Coating of nuts with a slurry and roasting
A dry-roasted nut is a nut delivering an extra crunch thanks to the addition of a coarse and flavoured layer. The process is now best implemented in large capacities on industrial lines.
The core is a nut – typically peanuts. Medium to large sizes are preferred as coating does not essentially increase the size of the nut.
Nuts such as cashews, almonds are used in premium mixes.
The raw nuts are primarily coated with a slurry acting as a glue.
This slurry is made of hydrocolloids which have swelling, filming and adhesive properties such as:
- Gum arabic
- Pre-cooked starch
- Modified starch
Then a flavoured powder mix is added.
Again, the mix is made of powders combining adhesive, filming and flavouring properties:
- Taste enhancers such as glutamate, yeast extract
- Heat-resistant flavour or ingredient developing flavour when exposed to heat
- Native flour or modified starch
Due to the extent of different ingredients, the flavoured mixes are usually supplied ready-to-use by specialist producers.
How does it work?
It is a batch or continuous operation depending on the size of the batch and flavour range.
The process may varies to obtain a specific aspect such as with the Japanese-style snacks: after the biscuit forming and roasting, the nuts are coated with a starch solution. After drying, it displays a shiny surface. Visibles can be included in or stuck with the solution: spices, herbs, algues.
Due to the difficulty to handle a mass of liquid- and powder-coated product, the process is best run continuously.
It allows for more even dispersion and roasting together with lesser damage of the nuts. The coating sequences run as follows
Preparation of the slurry by dilution/heating of a powder in water (10 to 40 % dry ingredient depending of the hydrocolloid)
Coating of the nuts with the fluid slurry
Addition and coating of the wetted nuts with the flavoured powder mix
Dry-roasting in an oven until a crispy layer develops on the nuts
Cooling before packaging
The successive liquid and powder applications can occur batchwise or in a continuous way.
The slurry is simply dripped over the nuts and dispersed by the motion.
The flavoured powder mix is dosed and dispersed with a vibrating scarf plate.
As the powder tends to set quickly on the wet surface and form agglomerates, it is recommended to disperse the powder finely and progressively over the nuts.
In a simplified version, the process applies to the coating of sunflowers seeds to obtain a favourite Spanish treat : the “Pipas”. The solution is a glue made of cooked starch onto which salt is added before roasting.
The successive liquid and powder applications occur in a pan, or a continuous rotary tumbler or screw coater.
How do you measure your success?
Key quality features
The product is a combination of features
Choice of standard or premium nuts to enhance the taste.
Crunchiness, heat-stable or developping flavour and colour combination, degree of roasting.
Limited nut fines, skin, coating residue, lumps all along the process
Key quality parameters
Some aspects of this type of coating are critical
Viscosity (thickness) and filming property of the slurry,
Particle size of the powder to facilitate adhesion without causing lumps
Timely and quantitative control of the liquid and powder addition to prevent excessive humidity or agglomeration
Roasting time and temperature profile to remove moisture while preventing burning.