GEA TDS
offers different engineering concepts
for ESL treatment
processes:
direct heating, indirect heating, microfiltration and deep-bed
filtration
Process sequence of the ESL direct heating plant
The starting
product for the ESL direct heating plant is standardized and heat-treated milk
or just milk with a standardized fat content buffered in a storage tank. In the
ESL direct heating plant, the product is first regeneratively preheated to 70 °
C – 85 °C and then heated to max.
127 °C by direct steam injection. The
milk is held at this temperature for approx. 3 seconds and is then cooled down
to 70 °C - 85 °C in a flash cooler. To ensure the product is well stabilized,
aseptic homogenization is carried out at a temperature of approx. 70 °C.As a
result of these extremely short heating and cooling times at a high heating
temperature, the direct process offers the advantage of top product quality.
Taste tests have shown that from an organoleptic point of view the product is
virtually comparable with conventionally pasteurized fresh milk.
Milk can
be supplied to the filling line as finished milk via modified sterile tanks or
via high-hygienic milk tanks designed especially for ESL milk. The storage
temperature in the tanks should never exceed 5 °C. For the “ESL indirect
heating plant” process a similar intermediate storage approach is required.
Process sequence of the ESL indirect heating plant
The
pre-treatment of the milk corresponds to the process for the ESL direct heating
plant. The product is supplied to the ESL indirect heating plant from storage
tanks. The milk is first heated to 70 °C by regenerative heat exchange and then
specially homogenized. Next, the product is preheated to approx. 105 - 107 °C
by regenerative heat transfer and then heated to 124 °C
in the heating
section. The temperature holding time is approx. 2 seconds. Special tubular
heat exchanger sections have been developed to ensure that the product quality
achieved is comparable with the quality of ESL milk produced by direct steam
injection.
Indirect UHT heating plant for ESL milk production
Microfiltration process sequence
For the microfiltration
process, ceramic membranes with pore sizes of 0.8 μm – 1.4 µm are used.
Bacteria removal rates of more than 99.5 % can be achieved. The process used
here is cross-flow filtration, which produces a bacteria-reduced permeate and a
bacteria-enriched retentate. The bacteria concentrate is 20 times or 100 – 200
times concentrated. Retentate that has been 20 times concentrated is high-heat
treated and added to the permeate. 100 - 200 times concentrated retentate is
not used for the production of ESL milk. In the first heat transfer section of
the milk heat exchanger the raw milk is preheated and then cleaned and skimmed
in the separator. The skim milk is then microfiltrated at skimming temperature.
The cream required for fat content sterilization is high-heat treated
together with the retentate yielded from microfiltration at approx. 105 °C -
125 °C for 4 - 6 seconds.
After high-heat treatment the cream is mixed with
the skim milk and homogenized in a separate stream. The standardized milk is
pasteurized in the milk heat exchanger, then cooled down to 4 - 6 °C and made
available for filling in the filling tanks.
Deep-bed filtration process sequence
For the deep-bed
filtration process, polypropylene filter cartridges are used. The systems
consist of a pre-filter with a nominal pore width of 0.3 µm and a main filter
with 0.2 µm. The micro-organisms are retained in the depth of the filter
medium. Filtration is carried out at separation temperature. Due to the low
pressure drop in the filtration system, no additional pumps are required. No
retentate is produced as is in microfiltration. Apart from retentate treatment,
the process sequence in the heating plant corresponds to microfiltration
processes. As the level of thermal stress to which milk is exposed is lowest in
the filtration processes described, this milk is closest to the conventionally
pasteurized fresh milk from an organoleptic point of view.
Published: 17 January 2009 / 19 February 2009