Thermometer oddity

[Jethro_uk]

For reasons not relevant here :) I'm trying to make invert syrup.

It seems the trick is to get the fully dissolved sucrose (i.e. granulated sugar) to a temperature of 114C before killing the heat and letting cool naturally to a (literally) Golden syrup.

To my pleasant surprise I discovered a true candy thermometer at the back of the drawer. However (a) it misted up inside (I know it's not supposed to) and (b) trying to read the scale to the exact 114C was a PITA.

So I reverted to my plan A which was to use a digital thermometer I have for brewing. Long probe and reads C/F with a max/min hold feature.

The problem is that the display dances all over the place with (what I presume) is the sampling rate = about once every 2 seconds. So I've seen

108/112/109/107/113/110/107/109 before getting 114 ... and then 110/109/114/115/109 etc ?

Just curious as to what is happening and more importantly what it says about the accuracy of the thermometer ????

[Martin Brown]

It probably says that you are not stirring the sugar solution vigorously enough to keep its temperature uniform and so the sensor is seeing convection currentss of hot liquid from the base of the pan going past.

Analogue thermometers tend to average these things out rather better.

[Jethro_uk]

Well you aren't supposed to stir it once the sugar has dissolved ...

I guessed there was some convection happening but to jump around 3-4C in a couple of seconds ?

I've started nosing on Amazon ... not really sure a digital jobbie is the best tool here. As you suggest an analog one (that doesn't steam up and can be read to the accuracy required) might be in order.

I've had 3 stabs at making it. The first with 1Kg sugar - came out fine (a true golden syrup). Second time with 2Kg and again - came out fine. However the last was a bit thicker when cool and more problematically it crystallised in storage. Not sure if that's because it was too hot, not hot enough, or there was something in the glass jar I used for storing that triggered a crystallisation.

It never ceases to amaze me how much "science" there is in even the most basic of kitchen activities :)

[Chris Hogg]

Beekeepers heat sucrose solution to convert it to fructose and glucose, i.e. to hydrolyse the glucose. Additions of citric or acetic acid speed up the conversion. They call it 'ragus' ('sugar' inverted, ha, ha!). They feed it to their bees in winter, if the bees' own stores are getting low. Being invert sugar, they're able to digest it better than sucrose. Some details of how to do it suggest 112°C but it may depend on the sugar concentration. There's a wikihow here

They say 114°C.

I agree with MB, almost certainly there are temperature variations throughout what must be a fairly viscous solution. I can't see that stirring is going to make any difference to the inversion process, other than possibly causing the solution to crystallise on cooling, which isn't what you want by the sound of it. Your last batch may have had nuclei of undissolved sucrose in it, to initiate crystallisation. Beekeepers usually like it solid - as it's solidifying they pour the solution into dishes to make 'cakes' to place inside the hive, and call it 'bee candy'.

[Brian Gaff (Sofa]

Well normal thermometers are very slow to react to changes. I remember having a realistic one built on a large moving coil meter. That was quite fast, but did not wobble. I'd imagine the digital kind are basically doing a sample and the accuracy is averaged over time but if you need precision then you may be out of luck. Does it really need to be that temp exactly? Brian

[Thomas Prufer]

This uses acid, and then neutralizes it:

Thomas Prufer

[Jethro_uk]

Well the guides seem quite emphatic. And I know from childhood experience how difficult it can be to make fudge as opposed to some sort of crystal candy confection.

[Jethro_uk]

Thanks - that's fascinating.

I'm using the resulting syrup to make liqueor - specifically Limoncello, since the prepacked "just add alcohol" sachets are currently unavailable.

I've got damn close to how they tasted though. So I won't be getting them again anyway.

[Fredxx]

Have you thought of using a thermocouple type probe?

I would suggest probably more reliable than a kitchen thermistor thermometer.

[Paul]

Maybe you could observe the uniformity of the "liquid" with polarimetry. A glass beaker, and light from the side might work.

If the digital thermometer does not exhibit dual-slope converter noise while measuring room temperature, then what it is showing you is real, moment to moment, temperature variation. The polarimeter might show swirls of material with various concentrations in it.

Paul

[Jimk]

Kendal mint cake wout the mint?

[Chris Hogg]

Quite probably.

I note that it says "The mixture is continuously stirred (without stirring, the resultant product would be clear)". Apparently KMC was discovered when a batch of Glacier Mint went wrong, and it crystallised. Whether bees would take either, or whether the mint flavour would put them off, I've no idea. I doubt it would lead to mint-flavoured honey, though - it would all have been digested long before the main honey crop started to be accumulated in the following summer.

[Jethro_uk]

To be honest, I think this is one of those ocassions when old-school makes sense. A decent analogue thermometer which inherently integrates over time to give a rolling average.

[Andy Burns]

You're not going full Heston Blumenthal with your limoncello then?

[Adam Funk]

You should get these books if you don't already have them:

• Harold McGee: On Food and Cooking: The Science and Lore of the Kitchen

• Jeff Potter: Cooking for Geeks: Real Science, Great Cooks, and Good Food

[Andy Burns]

I was thinking jethro might be making home-made after eights, time for the enzyme to liquidize the filling before xmas?

[Jethro_uk]

Looks like some interesting lockdown reading !

[Adam Funk]

If you're interested in linguistics too, _The Language of Food_ by Dan Jurafsky is also good.

[Fredxx]

I suppose I have used a few thermocouples with this type of cheap meter. They work well and give a stable consistent output. I like the idea of a digital display as it removes any guesswork of needles and parallax.

They are inherently linear whereas thermistor types are designed for a specific temperature range.

YMMV

[AnthonyL]