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I was wondering if honey's viscosity has any correlation with its acidity and if not, it would be very helpful if anyone could tell me what variables could affect the viscosity of honey?

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  • $\begingroup$ Water content will be a big factor $\endgroup$ – Waylander Apr 18 '20 at 11:02
  • $\begingroup$ I agree, water is a way to decrease the viscosity of honey but what else? $\endgroup$ – General MO7 Apr 18 '20 at 11:24
  • $\begingroup$ If a question is asked on Chemistry SE site, then, in contrary to sites like Quora, it is expected from the author to elaborate the topic in the question, doing at least basic research oneself, writing what he/she found and understood, and what is the stumble stone. The quick questions without explicitly expressed particular effort are not very welcome, and may be closed. $\endgroup$ – Poutnik Apr 18 '20 at 11:36
  • $\begingroup$ another factor can be glucose : fructose ratio, typically 1:1 - 1:1.7 ( remembering from honey keeper books of my father ). The former ratio is more vulnerable to crystalization. $\endgroup$ – Poutnik Apr 18 '20 at 11:39
  • $\begingroup$ Can you please be more specific? and also what do you think of the main question about the correlation between honey's viscosity and acidity? $\endgroup$ – General MO7 Apr 18 '20 at 12:32
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Not much. Given all other variables which influence the viscosity of honey (sugar composition, including polysaccharide content, humidity, degree of crystallization), pH would not be regarded as a key determinant.

Honey has a pH of 3.4-6.1 (average around pH 3.9) according the National Honey Board of America. This translates roughly into a $\ce{H+}$ concentration in the mmolal-$\pu{\mu molal}$ range (because the water content is so low, strictly one should discuss activities). The main organic acid responsible for the low pH of honey is gluconic acid, which looks, as described by the Wikipedia, like the "open chained form of glucose, terminating in a carboxylic acid group". The chemical structure and MW of gluconic acid are so similar to those of glucose, and its concentration in honey is so low, that it does not alter much the viscosity of honey.

As concluded in Ref (1):

Variation in viscosity measures of honeys of similar moisture contents is usually attributed to non-sugar, colloidal material. Honeys of similar moisture content but different absolute viscosity (Table 1) all obeyed Eq. (3). Thus, any contribution of non-carbohydrate, colloidal material to concentration dependent viscosity was minimal in the honey samples assayed. These findings agree with White,[10] that the types and concentrations of carbohydrates largely determine the gross physical attributes of honey.

References

  1. Brenda Mossel, Bhesh Bhandari, Bruce D'Arcy & Nola Caffin (2003) Determination of Viscosity of Some Australian Honeys Based on Composition, International Journal of Food Properties, 6:1, 87-97, DOI: 10.1081/JFP-120016626
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OK, speculation here: time. What?

Well, consider asphalt, as in asphalt concrete. The mass of asphalt (petroleum residue) plus stone is heated in a large mixer, quite hot, then loaded into a truck for delivery to some location for deposit onto a street. It is dropped and rolled to about 2 inch thickness, then rolled again to insure flatness and good compaction, and it rapidly cools down to ambient temperature. (A few hours.) Then we are told to keep off it for a day or so and even then to avoid parking a car on it for a week or so, to prevent forming unsightly depressions that will catch water. The reason is that the asphalt molecules are quickly frozen into some not completely stable configuration as they cool down, and will continue to deform and rearrange until they reach a final structure that then resists further deformation: i.e., has considerable rigidity and strength.

Now, honey. I have noticed, in a qualitative way (having no viscometer at home), that when I take the jar of partially crystallized honey and heat it to get it all redissolved, and tilt the jar, it seems quite fluid , not only when hot, but also when it has cooled (I have thermometers at home, but didn't think it was worth measuring the temperature, till now!), and even an hour or so later... But the next day, the honey seemed considerably more viscous than I remembered, almost as viscous as it should be...And the next day, everything seemed quite normal, so I did not investigate further.

So I wonder if the fructose and glucose (in a concentrated solution: ~85% sugars, 15% H2O) form some metastable hydrogen-bonded polymers with a long-range structure that persists for a long time before sorting out to crystals. The viscosity should then be a function of how long since it had been at a high temperature, and this function should also be influenced by the original ambient temperature viscosity (related to water content), since a lower original viscosity suggests that the original viscosity will be regained faster after heating if the molecules are not tied up so much.

And if the long-range polymer structure is applicable, then perhaps mechanical disruption is also a variable. On the one hand, whipping air into the honey will disturb the viscosity. But perhaps even mild stirring will cause the polymer structure to deform or break down. This could be measured by viscoelasticity (a high elastic component indicates no breakdown of the polymer structure), but if the elastic component of the viscosity diminishes as stirring continues, the polymer structure is being deformed, and the recovery time of the polymer structure could be measured by stopping the stirring and measuring the viscosity periodically.

A very sweet experiment, to be sure!

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  • $\begingroup$ Yes, honey is a metastable supersaturated solution (when not crystallized that is). $\endgroup$ – Buck Thorn Apr 18 '20 at 15:53
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    $\begingroup$ Note also that honey is non-newtonian liquid so it's viscosity is not constant, but is stress dependent. $\endgroup$ – Poutnik Apr 18 '20 at 17:35
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    $\begingroup$ @Poutnik You're wrong, but I'm worse! I did no literature search, and just speculated - mental mastication. So when I looked it up, I find that honey may indeed be thixotropic (shear thinning), but not very much. Honey is considered Newtonian! intechopen.com/books/honey-analysis/… $\endgroup$ – James Gaidis Apr 18 '20 at 19:25
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    $\begingroup$ But thixotropic does not mean Newtonian. $\endgroup$ – Poutnik Apr 18 '20 at 19:27
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    $\begingroup$ Thixotropic means non-newtonian, but if the thixotropy is small, or found only in certain strains of honey, as reported in the reference, and if it is then claimed to be Newtonian, I'll accept that. You can estimate that honey is Newtonian by the way it drips: it runs like water (if it doesn't dry out) down the jar, just slower. Honeys may vary in viscosity and acidity; both parameters vary only slightly in actuality, and I have not seen any reports about a correlation, and I can't think of a causal relation. $\endgroup$ – James Gaidis Apr 19 '20 at 13:44

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