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As the boiling point is a colligative property, one would expect that the boiling point of sugarcane juice should be higher than pure water because Sugarcane juice contains water (75–85%), non-reducing sugars (10–21%), reducing sugars (0.3–3%), organic substances (0.5–1%), inorganic substances (0.2–0.6%) and nitrogenous substances (0.5–1%)1. However, the boiling point of sugarcane juice is only 91-95°C [2]. Are there any chemistry theories that can explain this drop in boiling point?

enter image description here[3]

1 Sreedevi, P., Jayachandran, L.E. & Rao, P.S. Browning and bioactive composition of sugarcane juice (Saccharum officinarum) as affected by high hydrostatic pressure processing. Food Measure 12, 1962–1971 (2018). https://doi.org/10.1007/s11694-018-9811-7

[2] Tiwari, G. & Prakash, Om & Kumar, Subodh. (2004). Evaluation of convective heat and mass transfer for pool boiling of sugarcane juice. Energy Conversion and Management. 45. 171–179. 10.1016/S0196-8904(03)00143-2.

[3]: Canilha, Larissa & Chandel, Anuj & Milessi, Thais & Antunes, Felipe & Freitas, Wagner & Felipe, Maria & da Silva, Silvio. (2012). Bioconversion of Sugarcane Biomass into Ethanol: An Overview about Composition, Pretreatment Methods, Detoxification of Hydrolysates, Enzymatic Saccharification, and Ethanol Fermentation. Journal of biomedicine & biotechnology. 2012. 989572. 10.1155/2012/989572.

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  • $\begingroup$ Is there noted atmospheric pressure ? I am not sure it would be boiling, of placed in the boiling water bath. Unless there are volatile components or some decomposition occurs, e.g. decarboxylation. Could you quote broader context from [2] ? $\endgroup$
    – Poutnik
    Jan 5 at 6:35
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    $\begingroup$ I don't believe the values listed there in reference 2. It was a very very crude set-up and they do not mention what was their criterion of determining the boiling point. Boiling point of filtered sugar cane juice must be higher than pure water. $\endgroup$
    – M. Farooq
    Jan 5 at 6:37
  • $\begingroup$ See this page sugartech.co.za/bpe/index.php $\endgroup$
    – M. Farooq
    Jan 5 at 6:38
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    $\begingroup$ Evaporating water from syrup is industrially always done at reduced pressure. The boiling point of sugar solution at ambient pressure is definitely > 100°C. $\endgroup$
    – Karl
    Jan 5 at 8:23
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    $\begingroup$ @Karl: You hit it on the nose: engineers! I went back to the paper. The word boiling is used many many times, but "pool boiling" is specified, and that term has a specific meaning for heat transfer engineering: it's evaporation from the surface of a stagnant liquid hedhme.com/content_map/?link_id=598&article_id=192 For non-engineers, boiling is boiling, but for engineers, boiling can just be evaporation at near the boiling point. Amazing how difficult it is to communicate across interdisciplinary divisions! Those engineers were not careless; we chemists were ignorant. $\endgroup$ Jan 6 at 13:50
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A more plausible explanation is that it is not water that is boiling. The "organic substances" and maybe the "nitrogenous substances" in the juice could include some more volatile components that may pass selectively into the gas phase at a lower temperature than most of the solution. Such a vapor would not be the pure organic or nitrogenous substance; it would carry off some water vapor as the vapor pressure of the water is a large fraction of an atmosphere. The temperature at which such selective vapor begins to form is properly called the bubble point.

Of some relevance is the effect of the sugar itself on the boiling point. In the absence of other solutes this effect is very small, cfailing to reach $5°C$ boiling point elevation until we have nearly 80% sucrose in the solution (by mass). Sucrose has a large molar mass smd water has a high heat if vaporization, both of which factors attenuate the boiling point rise at concentrations relevant to sugar cane juice. This lack of an effect from the main nonvolatile solute would enable small amounts of more volatile components to yield a bubble point lower than the boiling point of pure water.

enter image description here From Ref. [1]

Reference

1. Willism Heffner and Hinanshi Jin, "Building a Low Cost, Hands-on Learning Curriculum on Glass Science and Engineering using Candy Glass", MRS Online Proceeding Library Archive (January 2009) 1233, https://www.researchgate.net/deref/http%3A%2F%2Fdx.doi.org%2F10.1557%2FPROC-1233-PP03-06.

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    $\begingroup$ Oscar, have you tried sugar cane juice? It is drinkable, in fact it is a popular drink in South Asia road side stalls. Why do you assume there are significant amounts of nitrogenous substances and organic substances there? It would stink or perhaps be bitter if it had those nitrogenous substances in significant amounts. $\endgroup$
    – M. Farooq
    Jan 5 at 18:31
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    $\begingroup$ @M.Farooq See OPs question. ;) And I would assume that means "residual protein". Nothing that would stink, or lower the Bp. $\endgroup$
    – Karl
    Jan 5 at 18:52
  • $\begingroup$ Proteins neither boil nor they are volatile. $\endgroup$
    – M. Farooq
    Jan 5 at 19:01
  • $\begingroup$ Also note that the sugar itself is not very potent in raising the bp to begin with. See the recejt modification to the answer. On another comment, proteins could become amino acids. $\endgroup$ Jan 5 at 19:02
  • $\begingroup$ This also explains the odd occurrence in DOI: 10.2298/TSCI150219213S where the boiling point is written twice. Once the study lists the boiling point as 107°C and once again as "boiling of water in sugarcane juice may start at 91-95C". $\endgroup$ Jan 6 at 3:22

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