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Asked on 20.06.2022.

Blisters and hollow bottom in rotary moulded biscuits

I´ve read the article about rotary moulded biscuits. It is really very interesting. I would like to go deep with the problem of blisters and hollow bottom in the finish product. Please, could you tell how the characteristics of the dough and the way of mixing affect this problem?

I think that the type of flour, the amount of water, the way we introduce some reworked material, the temperature of the dough are important, but I am not sure of that.

Please, could you help me?

Thank you in advance.

Best regards.


Answered on 24.06.2022.

Hi Eduardo

To me, the type of band and amount of water in dough impact this directly. It seems that softer doughs tend to show this behavior more than firmer ones. It is not clear to me what type of oven band you refer, but this is more common in solid steel bands, or dirty wireframe bands, since gases and steam could get trapped between band and dough piece. The amount of leavening power and temperature of the band prior to oven entrance could also impact this, so, not using pre-heating could help too. 


Answered on 20.06.2022.

Hi Eduardo

Problems like hollow bottoms in short doughs can be a result of the way the dough is mixed and processed as well as variations with raw materials especially flour. Both can result in gluten development which toughens the dough causing the dough pieces to shrink after forming and potentially closing the docker pin holes. If the docker pin holes close the steam formed during baking gets trapped between the dough piece base and the oven band resulting in cavities often referred to as hollow bottoms and potentially” checking”.

Dough mixing

For short doughs suitable for rotary moulding the dough mixing process needs to be designed so that the flour is kept separate from the water until the final stages of mixing which helps minimise the formation of gluten. This usually involves using a 3-stage mixing process as follows:

1. Mix the fat, sugars and any other dry ingredient (except flour) until a smooth paste is formed. The rework can be added at this stage

2. Add the water with any chemicals that need dissolving and mix until the water is well dispersed, and the sugar is partially dissolved. Best results are achieved when the water is dispersed as small droplets in a continuous fat phase.

3. Add the flour and mix on slow speed until a smooth dough forms, don’t overmix at this stage.



The consistency of dough fat is very important and is temperature sensitive. In short doughs plasticised fat binds the dry ingredients together and forms a continuous network which can be processed into dough pieces by wire cutting or rotary moulding. One of the most common dough fats used in biscuit making is Palm Oil which at room temperature is a mixture of crystalline fat dispersed in liquid oil with the quantity of the crystalline material reducing as the temperature rises. The optimum temperature for dough fat usage is approximately 20 ˚C because if the dough temperature is too low more fat will be crystalline, and the fat may form lumps during mixing. If the mixing temperature is too high the fat crystals will melt resulting in liquid oil which cant suspend the water and this may separate out of the emulsion potentially hydrating the wheat gluten and forming a tough dough.

Wheat Flour

The protein content for short dough flour should be in the range 8 -10% with lower protein levels giving more baked volume and more tender textures. The best flours are milled from soft wheats which results in flour with low damaged starch. It is the level of damaged starch which determines how much water is required to make a machinable dough with more damaged starch requiring more water in the dough.

I hope this helps

Andrew Hughes

Answered on 27.06.2022.

Dear Eduardo,

As involved in the Bake Oven Band business for many years I have had this topic on my table a few times. Strictly from a machine element point of view.l though.

Quite a few bakeries are using perforated steel belts in their ovens to prevent gas-blisters underneath the baked product.This steel band product allows baking of a wider variety of biscuits as well.

Perforated belts are easy to clean and stays flat over time, because of its stress relieving abilities.

Different hole patterns and open areas can be chosen depending on requirements.

One can say that perforated steel belts are combining the best of two technologies; solid bands and mesh bands.

Best of luck to you!


Answered on 07.07.2022.

Dear Eduardo,

Hollow bottom and blisters at bottom side of the soft dough biscuits mainly due to band on which it is baked secondly the ingredients used and mixing methods.

1.  If it is a steel flat band then change it to articulated S band.

2.  Please check the fat, if fat is having lumps then you have this problem

3.  Please check the cream before you add wheat flour.  You should have uniform homogenous cream without any separation of oil and water and should be a good emulsion of your ingredients.  If not then adjust the emulsifiers and other ingredients and water to get good emulsion also check the speed of the mixer blade.  It should have sufficient speed to homogenous the ingredients with emulsifiers.

4.   Do not give more bottom heat in initial zones.  Moulded soft dough biscuits to be baked from top heat and having around 15-20*C more than bottom heat.  Give bottom heat at colouring zone after baking done in the oven.

Hope this will give better result in getting the problem out.

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