Parameters to compare different millet grain hulling machines

This is a Q that keeps coming up again and again. And given how much impact this has on realizing more millet processing units, I am kicking myself for not having posted this earlier.

There are many individuals and organizations thinking of starting millet processing. And everyone arrives at a realization that the biggest challenge is in getting a good dehusking machine. This is only part of the challenge, but thats for another day. So how does one evaluate the performance of a dehusking / hulling machine? Broadly, we can look at these parameters under two categories.

What does it take to get it to work?

These are reasonably self evident and do not need too much explaining.

1. How much electric power will it require to run?
2. Does it require 3 phase or single phase electricity?
3. What is the minimum quantity needed to get decent output?
4. How many people are needed to get the design output?
5. How much floor space does it require?
6. Does it require an elevator to load grains into the machine?
7. How many times do the grains need to be processed before we get desired quality output?

When working, what does it deliver & how much of it?

There are four parameters to quantify this:

1. hulling efficiency: how many unhulled grains come through in 100 grains of the output after one pass through the huller.
2. rice recovery percentage: weight of the rice fraction in the output as a percentage of input material.
3. grain shattering percentage: the weight of the shattered grains in the output as a percentage of the input material
4. through put / capacity: What is the maximum quantity of input grains it can process in an hour?
5. bran loss & damage to bran estimation: how white are the millet rice kernels in the output? And what is the estimate on the extent of damage to the bran layer?

Note that the unhulled grains and the broken/shattered rice kernels in the output should be separated before weighing for the rice fraction or the shattered grains fraction of the output.

Do ask the machine supplier these questions / for the values of these parameters. This information could be in the product brochure or as part of a third party evaluation report. If you are looking to process a particular millet, make sure the numbers they share are for that millet. Yes, millets are rain fed crops and there will be variations based on variety, season, etc. But having these numbers and estimates is essential to understand how well the machine performs and what has gone into designing (and refining) it.

If you are looking for a machine to process multiple millets, please ask them to share information for at least three of them – either foxtail or little millet, kodo millet, and either proso millet or barnyard millet.

Immature millet grains – why remove when processing?

The primary motivation to eat cereal grains such as paddy rice, millet rices, or wheat is the nutrition we can derive from these. The largest component, in terms of weight composition of the grain are the carbohydrates in each of these grains. The other nutritional components we can derive from these grains are fibre, minerals and essential fatty acids – to varying degrees depending on the nutritional content of individual grains. In a recent post, I had mentioned about how we can use the carbohydrate to fibre ratio as a fairly good indicator to identify a grain that suits one’s dietary needs.

Note the immature grains in this foxtail millet sample. They are the thin ones distributed throughout, some with a greenish color.

When a grain is very light, it is not filled with enough carbohydrates in its endosperm – the hard part of the grain. These grains typically do not get dehusked properly during the hulling process. And even when they do, the millet rice kernel tend to shatter resulting in an increase in the grits among the millet rice kernels. These immature grains would also not taste good when eaten primarily due to the ill-formed starch component in the endosperm or the heart of the grain. So the cooking quality deteriorates dramatically even if we are able to process them to rice or rawa form.

The maturity of the oils – the fatty acids in the bran layer in such grains is also very low. This means that the oils go rancid very quickly in such immature grains even if

one is able to get the husk off without damaging the millet rice kernel. And once the oil on a few grains go rancid, it gives the entire package a foul odour and the rancidity spreads to the other mature grains too.

To summarize, removing the immature millet grains from the better formed ones during processing for the millet rice, improves  (i) the taste (ii) the cooking quality (iii) shelf life and (iv) the cleanliness of the product. Once separated, the light grains can be used for cattle feed as it is rich in cellulosic material.