Public Lab Research note

Refractometer for measuring nutrient density in food

by liz | June 07, 2011 15:52 07 Jun 15:52 | #339 | #339

Refractrometer from RealFoodCampaign

A refractometer measures how much light bends as it passes through a liquid. n = velocity of light in a vacuum / velocity of light in medium

Can a refractometer be used to check nutrient density in plant sap?

From the Real Food Campaign Website: Pure water at a specific temperature will bend light at a specific angle. A wide variety of biological substances can increase solution density, and thus cause light to refract (bend) more as it passes through a watery medium: 1. dissolved minerals (salts, acids, alkali) 2. carbohydrates (simple sugars) 3. amino acids (proteins) 4. lipids (special case: oil & water don't mix; form thin films) 5. almost any molecule with covalent bonding 6. very large suspended particles (colloids)

BRIX measures the percent solids (TSS) in a given weight of plant juice‚ nothing more‚ nothing less. BRIX is often expressed as the percentage of sucrose. However, the "sucrose" can vary widely. BRIX is actually a sum of the pounds of sucrose, fructose, vitamins, minerals, amino acids, proteins, hormones, and other solids in one hundred pounds of plant juice.


I'd hazard a guess that a spectrometer would yield much more information about the contents of sap -- there's only one dimension in BRIX data, just one number.

Like, does higher BRIX mean more sugars, or more minerals, or what? There doesn't seem to be any way to tell; and with grapes, the original use case, Mathew points out that sugar is the key factor for fermentation. So is BRIX just showing higher sugar? Maybe that's valid since many fruits are "better" if they are sweeter.

What are the specific compounds nutrient-rich growers are trying to maximize? We can ask Severine a bit on Thursday, but it seems not unlikely that you'd be able to use a spectrometer to identify a spectral curve for what people call "good" vegetation and use that to assess other examples of that particular species of fruit/veg. Or if you're more interested in specific nutrients, we could compare the spectra to some way of measuring those to see if there's a strong correlation.

I found this nice outline of how a refractometer is used by one organic farmer on this discussion forum:

A leaf sample is taken from the crop. crushed to express sap/plant juices and put on the prism face of the refractometer and the reading taken.

The prepared Foliar feed spray is applied to a given sq. metre at the same rate to be scaled up to cover the acreage.

about 1/2 to 1 hour later another reading is taken of the identical part to the previous reading ie. tip and first three lvs. or whatever.

If the resultant reading has raised the brix level 2points or more then thats the green light to go ahead and apply to the whole crop. If no increase it could mean that the foliar feed will not benefit the crop. foliar feeds work best when there is adequate calcium in the soil.

Limitations - if the crop is water stressed then the dissolved solids are less diluted and give a false high reading.

The refractometer wont reveal heavy metals, pesticide or herbicide residues and also wont give any difference in the reading if it is the case, possibly apart from lower readings due to a less healthy plant due to heavy metal particularly as they can intrfere with the uptake of essential plant nutrients especially if there are nutrient imbalances in the soil.

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Jeff - right, higher BRIX largely means higher sugars. It's interesting to consider building a reference library of spectra from "nutrient-dense" examples of particular crops to get measurements of more than suspended solids (largely sucrose). Or, alternatively, to start looking for specific minerals across multiple crops with a spectrometer.

I think the BRIX fans are imagining a day when there would be a simple-to-use consumer-level refractometer that someone would bring with them while grocery shopping.

By the way, it was very interesting to hear my farmer friend Deb Taft say that she was interested to use a refractometer in the field in order to train her eyes to recognize the signs of sucrose-dense crops. In other words, using a technological sensor to calibrate her own senses.

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I will tend to preface most of my participation on these forums with the caveat that I am far from an expert at most things, and will aim to bring more of the "citizen" perspective to PLOTS's science.

Lately, one of my mantras has been: when faced with a choice between two options - see if you can take both. As Jeff points out, BRIX measurements do not provide much information. Spectroscopy will provide a much richer portrait of any given sample. But "citizens" could not ask for a lower barrier to entry. E.g., take two drops of your tomato's juice, place it in the refractometer, and read the number. Higher is better, and here's a comparison chart for tomatoes generally, or for tomatoes in your region, or for tomatoes from this grower over the past several years.

That said, it's unclear to me where we could expect to see variation in the data, especially for fruits and vegetables that have been picked and transported. However, with enough data, you might begin to see interesting trends to bolster arguments that local produce is qualitatively better. But then there's that whole term "quality" - which is where I would hope more engaged citizen scientists equipped with spectrometers could provide some insight into the actual composition of the plant juices under test.

If the refractometer is cheap, simple to use, and easy to report with - I think this is a good introduction for citizen food science noobs.


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I'd love to see a series of spectrometer measurements of a tomato as it ages, as it's kept in the fridge, or frozen.

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