eli5: if fruit is natural sugar, how do scientists determine how many grams of sugar it has? which sugar do they measure? table?

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im reading my pack of dates and it says two dates have 25 grams of sugar, but if i put that on a scale itll be almost bigger than the dates themselves. so I dont get it, same for all the other measurements too, like protein or fiber? how do they know/measure it all?

In: Planetary Science

2 Answers

Anonymous 0 Comments

Fruit vary a lot in size and how much sugar they have in them, with ripe fruit usually having a lot more than fruit that isn’t ripe, so when you see something that says “this is how much sugar a date has in it” it doesn’t mean every date has that amount of sugar in it, just that most dates have *about* that much sugar in them, give or take.

How they actually measure it uses a branch of chemistry called “quantitative analysis” which is concerned with measuring how *much* of some chemical or class of chemicals is present in a sample. There are a wide variety of different methods that have been developed to measure how much sugar is in something, each method with its own advantages in drawbacks.

The Food and Drug Administration, for example, maintains [lists of these methods they recommend (or require)](https://www.fda.gov/food/science-research-food/laboratory-methods-food), both so they can use the best method for the analysis and so that when different groups do the measurements we can be sure they’re comparing apples to apples.

Most of these tests for sugar (and a lot of other food compounds) these days work in two steps:

1. extract the sugar and isolate it as best you can. This usually involves grinding up the sample and dissolving it in a solvent that sugars dissolve well in (usually mostly water). There may be additional purification steps, like filtration or dissolving it in a second solvent that sugar is also soluble in, but many other compounds are not, etc.

2. Analyze the extracted sugar (and other stuff) solution with [high-performance liquid chromatography (or HPLC)](https://en.wikipedia.org/wiki/High-performance_liquid_chromatography). HPLC forces the solution through a specialized tube filled with a material that allows some chemicals to pass through faster than others. This difference in how fast chemicals flow through it separates them by the time they reach the end of the column, so while a mixture was injected at the start, different pure compounds come out a different times from the end. This allows each pure chemical to be analyzed separately, so we can tell what it is and how much of it there is. [Here’s a “chromatogram” of a complex mixture of similar compounds separated by HPLC and detected at the end. Time is on the x-axis, and how strong of a signal the detector registered is on the y-axis. Each little peak is a separate, pure (or mostly pure) compound that can be evaluated individually.

The details of HPLC and their detectors are fairly complex (people dedicate entire careers to optimizing one little part of a specific method), but the general idea is simple: we get as much sugar out of the food as we can, then we use HPLC to further purify the sugar from other stuff *and* separate the different sugars from each other, and we then measure how much of each sugar we got after they were separated.

To get an average amount of sugar in a fruit, they do the analysis on a bunch of the fruit and take the average. So maybe they take like 50 dates and do the analysis on all the dates and then divide the final results by 50 to get the average amount in each date.

Anonymous 0 Comments

scientists use a standard weight of dates (say 500g) and then the package says “serving size 3 dates or per 500g”

not every date is the same size; and not every package has the same number or identical weight dates