Concentrating
on concentrated milk
Concentrated
milk products in India are largely used by the confectionary industry.
Dr Tej Kuchroo explains the basic technology that goes behind the making
of these products
Canned
sweetened condensed milk and evaporated milk are two concentrated milk
products that require the immediate attention of the industry. While
the former is widely available courtesy Nestlè and Amul the latter is
slightly elusive. They can both be used as delicious substitutes for
milk. Oddly though they have not really received the attention they
deserve.
Basic
technology
As
water constitutes 85 per cent or more of the milk, it is essential to
understand the basic technology involved in the removal of water from
milk. The present technology adopts three basic approaches:
1.
Evaporation – removal of water
2.
Ultra filtration and reverse osmosis
3.
Freeze Concentration.
The
difference in the three approaches lies in three different methods
a.
Removal of water as gas or vapour– Evaporation
b.
Removal of water as water – Ultra filtration
c.
Removal of water as ice or in solid state – Concentration by freezing
Evaporation
This
is an established method and the most commonly used technology in the
dairy industry. It is however relatively expensive due to the energy
spent and the heat damage incurred by the product due to exposure to
heat. The method involves a heat transfer process which is similar to
our domestic concentration of milk in a pan. The temperature of boiling
is determined by pressure (vacuum) and to a minute extent by milk solids
and hydrostatic pressure of the column of milk. The milk temperature
therefore is controlled by the control value when liquid and vapour
exist in equilibrium.
The
modern evaporator installation can appear to be complex but the technology
used is relatively simple and almost same for each plant.
1.
A vacuum evaporator acting as a heat exchanger
2.
A separator for the separation of vapour and concentrate
3.
Vapour condenser
4.
Equipment for the production of a vacuum and the removal of condensate
The
Evaporator is one of the most important equipment in the drying process
or concentration of milk solids. Its importance is also related to quality
of the concentrate and its operating efficiency. In recent years falling
film evaporators have achieved predominance in the dairy industry, while
other types like batch pan, plate, rising and scraped surface exchangers
have limited application. The advantage of the falling film designs
lies in its single construction design and very low capital cost with
low levels of thermal damage to milk constituents.
The
falling film evaporator consists of a bundle of tubes, surrounded by
a steam-heated jacket from which the milk flows. The diameter of these
tubes range from 25 to 80 mm and the proportional length is about 4
to 15 m. The milk is evenly distributed using perforated plates, and
flows by gravity. High vapour velocity is an important design feature,
which is influenced by the diameter and length of the tube, temperature
difference and level of vacuum. High velocity helps in reduction of
thermal damage of the milk. The necessary degree of concentration is
normally achieved through a single column evaporator, however a single
conventional unit evaporator consumes high quantity of steam. The common
types of evaporator presently in use are triple effect evaporators wherein
vapour from one evaporator column is used to heat the next column. The
effect is linked in series with a common condenser and the vacuum source.
The milk flow is usually high in low temperature conditions and the
difference is usually maintained at 1500 C between the evaporating columns.
Hence the water removed during each effect remains almost the same.
Energy
saved is marked when the columns are four in number or more. The heat
transfer co-efficient is lower in the later effects due to the increase
in viscosity which in turn increases the heating surface. These difficulties
could be overcome by the counter current flow of milk. This system however
results in an uneven retention time and possibility of lower quality
of product.
In
a single effect plant, heat content of the evaporated vapour is approximately
equal to the heat input. The reduction in steam consumption arises from
the use of the condensation heat of one effect to heat a second effect.
Saving in heat consumption depends upon the number of effects. The heat
consumption can be illustrated by comparing theoretical specific heat
consumption (SHC).
| SHC
= |
Amount
of steam used for heating |
|
|
| |
Amount of water evaporated |
Theoretical
values are; single effect, 1.0; double effect, 0.5; triple effect, 0.33
and quadruple effect 0.25.
Recompression
equipment is of two types, thermal and mechanical. Thermal systems
are the cheapest but thermal vapour compression is not efficient when
temperature differences are minimal. This reduces the steam consumption
and cooling water costs. Modern thermal recompression typically has
six effects with the theoretical SHC equal to 0.12 per cent and total
energy requirement is 360 kJ/kg water evaporated.
Mechanical
vapour recompression offers even greater thermal efficiency since all
the vapours are compressed. Though the capital cost is very high, single
stage centrifugal recompressors are widely used. They also provide compression
ratio of 1:1.2 and 1:2 but multistage recompressor requires a higher
compression ratio. Vapour condensation and production of vacuum is an
important aspect of the evaporating plant.
The
production of vacuum requires the removal of the following:
.....CONTD
TO
READ FURTHER... SUBSCRIBE TO YOUR
COPY TODAY!!!
