Brix does measure "nutrient density" but way too many people are reading this incorrectly to mean more brix = more nutritious. It's mostly a measure of photosynthetic efficiency in a plant.

If you have a baseline of what the variety/cultivar is supposed to do then you have a quick and easy way to make a rough guess of where the plant's production health is at (especially photosynthesis quality).

You learn nothing about nutrient content or types...you really only have a general snapshot of how the plant is doing compared to a known expectation and most of this is based on photosynthetic-to-sugar production rather than knowing the crop is chock full of Ca, K, Zn, etc...

This can all be thrown off by hydration/dehydration, lesser quality cultivar/varieties on comparisons (especially shipping types of fruits/veggies), and disease/pest pressure on the crop.

If someone is saying Apple 1 has better brix than Apple 2, it means very little compared to saying Apple 1 grown in one place has a higher brix to Apple 1 grown in another place. Variety and cultivar selection is highly important, especially when making comparisons.

This post was edited by nc-crn on Sat, Dec 27, 14 at 22:18

michael357 says," One could certainly suppose that a plant high in sugars may be healthier than one lower and that could infer the higher one has more nutrients in it. Until I see the research indicating this to be the case, I'll remain skeptical about using supposition on top of an inference to come to a conclusion on the usefulness of using Brix to indirectly estimate the nutrient density of a crop."

Well, don't get your hopes built up too high that you are going to get things "settled".

I remember testing a very delicious watermelon a few years ago. It tested 13½. A more average one was 11½. I tend to believe that the finer fruits test higher. Likely these fruits have laid in more goodies besides sugar. See? No see?

Brix testing is one tool in a growers tool box, but not the only one and one that may be late in the growing process. While it may be useful in determining when to harvest it is not the most reliable method of whether a soil is in good condition.

Brix measurements are useful in several growing areas, but one needs to remember that it is total soluble materials that are being measured, and not specific nutrients. You really are mostly measuring sugar content, and making the supposition that when sugars are higher, nutrients are higher. There is some reasonable research that does indeed show that other nutrients rise when brix rises for a given plant variety being grown. There is also the supposition that when sugars are higher, the plant is being grown in a better environment or soil. A fair bit of research and my own experience is that this is the case also. However, brix measurements are quite valuable in several areas. First, comparing the brix measurements from several plants of the same variety grown with different methods can help to characterize the growing methods. You might be able to determine, for instance, if the addition of a certain nutrient really did increase the production efficiency of a plant (as reflected by its sugar contents). Samples for such characterization can be expressed from a portion of the plant pruned off (such as a tomato branch). Care needs to be taken to sample from the same portion of each of the plants under observation; brix varies throughout the plant. You can use this method to help tailor future soil amendments or additions based on the impact of the amendment on a given plant. A second useful application of brix measurements is to compare one variety to another under the same growing conditions. For instance, some foreign markets require tomatoes to have a certain brix value. You would therefore want to start with a variety that had an intrinsically high brix value. One can somewhat impact brix values by growing techniques, but the major factor is the genetics of the variety being grown. nc-crn stated this fact well up above. There has been good work done at several locations aimed at increasing the brix (and presumably the nutrient levels) of some produce. The Controlled Environment program in Tucson has shown the results of some of this work at conferences. For most plants, you would need to wait until the plant has a leaf or stem that can be pruned in order to express the liquid needed. For a fast growing plant like squash, this might be 2-3 weeks after seedling emergence; for something slower like a pepper, you might need to wait 4-5 weeks before sampling. Since fruit sampling is destructive, it would only be done if there is a large crop of fruit at the same stage in order to determine if it is harvest time.
For total plant nutrient testing, if that is an interest, and you want to spend the money, there are tissue analysis labs who can do the work. You send in a small paper bag with leaves from the crop, and they provide a nutrient profile, and sometimes recommendations for fertilizer additions. Servi-tech in Dodge City, KS used to, and probably still does this kind of work.
In my experience, for a given variety it is indeed quite possible to significantly improve the quality of produce by proper fertilization. I, and people who eat my produce, can taste the difference readily. Certainly a healthy soil is an important start, as is a careful approach to cultural requirements. Note that many people claim, and it is my experience, that a high brix plant is more resistant to insect attack. There are probably just as many folks who would claim the opposite, so you'll need to experiment. I find, for instance, that if I grow peppers with more than the standard amount of nitrogen that I get a much larger plant with more succulent growth that tends not to be attacked by aphids as readily. My fruiting might be delayed a bit, but overall production will be higher, and the plant overall will be healthier. This kind of growing is one application I have for some soluble fertilizers in addition to organic ones. If you want to compare your produce to some nominal standards, http://www.highbrixgardens.com/pdf/brix-chart.pdf has a reference chart with a number of fruits and vegetables. The link attached below from Ohio has a good summary of the use of brix testing to help insure a quality harvest.
Renais

Here is a link that might be useful: Brix as an indicator of vegetable quality

Great link Renais, educational and informative, thanks for posting it. I see OSU doesn't think Brix is an indicator of nutritional value based on their extensive research. Anybody wishing to use Brix should read and know what's in that series of bulletins before taking a single sample. After that, sit down and come up with a reasoned out plan including any goals, sampling methodologies and needed equipment then go from there.

I'd still like to find some refereed journal articles on Brix, probably some in HortScience or ActaHort.

"I'd still like to find some refereed journal articles on Brix, probably some in HortScience or ActaHort." For a good start on getting references, you might try the following people whose science I find pretty exact and careful: At the controlled environment center in Arizona: Jensen (don't remember his first name), Chieri Kubota. A couple news note ideas are accessible through http://ag.arizona.edu/ceac/sites/ag.arizona.edu.ceac/files/Dave%20Eddy%20AZ%20GH%20Course%20for%20Mexico%20magazine.pdf. For a piece of work by Kuboto and her student see: Production of Hydroponic Tomatoes Rich in Flavor and Bioactive Compounds at http://www.researchgate.net/publication/267995405_Production_of_Hydroponic_Tomatoes_Rich_in_Flavor_and_Bioactive_Compounds. This paper may require a free signup. A very good discussion of brix is at: http://ag.arizona.edu/ceac/sites/ag.arizona.edu.ceac/files/ISHS%202004%20final%20version.pdf. The Arizona folks are world quality researchers. Not only are they working to improve the nutrient content of standard product, they are doing something I think they call farmacudical production to introduce certain other materials into a food stream for specific purposes such as vaccination. In the US we have access to ample vaccination opportunities; in other parts of the world, these foods might be the vehicle used to protect populations from devastating diseases. There is also good work going on at the University of Illinois, and (I think) Michigan State, but I don't have references right at hand.
Renais

I knew I'd remember something important once I posted. See Steven's article in Journal of American Society of Horticultural Science, somewhere in volume 102. This was one of the seminal articles. You can do a reverse citation search.
Renais

I mentioned the CEAC several times above, so thought I should give a link to them, and a little more background information. A short mission statement: "The CEAC’s Vision is: to develop Controlled Environment Agriculture as an economically, environmentally and socially sustainable agricultural option."

These folks are not striving to grow certified organic produce for the most part, but they are at the forefront of developing techniques usable around the world for sustainable agriculture in a protected environment. Their growing philosophy is closely matched to an organic philosophy focused on socially responsible, cost effective, nutritious and sustainable production. There is much an organic grower can learn from them; I certainly have benefited quite a bit. If you look at some of the web site you will notice the strong emphasis on environmentally benign growing, and a focus on producing food that really is better for you. Their methods are the kinds of things used in much of the hydroponic growing in North America. The produce coming out of these greenhouses is generally completely pesticide free, devoid of any herbicide materials, and grown with a focus on taste and quality as well as quantity. I particularly appreciate the effort they are expending to increase the content of certain nutrients in crops. I have seen the progress they have made over the years to provide better food for people around the world. In the US or Canada when you eat a great hydroponic pepper or tomato, there is a reasonable chance that CEAC helped.
Renais

Here is a link that might be useful: CEAC

We had a thread on this subject a couple of years ago, without changing anyone's mind, as I recall.

"One could certainly suppose that a plant high in sugars may be healthier than one lower and that could infer the higher one has more nutrients in it"

That is also what seemed like common sense to me, and what I have been going with. In any case, if brix does not indicate nutrition, and taste does not, then the ordinary grower who is not going to pay for ash-tests has no way to know, at all, so what's the difference?

Mike, to answer your original question, I use a garlic-press usually, and I try to express sap from whatever part I am eating.

However, I should use it more to assess plants while growing, such as testing foliage on tomato plants before and during fruiting. Still have not managed that 10-brix tomato fruit.

Pn: if you want to spend the big bucks on a press look at the link below, it is similar to the one I have made by Hach, hold on to your wallet :)

For the plants themselves, for Tomato and many other vegetable, petioles might work good for you to get sap from, I've used them a lot for nitrate sampling with the Hach press. You'd need to cut petioles up pretty small and sub-sample with your press Orr combine the sap from many pressings, mix and then analyze.

What's the difference? Here's a guess: the grower's crop is headed to a processor where extensive testing is done on the crop when it arrives that he/she can't afford to do. The grower is paid according to certain contractual parameters including the mass delivered an various qualities like Solubile solids, pH and maybe Brix too. There is only so much info one can get in the field, labs are where the bigger questions can be answered.

Here is a link that might be useful: Sap press