As you likely are aware, every nectar-producing plant species produces a nectar that is unique to that plant species. The nectar contains different concentrations of sugar, water, minerals, etc. All of this can affect the color of the honey made from that nectar. Also, heating honey can darken it. Some beekeepers heat their honey prior to or post extraction to get honey to flow out of the combs and through the processing equipment easier. You certainly know how thick and sticky honey can be at times. Thus, a little heat can help it get through the equipment quickly. However, one can heat honey too much, causing the sugars in the honey to caramelize.

Honey can even darken over time as it ages. I gave the first jar of honey I ever made to my grandparents, given I kept my bees on their property. They kept it in their pantry for years. I saw that jar not too long ago and …