Autogestión y defensa de los recursos

8.1 CONCLUSIONS

This work has identified the mechanisms for sticking and caking in dairy powders and the conditions under which these phenomena occur. Two main mechanisms for sticking and caking in dairy powders were identified. Milk fat was found to contribute to sticking and caking problems when present in high amounts, that is, greater than 42% total fat. This mechanism was found to be related to the surface fat content of the powder and hence sticking and caking problems were present in powders where the surface fat content was greater than 1.95g/m2. When a high fat powder is exposed to conditions of high temperature, the surface fat becomes completely molten and liquid bridges between particles form. These fatty liquid bridges can later partially solidify, due to the crystallisation of the milk fat, if the temperature of the powder is reduced to some temperature below the final melting point of the milk fat. The liquid bridging stage did not cause any increase in the caking strength of the powder, but the caking strength did increase when the temperature was reduced and the bridges solidified. The amount of surface fat that solidified/crystallised after being in a molten state was directly related to the caking strength of the powder. In order to prevent sticking and caking problems in dairy powders due to the milk fat during storage, it is recommended that temperature fluctuations be avoided.

The second mechanism responsible for sticking and caking problems in dairy powders was found to be due to the amorphous sugars present in the powders. This mechanism is related to the glass transition temperature (Tg) of the powder. If the powder is

exposed to conditions where the Tg is exceeded, the amorphous glass becomes a rubber

and flow of the amorphous material between particles can occur. The liquid bridges that form give the powder some strength and with time crystallisation of the amorphous sugar will proceed and the bridges will solidify, causing the powder to cake. Once caking has occurred, the powder can only revert back to a free flowing state by breaking the bridges between the particles. The rate of sticking of amorphous sugars with time was found to be related to the T-Tg of the powder. Therefore, it is only important what

the T-Tg of the powder is and not the temperature and relative humidity conditions used

to achieve the particular T-Tg. The sticking of amorphous sugars was found to be a

viscosity related mechanism. This was found to hold for amorphous glucose, galactose, sucrose, fructose and maltose.

In order to predict conditions for sticking and caking in dairy powders, the isotherm and Tg profile for the powder are needed. This work also investigated methods for

predicting isotherms and Tg profiles for powders of a known composition. It was found

that the isotherm could be successfully predicted by the weighted addition of the amounts that each component would sorb alone. That is, the isotherm for a powder can be predicted from the weighted addition of the isotherms for each component. A new method for predicting the Tg profile for a powder was proposed and validated. This

amorphous sugar components. Methods currently in literature use the weighted addition of the dry Tg values for each component and then apply a plasticisation by the

moisture effect related to the amorphous sugars. The proposed method was found to be more accurate than traditional methods given in the literature for predicting the Tg of

multicomponent powders. In order to predict the Tg profile for a powder, it is necessary

to determine the water activity of the powder and hence the amount of moisture that is associated with the amorphous sugars in the powder. The methods for predicting the isotherm and Tg profile require that accurate isotherms and Tg profiles for each

components be used. Isotherms and Tg profiles for the components were determined,

either experimentally or from available literature, and equations for the prediction of the isotherm and Tg profile for each component are determined.

Finally the prediction methods used in this work, that is, that the isotherm and Tg

profiles for a multicomponent powder can be predicted from the weighted addition of the isotherms and Tg profiles of the components, were combined in a model and

validated. The validation was done using a number of dairy powders of different compositions. A comparison was made between the measured and predicted isotherms and Tg profiles for these powders. It was found that the isotherms and Tg profiles could

be adequately predicted using the methods used. In general, the agreement was within

±10% for the isotherms and ±5°C for the Tg profiles. The model can be used to

determine the water activity and hence moisture content that a powder should be dried to in order to avoid sticking and caking problems during storage. It can also be used to determine temperature and relative humidity combinations during drying so that the powder can be dried under conditions that do not exceed a particular T-Tg. The T-Tg

required for instantaneous sticking during drying was not found. It is thought that the T-Tg for instantaneous sticking will depend on the amount of amorphous sugar and the

composition of the amorphous sugars present.

8.2 SUGGESTED FUTURE WORK

This work has highlighted several areas that require further research before the model can be accurately applied in the dairy industry. These are listed below:

1. Investigation of the rate of sticking in dairy powders as related to the T-Tg and the

amount and composition of amorphous sugars present. It is proposed that the T-Tg

required for sticking during processing depends on the amount of amorphous sugar present.

2. Further investigation into the validity of the methods used in isotherm and Tg

prediction, particularly for dairy powders with a number of amorphous sugars present.

3. Development of a standard method for measuring the moisture content of a powder so that accurate predictions of the water activity and hence Tg can be made. The

moisture content determined on site should not remove the bound moisture or residual moisture as this is not included in the isotherms and therefore predictions using the isotherms cannot be made. Alternatively, the bound and residual moisture for each component could be determined so that a correction could be made to the moisture content.