(585y) Study Of Spray Dried Yoghurt Production In A Pilot Scale Equipment

The search for
new solutions in both products and processes guide the development of the food
industry. When compared to their hydrated forms dehydrated products have
advantages for ease of handling, transportation and storage costs, and usually
an extended shelf life. Yogurt is very well accepted by Brazilian society and
is consumed in its natural state in various formulations. If dehydrated it may
be used as an ingredient in many processed foods, stimulating the development
of new products. This paper attempts to establish suitable conditions for the
production of yogurt powder using the technique of spray drying in a pilot
scale equipment, studying the influence of the rotation speed of the atomizer
disk (25,000, 27,500 and 30,000 rpm), the temperature of the air entering the
drying chamber (160, 170 and 180 °C) and the feeding flow of the product (30,
40 and 50% of the rated capacity of the feed pump, respectively, 5.2, 6.9 and
8.7 kg/h) by a randomized complete 23 factorial design with central point. The
results showed that there are significant interactions between the three
studied parameters. The effect of the feed flow rate of yogurt in the dryer is
very important to control the particle size distribution of the dried product,
affecting powder homogeneity.

The design of
experiments technique proved to be a suitable choice for the study of the
process, since there are interactions between the parameters in system
responses, a fact that is hardly observed by using a strategy of changing one
condition at a time.

Plain yoghurt
from Fazenda Bela Vista (Brazil) was used in the experiments (88.2% water, 5.8%
carbohydrates, 3.7% proteins, 2.0% fats). Equipment was a spray dryer from
Tecnape (Brazil) with nominal evaporation capacity of 5 L of water per hour
(Figure 1). Hot fumes from a LGP burner with temperature control is feed
co-currently in the drying chamber (0.63 m3). Powdered product is collected
from the bottom of the chamber and also from a cyclone separator. The process
conditions are suitable to be scaled-up to equipment of larger size.

Figure
1. Spray dryer SD-05 from Tecnape (Brazil)

There was
considerable deposit (20 to 30%) of product in the wall of the drying chamber,
indicating that the optimum conditions for drying have not been determined yet.
This is due to the low value of the glass transition temperature of yoghurt
with respect to drying conditions of the process. It was possible to know the
relative importance of each parameter in the drying process, however, the sticking
of material on the wall of the drying chamber is probably the major challenge
to be addressed for deemed satisfactory drying process as a whole. Results
suggest that a reduction of inlet air temperature in the drying chamber should
contribute to the reduction of the deposit. Nevertheless, equipment operating
limits do not allow temperatures lower than 160 °C for stable steady state
operation.

The feed flow rate
of yoghurt proved to be the most important parameter since any excess
compromise the performance of the whole process. The best conditions to produce
a powder having greater homogeneity and less amount of material deposited on
the wall occurred at 30% of the capacity of the pump (5.2 kg/h), the lowest
feed flow value studied, and at the upper value of the atomizer disk speed, rpm
30,000. With the inlet air temperature of 160 or 180 °C results were
satisfactory. However, because of the material that adheres to the drying
chamber wall one cannot consider that the drying process has been successfully carried
out.

The particle
diameter analysis was done by laser diffraction using a Mastersizer 2000
(Malvern, UK) and the difference [d(0.9) ? d(0.1)] can be used as an index of
homogeneity of the distribution of particle diameters. It is understood that the
more homogeneous the sample with respect to particle size, the better the
performance of the process.

In an ideal
dryer, the drying is an isenthalpic process, so that the energy required for
the evaporation of water from the wet material is achieved by reducing the air
temperature entering the device. If satisfactory moisture level in the final
product is achieved, the lower the temperature of the outlet air, the smaller
the heat effects such as color change and microorganisms destruction.

The sticking on
the wall of the drying chamber is a very undesirable effect and suggests that
the chosen material to dry or process conditions chosen are not the right ones.
The sticked material on the wall was evaluated by weight percentage of brushed powder
from the drying chamber with respect to the total mass of product recovered in
the process. It is understood that the smaller the amount of material sticked
on the wall, the better the process of spray drying.

The color
difference analysis was established against Molico skimmed milk powder (Nestlé)
by means of CIELAB color values under D65 illuminant and 10° angle using a
ColorQuest XE (Hunter Lab, EUA). The smaller the difference ΔE^*,
the better drying process performance. All variables studied were related to process
parameters to identify key trends and effects and the main results of the
experiments are presented in Table 1.

Table
1. Process performance indexes for plain shaken yoghurt spray drying for tests
in coded variables

By inspection of
Table 1 it is possible to verify that the desirable minimum values of process
performance indexes appear in four different study conditions (2, 3, 4 and 5).
Thus, for the study conditions there was a not a condition that would
accommodate the various desired goals. Thus, to define which conditions proved
the best, with the goal of guidance for further study will be necessary to find
a compromise solution, which should be verified in practice later. With this
aim other values besides the minimum ones were also considered to settle the
best possible process conditions. So, the conditions of test number 2 showed a
minimum percentage of material sticked on the wall and were reasonably
satisfactory in two other parameters: sample homogeneity and color difference.
Therefore, one can conclude that, in this project, the best working conditions
were: speed of rotation of the atomizer: 30,000 rpm; inlet temperature of the
air in the drying chamber: 160 °C and feed flow rate of pump: 30% of the rated
capacity, or 5.2 kg/h. These conditions will be used as basis for further
study.

The sticking on
the wall phenomenon is occurring, probably, but maybe not exclusively, on
account of having exceeded the glass transition temperature of the yogurt. This
problem can be solved partly or wholly with the addition of maltodextrins in
the shaken plain yogurt. Maltodextrins with DE = 5 or 10 have high glass
transition temperatures and thus can help on decreasing the gumminess condition
of yoghurt, hindering its grip on the wall and on other particles also. Based
on literature review it is evident the importance of obtaining a product with
viable microorganisms and, accordingly, both the addition of skimmed milk
powder and gum acacia suggest the need for research.