Refractometers for cider apple juice

[David Pickering]

This season just past I tried working with an optical refractometer and whilst it was easy to read the scale for zeroing/calibration (using water) when apple juice was being assessed the reading of the scale was little better than a guess. What has been the experiences of others using these optical instruments?

Having returned the optical instrument to the supplier I’m now thinking about going to a digital instrument and am wondering whether other people have used them and what their thoughts are? The fact that a digital instrument gives a number on the read-out does not indicate how “easy” it has been for the instrument to decide on a value.

Searching the Workshop archives in relation to refractometers the following appeared which I think is a revision for Andrew’s book. 

p.59  You may sometimes see sugar levels referred to as „degrees Brix‟, and many refractometers are calibrated in this way. This is in theory the percent of sugar in the juice by mass (weight). However, the refractometers are calibrated against pure sucrose, which apple juice is not, and its refractive index is dictated by things other than just sugars. So the true sugar content is lower than the Brix reading would imply. For practical purposes, dividing the last two figures of the SG by 4 gives approximately the Brix value whereas dividing it by 5 gives closer to the true sugar value. 

The procedure of using sucrose solution for calibration is confirmed in some digital refractometer manuals.

Reading the archive text from a different perspective makes me wonder whether it would be feasible to perform the calibration on a digital instrument using water as the zeroing solution and a formulated solution for “apple-assessing” the instrument scale.

An example of the sugar components for 248g of apple juice is quoted at

http://nutritiondata.self.com/facts/fruits-and-fruit-juices/1822/2

Sugars

23.9

g

 

Sucrose

3125

mg

 

Glucose

6523

mg

 

Fructose

14210

mg

 

Lactose

0.0

mg

 

Maltose

0.0

mg

 

Galactose

0.0

mg

But this takes no account of other compounds that may affect the refractive index.

And does unconverted starch affect the refractive index? 

Cheers - David

David Pickering - "Linden Lea" 681 Huntley Road, ORANGE NSW 2800

http://www.cideroz.com/

http://www.cideraustralia.org.au/

dav...@cideroz.com

mobile: 042 727 1477

home: 02 6365 5275

[Dick Dunn]

just to David's first question...

On Thu, Jun 05, 2014 at 09:46:19PM +1000, David Pickering wrote:
> This season just past I tried working with an optical refractometer and whilst it was easy to read the scale for zeroing/calibration (using water) when apple juice was being assessed the reading of the scale was little better than a guess. What has been the experiences of others using these optical instruments?

I've generally found it easy to read juice on my refractometer. It's not
as sharp as plain water of course, but acceptable. Did you have trouble
perhaps because of suspended solids in the juice?

--
Dick Dunn rc...@talisman.com Hygiene, Colorado USA

[greg l.]

David, I find the optical refractometers work well if you have a good light source, I stand near a window to read mine. Some models are easier to read than others.

I am not sure why you need to know the actual sugar content instead of the reading. I thought the brix reading was considered accurate enough for practical purposes. It will give you a good indication of the ripeness as well as a figure for abv if you divide by 1.8. The Baume scale was based on actual observations, rather than theoretical sugar values.

Greg

On Thursday, 5 June 2014 21:46:25 UTC+10, David Pickering wrote:

[Alex Slater]

I don't have any problem reading mine, but as Dick was saying I think it all depends on the light source - plus of course setting up the focus correctly. That said though, I mainly use mine out in the orchard to do spot checks on apples - which is where it really shines as such a small juice sample is needed.

[Andrew Lea]

I use mine in much the same way and have never had a problem reading the
scale, so long as the prism is fully covered with liquid and it points
towards the light (and it's focussed). Mine is a hand-held refractometer
of this type http://www.bs-ltd.com/ltd/elineatc.html although not that
exact model.

David, I wonder what sort of kit you were using? You talk about
calibration - but the hand-held types are fixed and cannot be
recalibrated AFAIK. Can you point us to a web link?

Andrew
--
near Oxford, UK
Wittenham Hill Cider Portal
www.cider.org.uk

[Dick Dunn]

On Fri, Jun 06, 2014 at 07:22:04AM +0100, Andrew Lea wrote:
...

> David, I wonder what sort of kit you were using? You talk about
> calibration - but the hand-held types are fixed and cannot be
> recalibrated AFAIK. Can you point us to a web link?

Andrew - Please double-check your refractometer on this. The one that I
have is substantially the same as the photo of the one you mentioned at
http://www.bs-ltd.com/ltd/elineatc.html , and it -can- be recalibrated.

Immediately in back of the hinge (you can barely see this in the photos)
there is a black rubbery button which is a cover. Remove it and you find
a squat knurled metal cylinder with a slotted screw in the center. That
is the adjustment. I'm describing mine; I imagine others are different in
detail but similar in concept.

[Andrew Lea]

On 06/06/2014 14:34, Dick Dunn wrote:
> On Fri, Jun 06, 2014 at 07:22:04AM +0100, Andrew Lea wrote:
> ...
>> David, I wonder what sort of kit you were using? You talk about
>> calibration - but the hand-held types are fixed and cannot be
>> recalibrated AFAIK. Can you point us to a web link?
>
> Andrew - Please double-check your refractometer on this. The one that I
> have is substantially the same as the photo of the one you mentioned at
> http://www.bs-ltd.com/ltd/elineatc.html , and it -can- be recalibrated.

Fair point. I was obviously wrong. Looking around, it seems that many of
them can be recalibrated. Mine can't. It has some sort of automatic temp
compensation built in but it is over 25 years since I bought it and the
instructions are now lost ;-)

[greg l.]

If you can afford it something like this Atago digital refractometer would be the bee's knees.

http://www.ebay.com.au/itm/NEW-Atago-PAL-1-Digital-0-53-Brix-Refractometer-32-/220443305213

No more peering through eyepieces.

[David Pickering]

Thanks to the various people who have responded to my question.

Dick - yes it’s possible that suspended solids or even minute bubbles from the sampling process caused the difficulties. Reading was not easy - the correct value could have been over a range of many units it was so fuzzy.

I should add that in an earlier life I used optical refractometers ad nauseum for field assessment of sugar (i.e. sucrose) in sugar cane as part of the cross-breeding and variety selection process.

Greg and Alex - the focus was sharp. Lighting was good: used in the orchard it was either in direct sunlight or in diffuse light, neither improved matters.

Andrew and Dick - yes calibration of the zero point was via that small screw under the rubber cap.

Greg (again) - yes it’s tempting to go down the digital path. There are various units out there Atago and Hanna feature a lot. My concern is:

"...how “easy” it has been for the instrument to decide on a value.” I know how difficult it was for my eye to discern a read-off point on the scale of the optical instrument but can I be confident that a digital instrument will be giving an accurate reading or just returning a reading of +/- 10 or 15% of the true value.

That is one aspect of a digital unit, the other is what led me to mention the possibility of creating a standard solution (or multiple standards) to assess the accuracy of a digital unit when the different sugar types in apple juice are considered. By this means it might/would be possible to develop corrections or offsets to the indicated readings.