Ash/Dry Matter

What is dry matter?

Dry matter is the true dried component of a sample. Most stored samples will collect a small amount of water (generally less than ten percent of the sample weight) from the air, even if already dried. In Fairbanks it is relatively dry all year long – but imagine if these samples were stored in a lab located in a rainforest, where the humidity is always high, or in a location where humidity in the air can vary by season. We need to make sure all of our analyses are conducted only on our sample, and not on any residual moisture content which could change.

What is ash?

Every dried food item can be divided into two components – ash and organic matter. Ash is composed of all of the minerals – the iron in the hemoglobin in blood, and calcium in bones, for example. Organic matter is composed of everything else – all the carbon based material, such as fatty acids or proteins. In a dried sample, if it’s not ash, it’s organic matter, as shown in the following equation:

ash + organic matter = 100% of dried sample.

By subtracting the ash content from the total dried sample (100%), we can easily calculate the organic matter content too!

The relationship between dry matter, organic matter, and ash can be pictured like this:


Each sample can be divided into at least three parts – the ash (mineral) component, the organic matter (carbon-based material) component, and the residual moisture content that a sample accumulates from just sitting around the lab.

How is dry matter and ash measured?

Measuring dry matter and ash content are some of the easiest lab analyses to do. We simply put a small amount of each sample in a small glass vial, let it dry in the oven overnight, and weigh it back the next day to see how much moisture was removed. From there we can place the sample in a furnace and slowly heat it up. This will burn off (in a controlled manner) any organic matter content, leaving the ash (mineral) fraction behind. Weigh it back again and viola! You can calculate the ash content.

Why is it important to measure ash and dry matter?

Dry matter is necessary to measure so that we can figure out how much water each sample absorbs just by sitting around in the lab. We can use this to correct any further analyses such as protein content, digestibility, etc… for the true sample amount in each assay. For example, let’s say you want to analyze 100 grams of sample for protein content. If you remove 100 grams of sample straight from the bag it’s stored in, you are not getting 100 grams of sample exactly – you are getting maybe 92 or 95 grams – whatever the difference is in moisture content of stored samples. We need to know exactly how much sample (not sample and water) we are getting for each lab analyses to ensure the most accurate calculations.

Ash is relatively easy to measure if you are already measuring dry matter. Ash is also important because the total minerals in plant tissue may change over the course of a growing season, as plants accumulate new tissue and remove materials from the soil. In addition, ash and protein are often linked together in the same molecule in organisms, and tracking one component may be helpful in looking at the other.