The last concentrate leaving the evaporator is liquid. This concentrate may however have different viscosity depending upon the composition, heat sensitivity of the proteins, pretreatment, temperature and solids content.

THE EFFECT OF VISCOCITY
Whole milk concentrates are generally less viscous than skim milk concentrates, and as a general rule the viscosity should not exceed 60 and 100 cP, respectively, if the atomization should be optimal.

Higher viscosities can of course be handled in the dryer, but not without losing capacity (bad atomization - big droplets) and the result is an inferior product.

The composition will influence the viscosity, especially on the protein (P) content in relation to the lactose (L) content. When the ratio P:L is high the concentrate will get a high viscosity. This is especially a problem with jersey cows during the entire year, while other breeds tend to give problems during the beginning of the lactation period. The ratio P:L can be adjusted by adding lactose. As a general rule it can be concluded that a higher fat and lactose content will result in lower viscosity. Higher protein content will give higher viscosity.

HIGH HEAT TREATMENT
When milk is exposed to a high heat treatment, especially in indirect pasteurizing systems, prior to the evaporation, the viscosity of the concentrate will be higher.

The concentrate temperature will naturally have a direct influence on the viscosity and higher temperature means lower viscosity.

The solids content of the concentrate will have a very significant influence on the viscosity, and the higher the concentration the higher the viscosity.

PARAMETERS AFFECTING VISCOSITY
However, the above only states the direct influence of some parameters on the viscosity. One of the main influences on viscosity is time, i.e. the viscosity is a function of time, also known as age-thickening. This means that the viscosity will increase if the concentrate is left for some time. The increase depends on composition, mainly proteins binding to each other, temperature and concentration. The age-thickening is only partly reversible by agitation.

A temperature increase will naturally result in a viscosity drop; but as the age-thickening is more pronounced at higher temperatures, the viscosity will soon increase to the same level and higher as the time passes. See Fig. 36.


Fig. 36  Age-thickening as a function of temperature (skim milk 48.5% solids)

The age-thickening will also be influenced by the content of the solids and will appear more pronounced the higher the solids content in the concentrate. See Fig. 37. The composition will have same influence on the age-thickening as on the viscosity. If the concentrate should be kept for some length of time, or transported over long distances before further processing, the concentration and temperature should be low. The low temperature will at the same time prevent bacterial growth.


Fig. 37  Age-thickening as a function of solids content (skim milk 55°C)

The quality of the concentrate for spray drying should comply with the following guidelines:

Skim milk

  • Solids content:
    48-50% TS
  • Viscosity:
    max. 100 Cp at 40ºC measured on the feed to be atomized. Measurements on concentrate not older than 15 min. and kept under vacuum between evaporator and sampling for viscosity measurements. Method Brookfield viscometer model LVT with spindle 2, rotation 60 rpm, measured at 40ºC.
  • Protein denaturation:
    For instant products the WPNI should be 2.5-3.5 mg. For high bulk density powders the WPNI should be max. 1.0 mg.
  • Solubility index: 
    no measurable amount.
  • Sieving test:
    no visible insolubles (cheesy flakes) on 250 micron mesh after passing 1 litre of concentrate through the mesh and washing with water.
  • Scorched particles:
    no measurable amount.

Whole Milk

  • Solids content:
    48-50% TS
  • Viscosity:
    Max. 60 Cp at 40ºC measured on the feed to be atomized. Measurements on concentrate not older than 15 min. and kept under vacuum between evaporator and sampling for viscosity measurements. Method Brookfield viscometer model LVT with spindle 2, rotation 60 rpm, measured at 40ºC.
  • Protein denaturation:
    For instant products the WPNI should be 2.5-3.5 mg. For high bulk density powders the WPNI should be max. 1.0 mg, based on SNF.
  • Solubility index: 
    no measurable amount.
  • Sieving test:
    no visible insolubles (cheesy flakes) on 250 micron mesh after passing 1 litre of concentrate through the mesh and washing with water.
  • Scorched particles:
    no measurable amount.

Whey

  • Solids content:
    min. 52% TS
  • Crystallization degree: 
    > 75% of the lactose to be crystallized,
    mean crystal size: 30 microns
    biggest crystal size: 50 microns
  • Viscosity:
    max. 1000 Cp at 40ºC measured on the feed to be atomized. Method Brookfield viscometer model LVT with spindle 2, rotation 60 rpm, measured at 40ºC.
  • Protein denaturation:
    Whey proteins should be max. 25% denatured (approx. 80ºC, 15 sec. pasteurization in evaporator)
  • Solubility index: 
    no measurable amount
  • Sieving test:
    no visible insolubles (cheesy flakes) on 250 micron mesh after passing 1 litre of concentrate through the mesh and washing with water.
  • Scorched particles: 
    no measurable amount.
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