Blisters on marie biscuits

Large crust blister are the results of various reasons and some of them are well discussed by the previous members.
I want to add some more points,
1. Hard crust means the top layer is getting hardened very fast and that may be due to the absence of enough moisture in the proofing zone. If the problem is not getting solved by adjusting the top bottom temperature, check for the presence of moisture in the zone. The suction motor also plays a great role in that.
2. The gluten quality is very vital for hard dough. Very high and hard gluten may be the cause.
3. Long mixing results very tough dough, is another reason for blister.
4. Dough may requires more standing time after mixing to get stabilized.

- increase the sms content and increase the mixing time to get stable dough. the thickness will be less but increase the raising agent 

-use an enzyme in the recipe and make resting time to get stable dough

When blister appear in the product and it is necessary to identify in 

which part of the oven the problem is evident, when this occurs it 

can be recipe with wrong balancing or cooking, the ballast with very high

 temperatures can show the blister, this can be solved with reduction of

 ballast temperature throughout the oven without losing cooking zone temperature.
The other alternative is to better homogenize the mass without raising the temperature too 

much and making adequate rest for better stability.

We should be trying to dissolve baking soda as well as Ammonium Bicarbonate in water prior the addition in the Mixer creating homogeneous disperse in the dough.

Increase ur creaming phase of mixing from 1 to 2 mints and same time reduce in SMS phase addition,

Have you separate phases of mixing dough like as follows:

1. in phase 1 you add all flour, shortening, sugar and other materials except SMS u can increase this time 1 to 2 mints

2. In 2nd phase you add SMS and give mixing from about =6-10 mints depending upon ur flours

Adjust ur timings ur problem will be resolved this is main reason.

You can revise your mixing sequence in 3 phases for proper homogeneous mix as follows it is proven and will solve ur problem:

1.1st phase shortening+sugar+all other ingredients except flour and SMS mix time 4-5 mints

2. 2nd phase add flour mix time 2-3 mints

3. 3rd add SMS and mix 7-10 mints depend upon u. 

apply it and share ur feedback with me i hope ur problem will be resolved.

Besides what is written above, try lowering dough temperature (by reducing mixing time or/and ingredient water temperature) and try using a DATEM emulsifier. Should work.

Blisters in hard dough owing to following reasons.

1.  Very high gluten flour.

2.  SMBS level

3.  Standing time fixation with respect to flour quality and smbs level.

4.  Initial zone temperature while baking

5.  Chemicals getting homogeneous mix before wheat flour additions.

6.  SMBS to added after flour added and it will be best if you add after one minute of mixing.

If Wheat flour dry gluten between 10-12, and sedimentation value between 20-21 and accordingly the smbs level if keep at around 0.5 to 0.7% and keep around 25-35mts standing time and process the dough,  

In initital zones of oven keep the maximum steam inside the oven and final zones you can open fully.

also Make sure that ammonia is not having lumps or it is completed dissolved in water and added.

See the docker pins of the cup..  It should give proper hole in the wet biscuits and finally dry biscuit also checked for light passing through hole when you look the biscuit against light.  I think by following above steps you can very well avoid blisters on the hard dough biscuits.