Moisture Content in Walnuts

by Clive Marsh, Autumn 2007

It is universally recognised that walnuts are best stored at 8% moisture content. This is the standard accepted in both USA and Europe. There is good scientific reason for this: at higher moisture content, yeasts and moulds can grow and spoil the nuts in a relatively short time (a few months or less). Whereas nuts with a lower moisture content don't store as well and have used more energy than is necessary in the drying process.

What does 8% moisture content mean?

Moisture content is a measure of how much water is present in the in-shell nut (i.e., including shell, partitions and kernel) relative to the total weight. So, 100 kg of nuts at 8% moisture will contain 8 kg of water and 92 kg of dry material (shell, partition, kernel, etc.)

How can you measure moisture content?

Qualitatively, the moisture content of the nut is too high if the partition doesn't 'snap' when bent with the fingers. If the partition does snap, then the moisture content is close (enough) to 8% or less. So if you are drying recently harvested walnuts, monitoring the partitions for 'snappiness' will tell you when you can stop drying.

Quantitatively it is not easy to measure moisture content, unfortunately. The standard method is to take a sample of the nuts, weigh them as-is, then weigh them again after thorough drying in an oven. The difference in weights is assumed to be the amount of water that was there at the start. The moisture content is the weight of water divided by the wet weight of the nuts.

Say you are monitoring the moisture content of nuts on your drying racks, and want to know whether they are sufficiently dry. You take, for example, a sample of 100g of nuts from the racks. You put them in an oven and, after drying, find that the weight is now 91g, meaning that 9g of water has been removed in the drying process. The moisture content of the nuts on the racks is therefore 9g divided by 100g, or 9%.

The oven temperature for drying your sample must be over 100ºC to drive off all the water but it should not be too hot (105ºC is usually suggested). The time required in the oven is 'long enough to drive all the water off' (i.e., to get the nuts to 0% moisture content). This is likely to take at least 3 hours. However, to be sure, you should weigh your sample after 3 hours in the oven and then again after 4 hours. If there has been no weight change from 3 to 4 hours, you can be sure that all the water has been driven off. The main problem with the method is that other parts of the walnut (e.g., oils) can be vapourised and their loss 'adds' to what is believed to be moisture. This effect shouldn't be too bad at 105ºC but if the oven is hotter, quite a lot of non-water will be driven off in the oven. So, it is not 'simple' to quantitatively measure the moisture content.

What is the moisture content of a nut at harvest?

The moisture content of walnuts at harvest varies between about 15% and 25%. The wettest nuts will be ones whose husks haven't split (and thus the nut is still in a completely saturated environment) and ones that have been lying on damp ground. The driest will be ones that have been hanging on the tree by a thread from a well split husk through a couple of nor'westers. There will be some moisture take-up during washing, but if the contact time with the wash-water is brief, only the outside part of the shell will be wet and this is easily dried again.

How much weight (yield) will be lost in drying?

If the nuts are at the wet end of the range when harvested (25% moisture content)then 100 kg of freshly harvested nuts will comprise 25 kg of water and 75 kg of dry matter. After drying these nuts to 8% moisture content, the dry matter comprises 92% of the total weight. Therefore the total weight after drying is 81.5 kg (75 kg divided by 0.92). Thus you have lost 18.5 kg of weight (water) in the drying process (100 kg harvest weight minus 81.5 kg dried weight), or 18.5% of the original wet weight at harvest.

If the nuts are at the dry end of the range when harvested (15% moisture content) then 100 kg of freshly harvested nuts will weigh 92.4 kg after drying to 8% moisture content, so 7.6 kg (or 7.6%) of the weight will be lost.

In other words, you can expect to lose (in round figures) between about 5% and 20% of the harvest weight during the drying process.

Clive Marsh