3 Processing plants and equipment

Processing plants and equipment
The process of yoghurt production has evolved through the ages from a simple
preparation cai-ried out in the home on a veiy small scale to medium and large-scale
production centres handling many thousands of litres per day. The utensils and equipment
required vary in relation to the type of yoghurt produced, scale of production and the
level of technology adopted. Hence, it would seem logical to review the available
equipment and plant against a scale of yoghurt produced per day:
Home or small-scale production.
Medium-scale manufacture by a small produceriretailer.
Large-scale production.
Home or small-scale production
Traditionally, yoghurt is prepared at home, and ordinary kitchen utensils are used. The
milk is heated in a cooking pot and the production of the coagulum takes place in the
same container; Fig. 2.1 showed the overall process in brief. However, one factor that is
critical during the incubation period is the maintenance of a uniform temperature. This is
achieved by wrapping the pot in a woollen blanket and placing it in a warm place, for
example near a cooker. Although the traditional process could still be recommended to
individuals producing their own yoghurt, a simplified recipe is illustrated in Fig. 3.1.
The ‘airing’ cupboard (i.e. area beside the hot water cylinder in a modern house) is
sometimes used during the fermentation period, although yoghui-t ‘makers’ (Fig. 3.2)
have become available for enthusiasts to produce yoghurt under controlled conditions
(see also Taylor, 1981; Davide, 1988; Light, 1993; Hyman et al., 1996). Altei-natively,
warm milk inoculated with the starter culture (or natural yoghui-t) is placed in a widemouth vacuum flask and left undisturbed, allowing the milk to ferment and coagulate.
Cooling is carried out directly after coagulation has taken place and fruit and/or sugar are
normally added to the cold yoghurt.
Processing plants and equipment
Place 1 I of whole milk in a saucepan and heat to near boiling (for
the production of thick viscous yoghurt add 20-40 g of shimmed
milk powder 2 5-5 tablespoons), the addition of sugar is optional
Cool to 45 IC (or just above blood temperature) and add 1 level
tablespoon of plain unsweetened yoghurt
Pour into the containers of a ycghurt 'maker' and seal with snap
on lids, alternatively wrap the pot with a blanket, or pour the
inoculated milk into clean, wide mouth vacuum flask.
Depending on the activity of the yoghurt organisms andior
temperature of incubation, the milk should coagulate in 3-18 h
Cool the yoghurt as quickly a s possible - preferably overnight in
the refrigerator.
Blend in fruit and sugar (fresh fruit! puke or jam) in accordance
with personal preference and stir gently.
Maintain product under refrigeration until consumed
Fig. 3.1 Production of yoghurt at home.
Note the following: (a) one pot of the natural yoghurt produced could be used as a
starter culture to inoculate the following batch, (b) excessive subculturing can lead to a
prolonged incubation period, and hence it is recommended that a fresh yoghurt should
be introduced weekly, and (c) short incubation periods are obtained using fresh, active
starter cultures. an approach which is highly recommended.
3.1.1 Miscellaneous systems
The processing steps involved, including the equipment required, in the manufacture of
set or stirred yoghurt by this simple procedure are summarised here:
Milk base is prepared in cans/churns.
The cans are immersed in a water bath which is required for the heat treatment of the
milk; the heat source could be steam or electrical. At the cooling stage, the hot water is
replaced by cold water from the mains.
164 Tamime and Robinson's Yoghurt
Fig. 3.2 Yoghurt maker where glass jars with screw-on plastic caps are used.
At 45 "C milk is inoculated with starter culture and incubated in bulk (stirred yoghurt),
or for set yoghurt the milk is dispensed into cups prior to incubation; special cabinets
can be used for the fermentation, or alternatively the temperature in the water bath can
be maintained at 42-45 "C to feiment the milk in bulk.
Fig. 3.3 Hand filling of yoghurt cups.
Processing plants and equipment
At the desired acidity the cansichui-ns are removed from the incubator unit(s) and
stored overnight in the cold store.
Fruit is added separately to each cadchurn and mixed gently using a milkicream
Filling and packaging is cai-ried out using hand-operated units (see below).
3.1.2 Packaging system
For this scale of yoghurt production, it is inappropriate to install a proper packaging
machine owing to the high capital investment required. Subsequently, the yoghurt is
packaged using hand-operated unit(s), but extreme care should be exercised in order to
minimise contamination of the product. Figure 3.3A shows how yoghurt can be produced
in a 10 litre stainless steel churn, followed by the addition of fruit on top of the cold
yoghurt and mixing. The fruit-flavoured yoghurt is dispensed into plastic cups manually
using a stainless steel jug, and finally the aluminium foil lids are crimped in place (Fig.
3.3B-E). Incidentally, an improved method of closure of the yoghurt cups uses a handoperated heat sealer.
An alternative method of packaging very small volumes of yoghurt per day involves
use of a small-scale cup filler. A typical example is the CD 500/1000 machine (see Fig.
3.4). This unit is capable of filling yoghui-t cold or hot, and the filling head is fitted with
an antifoam nozzle. The capacity of filling ranges between 85 and 600 ml or g, and the
piston used for filling the yoghurt has an easy measure adjustment with a fine setting.
Fig. 3.4 Small filling machine (A) and a thermostatically controlled heat sealer for aluminium
foil lids (B).
Reproduced by courtesy of CKX Engineering, Sudbury, UK.
166 Tamime and Robinson’s Yoghurt
The sequence of operations could be described as follows: place the yoghurt cup on
the tray and press the foot pedal; the machine will dose out the set measure of product.
The filling head automatically resets when the cup filling sequence is complete and the
filled cups are then heat sealed using a separate unit (Fig. 3.4B). The speed of filling
depends on the cup capacity and the speed of the operator but, in general, the cup filling
speed ranges between 10 and 20 containers min’.
Alternatively, paperboard cartons could be used for the packaging of yoghurt using a
hand-operated cartoning and filling machine (Fig. 3.5). This method of filling yoghurt
could be referred to as a hand form-fill-seal operation. The hand-operated bottom carton
sealer (Fig. 3.5A) preforms, crimps, heats, folds and bottom seals all sizes of carton, and
pre-breaks the tops in preparation for the ‘top sealer’; incidentally, a similar unit was
illustrated in the first edition of this book and the design has been changed to include an
air-operated base sealing plate.
The hand filledsealer is basically designed for liquid milk but, by slightly modifying
the filling head, it becomes feasible to fill a viscous product such as yoghurt (see Fig.
3.5B). The preformed cartons are placed under the filler and a microswitch operates the
fill time. Then, the carton is pushed under the sealer and the handle is pulled to seal it.
The speed of both the hand cartodsealer and the fillingisealing machine is about 10 units
Fig. 3.5 Hand-operated packaging equipment for filling yoghurt into cartons. (A), Carton
maker/sealer, (B), hand filler sealer.
Reproduced by courtesy of CKX Engineering, Sudbury, UK.
Processing plants and equipment
3.2 Medium-scale production
The volume of yoghurt production in this category is rather low, perhaps in the region of
a few hundred litres per day, and such small producershetailers aim to market their
yoghurt within a limited area (see also Muller and Weijenberg, 1991). The different types
of equipment that could be used at this level are described below (see also IDF, 2001).
3.2.1 Hand-operated vat
In some parts of the world, equipment manufacturers may produce specially designed
small processing vessels (i.e. hand-operated, multi-purpose tanks) where the agitation of
the milk base during heating and cooling is done manually. The different steps involved
during the production of yoghurt can be summarised as follows:
Sanitise the equipment directly using chemical sterilising agents, drain and rinse with
clean water.
Pour the milk into the vat, add the required amount of dried ingredients (milk powder)
and mix with the aid of a stainless steel wire whisk.
Start the heating cycle using an electric element to heat the insulated water jacket and
hand agitate the milk.
After reaching the desired temperature, the heating element is switched off and the
milk is held for 10-30 min (depending on temperature), prior to cooling.
During cooling, the water in the jacket is replaced by circulating mains water. At 4045°C the milk is inoculated with starter culture and left undisturbed during the
fermentation period.
After a few hours, or at the desired acidity, mains water is circulated through the jacket
to cool the coagulum, a process that may be assisted by gentle agitation.
At around 15-20 "C, a known volume of yoghurt is drained out, mixed with fruit/
flavouring additives and hand-filled into plastic cups.
3.2.2 Multi-purpose vat
This type of vat is really a batch pasteuriser which is slightly modified to meet the
requirements of yoghurt manufacture. It is widely used for the production of viscous
yoghurt (Fig. 3.6). These vats are usually made of stainless steel and insulated with a
water jacket. The capacity may be in the region of 50-22501. When this type of vat is
used, the processing stages of stii-red yoghurt production usually follow two altei-native
patterns. In the first approach the vat is utilised for all the different steps necessary for the
preparation and production of yoghurt (Fig. 3.7, process A). However, in the second
approach the vat is merely used for the preparation of milk, that is, mixing the dried
ingredients with milk, heat treatment and cooling to incubation temperature (Fig. 3.7,
process B).
Processes A and B described in Fig. 3.7 illustrate clearly the steps necessary to
produce stirred yoghurt, but for the manufacture of set yoghurt, process C (Fig. 3.7)
should be followed. Processes B and C are similar except that in process B the milk is
fermented in bulk, while in process C the milk is incubated in the retail container. The
major differences between set and stirred yoghurt are illustrated elsewhere (see Chapters
2 and 5).
The multi-pui-pose vat (Fig. 3.6) can be heated using different sources of energy (e.g.
electrical, steam or gas) and this versatility makes this type of processing equipment very
168 Tamime and Robinson’s Yoghurt
Fig. 3.6 Typical batch pasteuriser which can be used as a multi-purpose vat for the production
of yoghurt. Gusti-steam, electric or gas heated ‘Pastomix’ - ‘Pastolux’ vat for heat
treatment of the milk base or cold storage of yoghurt (see text).
Reproduced by courtesy of 7. Gusti, Wellingborough, UK.
popular with the small producer. During the cooling stages, mains water can be used or a
closed-circuit cooling system circulating chilled water may be employed. However, if intank cooling is used for cooling the yoghurt, a slow-speed agitator (i.e. <45rpm) is
operated to mix the coagulum gently and assist cooling but, at the same time, inflict
minimum reduction in viscosity on the product. The diameter of the outlet valve must be
>5cm in order to facilitate ease of drainage of the yoghurt. On such a small scale of
production, the stages of fruit mixing and filling can be carried out manually, but great
care must be taken to minimise post-production contamination. Figure 3.7, process B,
illustrates this approach. The fruit is added to each cadchurn and gently mixed with the
yoghurt by means of a milwcream plunger.
3.2.3 Mini dairy science and technology
The ‘mini dairy’ is a small compact processing plant that was developed in the late 1970s
by Alfa-Lava1 A/B, Lund in Sweden - a project sponsored by the Swedish government to
establish small-scale milk processing units in the developing countries. At present, Tetra
Pak and Alfa-Lava1 Agri are responsible for marketing of these units in different parts of
Processing plants and equipment
Process A
Process B
Process C
Fig. 3.7 Small-scale production of yoghurt using a multi-purpose tank.
1, Inlet for liquid milk; 2, dried ingredients (milk powder(s) and sugar) added
manually; 3, starter culture added manually; 4, stainless steel chums (process A contain cold yoghurt
or process B
processed milk base inoculated with starter
culture); 5 , fruit added manually; 6, small-scale filling machine; 7, hand-filling
machine; 8. cold store; 9, t ~ 7 0small tanks (in parallel) used for the addition of fruit
with yoghurt so that filling can be continuous; 10, positive pump; 11, incubation
cabinet for set yoghurt (process C).
the world. The mini dairy unit is basically designed for processing market milks, cheese
and fermented milks. For yoghurt, for example set, stirred and/or drinking type, the unit is
capable of producing 1000 1 per batch over an extended 8 h shift. All such units are preassembled and tested to give a short and efficient installation and start-up time. The
energy required for heating and cooling is provided by mains electricity or a dieselpowered electric generator and hot water is generated by an oil- or wood-fired furnace.
Figure 3.8 illustrates a unit for processing milk for the manufacture of the products
mentioned above (Gandhi, 1986; Caviezel, 1987; Briem, 1992; Olivetti, 1993; see also
Capogna et al., 1997; Gran et al., 2002).
3.2.4 Small-scale packaging machines
Although hand filling has been adopted by many small dairies, the use of a proper filling
machine does offer some advantages. A wide range of fillers is available on the market,
and these filling machines are equipped with a diversity of sealing mechanisms, for
example the ability to heat-seal foil lids, crimp foil lids or snap-on plastic lids. The
ultimate selection of a particular type is largely a matter of personal preference (see Platt,
1990; Anon., 1998). Most manufacturers of packaging machines also produce small-scale
equipment to meet the demand from small dairies. Some examples follow.
170 Tamime and Robinson's Yoghurt
Fig. 3.8 General view of a mini dairy processing plant.
Reproduced by courtesy of Tetra Pak AIB, Lund, Sweden.
Regal RP/SA2
This machine is semi-automatic and consists of:
stainless steel hoppers that hold the yoghurt and the fruit base;
stainless steel rotaiy table;
a foil dispensing assembly with a spot sealer;
heat sealing assembly.
An illustration of this machine is shown in Fig. 3.9. The sequence of operation is as
follows: (a) the preformed containers are loaded into the machine by hand and a photoelectric cell (PEC) detects their presence, (b) the operator indexes the rotary table
clockwise to the filling assemblies, (c) when the container is filled (i.e. with fruitflavoured yoghurt or, in a two-step sequence, with fruit and the yoghurt base, separately),
the operator indexes the rotary table clockwise to the foil dispensing assembly where a
foil is placed automatically and spot sealed in position, and (d) the operator then indexes
the table to the heater assembly where the aluminium foil lid is heat sealed automatically.
As the operator indexes the rotary table once more, this allows removal of the filled
yoghurt containers. However, every time the table is indexed, another container should be
loaded to repeat the cycle. The volume of the fruit dispensing unit ranges from 10 to
80m1, and for yoghurt 60 to 300ml. Incidentally, the machine is fitted with a fully
interlocked stainless steel mesh safety guard. The same manufacturer produces fully
automatic filling machines up to 12 000 cups h-l.
Waldnel- Dosonzat 1 Eco, 1, 2 arid 10
These are rotary cup filling and closing machines that cover capacities ranging from 1000
to 20 000 cups hK'. These machines are fully automatic with the dosing unit mechanically
driven; this unit operates on the piston principle, which ensures filling with absolute care
Processing plants and equipment
Fig. 3.9 General view of the Regal semi-automatic.
Note: Arrow indicates stainless steel table specially designed for filling twin chamber
and accuracy. For viscous products such as yoghurt, product aspiration is realised by
direct feed via equalising pistons and the dosing range is regulated by handwheel. A
range of containers (e.g. cartons, plastic pots or glass bottles) can be used on this machine
for packaging yoghurt. Figure 3.10 illustrates one example operated within a laminar
flow cabinet hood. All models of the rotary Dosomat machines are fitted with a coding
system of one of the following types:
Fig. 3.10 Waldner Dosomat rotary-type yoghurt filling machine.
Reproduced by courtesy of Ultrapak, Aldershot, UK.
172 Tamime and Robinson's Yoghurt
coding with quick drying ink or hot stamping with ink ribbon on the lid or cup bottom;
heat or cold embossing into cup bottom;
The closure of the container (i.e. heat sealing with a snap-on lid) can be achieved by
heat, ultrasonic or high-frequency sealing. All models are suitable for clean-in-place
(CIP). Incidentally, the number of filling lanes on the rotary table ranges from one up to
eight, depending on the model and throughput.
GEI Turbo RotuJl
This is a multifunctional compact system of filling. The machine is available with
different sizes of interchangeable indexing table for packaging into a wide range of
container sizes. It can be supplied with many optional features such as:
automatic container dispenser and discharge systems;
multi-station or filling head facilities;
automatic closure, heat sealing and securing of anti-tampering devices;
date and price coding system.
The filling speed is around 8400 pots h-l on a four-head production system. However,
the specially designed filling head (see Fig. 3.1 1) ensures that there is a regulated speed
of filling, capacity to deliver fi-uit pieces intact into fruit-flavoured yoghurt, and virtually
drip-free cut-off between the fills.
Fig. 3.11 Filling heads on Turbo Rotafil packaging machine.
Reproduced by courtesy of GEI International, Wobum Sands, UK.
Processing plants and equipment
Cockx R 4000
The machine is a 16 pocket, eight station unit with options of pre-fill and over-lid (Fig.
3.12). In general, it is fitted with cup magazines, mechanical main piston fill, lid appliers,
heat sealers, date coders and cup ejection onto a conveyor with an extended collection
table; the filling speed is about 4000 cups hK'.
The machine has been designed to allow, if required, two different products to be
filled at the same time as the starwheel indexes two pockets at a time. The filling valves
are independent and, as an extra, two hoppers can be fitted as an alternative to the single
unit. If the pre-fill extra is used, then larger capacity cups can be filled faster with a predose prior to the main fill. The nozzles can be changed for different products and have a
positive cut-off. The measure adjustment is inside the main frame of the machine, easily
accessible through the interlocked doors. The lid magazines can be switched
independently and can be changed for containers of different rim size. The heat seal
heads have easily changeable seal plates and the date coders can be quickly adjusted for
height and position. The filled and sealed cups are raised out of the pockets and swept
onto a deadplate prior to being pushed onto a small conveyor, where they are guided onto
a collection table for packing.
The fill, lid application and heat-seal systems are all controlled by sensors and all
doors are fully interlocked for safety. There are no process controllers fitted to the
machine and the mechanical variable speed drive is connected to the piston fill drive
system by a chain and is also connected to a camshaft. This camshaft has a series of
roller-operated valves operated by individual cams which are easy to set up or adjust. In
this way, it is easy for the customer to understand the working of the machine at each
station. Lubrication ports are on one panel with the feeds through copper tubes to the
Fig. 3.12 Cockx rotary cup filler and sealer.
Reproduced by courtesy of CKX Engineering, Sudbury, UK.
174 Tamime and Robinson’s Yoghurt
Large-scale production
In this category, the equipment employed for the manufacture of yoghurt is specially
designed to handle thousands of litres per day and a highly sophisticated technology has
evolved which offers a dairy both improved mechanisation and automation. Since the
publication of the first edition of this book, few technical developments have occui-red
with respect to yoghurt technology and the latest technological progress in this field has
been reviewed in two International Dairy Federation monographs (IDF, 1988, 1992).
Driessen and Loones (1992) presented a comprehensive chart summarising the new
developments in technology including products with special microorganisms as follows:
Membrane techniques which make it possible to utilise the required properties and
avoid the unwanted properties of microbial metabolites.
Separate cultivation which makes it possible to combine microorganisms that need
differing conditions for their proliferation, for example, mesophilic and thermophilic
Applying automatic pH control to end the fermentation process and achieve a more
consistent product.
Mounting the cooler on top of the filler, to achieve better viscosity in stirred fermented
Applying in-line inoculation which makes manufacture of set fermented milks more
Overpressure of sterile air which has proved to be effective in protecting stai-ters
against contamination with other micro-organisms and bacteriophages.
The topic has been extensively reviewed elsewhere (Anon., 1981a,b, 1983a; Salji et al.,
1985; Evavoll, 1985; Nicolaus, 1987; Bianchi-Salvadori, 1989; Driessen and Loones,
1990, 1992; Nilsson and Hallstrom, 1990; Robinson and Tamime, 1990, 1993; Puhan et
al., 1994a,b; Nilsson, 1994; Strahm and Eberhard, 1994; Karagozlu and Gonc, 1996;
Gardini et al., 1996; IDF, 1998, 2003; Tamime et al., 2001; Storro, 2002; Anon., 2003;
Tamime, 2006). As a consequence, it was decided that only up-to-date information will
be provided here.
The diversity of these technologies can be discussed most easily in relation to:
type of yoghurt produced (e.g. set or stii-red);
effect of mechanisation on the quality of the yoghurt;
application of automation to the manufacture of yoghurt.
There are several approaches that can be employed for the production of yoghurt and, as
each yoghurt manufacturer has their own specific requirements, each plant is supplied, in
effect, tailor made. It is evident that plants that produce set and stirred yoghurt (or a
combined processing plant) have some stages in common (see Fig. 3.13), for example,
milk reception and handling, preparation of the milk base, homogenisation of the yoghurt
milk and heat treatment, and hence it is appropriate to review the relevant equipment in
relation to the different stages of manufacture; more specialised units are discussed
3.3.1 Milk reception, handling and storage
At present, milk collection from farms in developing and industrialised countries is
carried out in bulk using a road tanker although, in some instances, rail tankers or chui-ns
could be used. The facilities provided at a typical dairy for reception of this bulk milk
Fig. 3.14 Milk reception, handling and storage at a large factory.
1, Air climinator; 2, liltcr; 3, milk mctcr; 4, intcrmcdiatc storagc tank; 5 , thcrmisation and cooling or cooling only; 6, silo tank.
Iicproduccd by courtcsy ol' Tctra Pak Am, Lund, Swcdcn.
Processing plants and equipment
have been described by Tamime and Kirkegaard (1 99 l), Anon. (2003) and Tamime et al.
(2006) (see Fig. 3.14). The milk intake can be either metered using a metering pump, or
weighed (e.g. at a weighbridge for road tankers or in a duplex weighbowl for churns).
When milk is accepted, and after a sample for chemical and microbiological analysis has
been taken, the general practice for handling the milk may include: (a) filtering the milk
to remove contaminants (e.g. straw, hairs, soil) with the most universal system used being
a stainless steel filter; however, an optional treatment to clean the milk is clarification
using a separator; and (b) cooling the milk to <5 "C using a plate cooler prior to storing in
a silo.
The reception of milk in chui-ns is somewhat different from reception from a road
tanker. Normally the churns are unloaded in the reception area and the lids removed. The
freshness of the product is quickly determined by sniffing the churns and if any unusual
smells are noted, the milk is rejected; a composite sample of milk from each farm is
further analysed chemically for bacteriological quality.
As already discussed elsewhere (see Chapter 2), the milk is subjected to a number of
preliminary treatments before it becomes yoghurt. These processes are standardisation of
the fat content, fortification of the solids-not-fat (SNF) and homogenisation and heat
treatment of the milk base. These treatments will be discussed separately.
3.3.2 Standardisation of fat content in milk
The fat content of milk can vary according to source and season, but in yoghurt the level
is prescribed by consumer taste or the Statutory Instruments of the countries concerned,
so that standardisation becomes essential.
The theoretical approach to milk standardisation can best be visualised as follows:
Whole milk
Surplus cream
+ Separator
Skimmed m i l k
t Cream
(slandiii-disd milk)
and the accuracy of the process is dependent on such factors as:
type of equipment used and the efficiency of fat separation obtained;
control system used.
The skimming efficiency of the available plant has greatly improved over the years, so
that residual fat in skimmed milk usually falls between 0.05 and 0.07 g 100 8-l; the
skimming efficiency of the separators is thus referred to as 0.05 or 0.07, respectively. The
control system employed in milk standardisation lines can be either manual or automatic,
and while the foi-rner may be recommended for small/medium size producers, the
automatic system is essential for dairies handling large volumes of milk per day.
178 Tamime and Robinson's Yoghurt
A number of different systems can be used for milk standardisation (Hellstrom, 1986;
Anon., 1992, 1996a; Bird, 1993). The efficacy of any one particular system depends on
its ability to ensure that:
the pressure of the skimmed milk at the outlet pipe is lower than the pressure in the
tank where the skimmed milk and cream are remixed;
the fat content in the cream remains constant; the proportion of cream remixing with
skimmed milk can be stabilised, i.e. there are proportional mixing controls;
the final fat content of the process milk is within preset limits.
This is an automatic system for standardisation of the fat content in the milk and sui-plus
cream (Fig. 3.15). This unit is directly connected to a separator; however, when liquids
are mixed continuously in volumetric proportions, the Compomaster can be used without
a separator. In this system of standardisation, combined mass flow meters, density meters
and temperature transmitters are used to measure the cream and skimmed milk,
respectively (Hansen, 1996). Thus, by knowing the density and temperature of both
skimmed milk and cream, it is then possible to calculate the fat content of the cream. The
unit automatically adjusts to the set points for the fat content in both standardised
skimmed milk (1-5 g 100 g-l) and cream (18-50 g 100 g-l).
The Compomaster has capacities ranging between 7000 and 450001hp'. It is
delivered as a compact unit ready for installation and connections need to be made to the
product inlet, air-line and the mains electricity supply. According to Hansen (1996), the
Compomaster type KCC standardising system needs to be calibrated only once every
second year reflecting the high precision of the unit. This system also contains in-line
mixers for special applications (i.e. mixing cream and skimmed milk) without the use of a
separator; furthermore, this system is suitable for CIP application.
Automatic direct standardisation (ADS) Systems
These methods of standardisation of the milk and cream are very accurate and depend on
a careful choice of components and the design and engineering of the system. A typical
system is shown in Fig. 3.16, where the components within the system are clearly
identified. In brief, according to Bird (1993) and Anon. (2003), the ADS system can be
described as follows.
Fig. 3.15 An illustration of fully automatic in-line standardising system.
1, Control panel; 2, flow meter; 3, density transmitters; 4, regulating valves; 5, onioff
Reproduced by courtesy of APV Nordic, Denmark.
Fig. 3.16 Illustration ol' an automatic dircct standardisation (ADS) system for milk and crcam
1, Density transmitter; 2, [low transmitter; 3, control valvc; 4, control panel; 5, constant prcssurc valvc; 6, shut-ol'f valvc; 7, chcck
Kcproduccd by courtesy of Tetra Pak N B , Lund, Swcdcn.
180 Tamime and Robinson’s Yoghurt
The set points for standardised cream (or surplus cream) and milk fat content are fed
into the process control unit. The pressure control system at the skimmed milk outlet (Fig.
3.16 (5), constant pressure valve) maintains a constant pressure, regardless of fluctuations
in the pressure drop over downstream equipment. The cream-regulating system maintains
a constant fat content in the cream discharged from the separator by adjusting the flow of
cream discharged. The ratio controller mixes cream of constant fat content with skimmed
milk in the correct proportion to give standardised milk with a specified fat content. The
accuracy of the system, based on standard deviation of repeatability, should be <0.03%
for milk and about 0.25% for cream (see also Hellstrom, 1986; Anon., 1992).
The application of these systems to the manufacture of yoghurt could be considered
under the following conditions: (a) if the solids content of the milk is fortified using an
evaporator (Figs 3.13 and 3.17), then it is necessary to standardise the fat content in the
milk before the concentration process commences, (b) skimmed milk could be
concentrated by evaporation and then before further treatments (i.e. homogenisation
and heat treatment) the concentrated skimmed milk could be standardised with cream, (c)
concentrated skimmed milk may be standardised with cream, and (d) membrane filtration
(ultrafiltration, UF, or reverse osmosis, RO) is sometimes used to concentrate the milk
base. Normally, the fat is separated from whole milk and the skimmed milk is
concentrated to the desired level of solids; the concentrated skimmed milk fraction is then
standardised with the cream.
In general, therefore, the milk base is standardised for fat content before evaporation
commences but, if the skimmed milk is concentrated in a UF plant, the addition of cream
takes place later. The reason for adding the fat to the concentrated skimmed milk in the
Fig. 3.17 View of an internal evaporation and de-aeration plant used on a yoghurt processing
Reproduced by courtesy of APV Nordic, Denmark.
Processing plants and equipment
latter method is that the high pressure used during the concentration process could
damage some of the physical properties of the fat, which in turn may affect the quality of
yoghurt (e.g. an oiling-off or a churning effect). Alternatively, if on-line fortification of
the milk base with the use of membrane processing or vacuum evaporation is used, the fat
content is standardised before concentrating the milk, taking into account the factor of
concentration. For example, to raise the protein content in the milk base from 3.2 to
5 g 100 g-', the concentration factor is c. x 1.56. Therefore, the fat is standardised to 1 or
2.24g lOOg-' during the manufacture of low- or full-fat yoghurt -1.5 or 3.5 g 100 8-l in
the final product, respectively (Tamime et al., 2001; see also Nissen, 1999; Heggum,
1999; Jensen, 1999; Andersen and Johansen, 1999).
OL- 7000 system
This is another method for on-line standardisations of the fat content in the milk base. It is
manufactured by On-Line Instrumentation Inc. in the United States. This model is the
third generation design and ensures that the fat content is within 1t0.02 g fat 100 g-' of
the target level in the final milk base with 95% confidence. Illustration and operation of
OL-7000 system have been reported by Muir and Tamime (2001), and the same system
can be used to standardise concentrated milk with cream to the desired fat content
(Tamime et al., 2001).
3.3.3 Fortification of milk solids
The level of milk solids in the milk base can be raised by one or more of the following
Traditional process
Boiling the milk can be carried out in a tank similar to a batch pasteuriser. The aim of this
approach is the evaporation of one-third of the milk volume under atmospheric pressure.
However, this method of concentration of the milk solids is not used under industrial
conditions, mainly owing to the high cost involved, but also because the generation of too
much steam in the processing area can be unacceptable to personnel.
Addition of milk powder
Different types of milk powder can be used to fortify the yoghurt milk (see Chapter 2),
although skimmed milk powder is used most widely. The dried ingredients are
incorporated into the aqueous phase which could be whole milk, skimmed milk or water,
and the available equipment is designed to provide: (a) complete dispersion of the dried
ingredients into the aqueous phase, (b) complete hydration of the dried particles with no
residual lumps, (c) minimal incorporation of air in order to reduce the problems of
foaming, and (d) easy cleaning and sanitisation of the unit.
The powder-handling equipment found in a dairy is dependent on the daily
throughput and the method of bulk delivery. Basically, milk powder is packed into either
small capacity units (25-50 kg multilayer paper sacks with polythene liners), medium
capacity units (up to a tonne in metal or plastic containers) or road tankers for bulk
storage in metal silos. The machinery available for emptying the powder also varies, so
that while the sacks (small quantities) may be emptied directly into reconstitution units,
larger volumes are emptied into a sifter for delivery into the mixing unit. The powder
stored in metal/plastic bins or silos is transferred using either a screw-feed (of variable
speed) or a blower; dust filters must be used to recover any fine particles, especially in
182 Tamime and Robinson’s Yoghurt
plants handling large capacities. Some examples of milk powder mixing units are given
Mixing funnelihopper
Reconstitution of the powder is carried out in batches and a ‘closed circuit’ consisting of
a tank, pipe connection, centrifugal pump and the funnelihopper assembly is required.
The tank is normally filled with the aqueous phase at around 40-50 “C and the circulation
started. The positioning of the hopper in relation to the centrifugal pump is important, and
two options are available (see Fig. 3.18):
First, if the hopper is assembled on the suction side of the centrifugal pump, it offers the
advantage of rapid dispersal and adequate dissolution of the powder owing to the action
of the pump; the disadvantage is that frequent blockages may occur in the hopper.
Second, by placing the hopper on the outlet side of the centrifugal pump directly after
a specially designed venturi unit, the problem of blockage is overcome, since the
venturi unit creates a vacuum within the pipe causing the powder to be sucked into the
recirculating solution; full dispersal of powder may be a little slower (Newstead et al.,
1979; Sanderson, 1982).
The former circuit is illustrated in Fig. 3.19. It is noticeable that, in the latter approach,
any suction of air is retui-ned to the tank rather than the suction side of the pump, because
if air is introduced into the system, the action of the pump’s impeller can increase the
amount of air incorporated into the product. Furthermore, a reduction in aeration and/or
frothing can be achieved by installing a special valve on the mixing hopper and ensuring
that the return line in the mixing tank is below the level of the liquid. If additional mixing
of the added powder is required, one of the following units could be employed: (a) in-line
static mixer, (b) high-speed agitator in the mixing tank or (c) high-velocity liquid jet.
Fig. 3.18 Illustration of a mixing funnel/hopper-used for reconstitution of milk powders.
The sack of milk powder is placed on the table and then emptied into the funnel. The
force of the circulating liquid causes the powder to be aspirated downwards and
mixed with the water. Circulation is continued until the powder is dissolved. Notice
that the funnel has a valve connection, which has a slight constrictiodrestriction in
the pipe to provide a venturi effect.
184 Tamime and Robinson's Yoghurt
An alternative method to the funnel/hopper installation is the in-line mixer, and some
examples of such units are as follows.
This mixing unit is supplied by the Tri-Clover Inc. of Wisconsin, USA. The principle of
this mixing unit is that the venturi jet mixer is replaced with a dual stage blending
process (see Fig. 3.20). The system is designed for continuous in-line or batch blending
of dry ingredients at a rate of up to 45 kgmin-l. The product passes through the initial
liquidldry ingredient blending chamber to a second blending chamber which effectively
serves as a discharge pump. This double blend feature improves end-product consistency
and provides a smoother and more uniform blend. With the discharge pump function
handled within the blender itself, it is possible to achieve significantly higher vacuum
rates over a wider range of process conditions. The increased vacuum rates contribute to
fast and consistent flow rates throughout an entire production run, and with such a
blending system, the additional strainers and a discharge pump are not required.
Incidentally, this unit is rather compact and occupies only 50 x 75 cm2 of floor space
(see Fig. 3.20).
Silverson mixers
These types of mixer operate at very high speed and exert an homogenising effect during
the recombining of dried ingredients. The models, which could be used for the
Fig. 3.20 Tri-Clovers dual-stage Tri-Blender'.
Reproduced by courtesy of Tri-Clover Inc, Kenosha, USA.
Processing plants and equipment
Fig. 3.21 The ‘Flashmix’ that can be used for in-line mixing of powders.
Reproduced by courtesy of Silverson Machines, Chesham, UK.
reconstitution of milk powder, are known as the ‘In-Line’ and the ‘Flashmix’. The latter
unit is shown in Fig. 3.21. These machines are designed for continuous operation at high
speeds and each has incorporated a high shear rotoristator processing workhead; the InLine mixer has one such head and the Flashmix has two. The upper head is normally a
general-purpose disintegrating unit, while the lower head is a square hole type with highshear screen. The operating characteristics of these workheads are briefly described by
the manufacturer:
The liquid is gravity fed or pumped into the hopper and is rapidly drawn down by the
two rotodstator workheads; a vortex is created by the flow of liquid through the
Flashmix, and it is into this vortex that the powder is added.
The liquid/solid mixture is drawn down the vortex into the mixing chamber and has no
way of bypassing the workhead(s) assembly, ensuring that all the solids are totally
dispersed before leaving the mixing chamber.
Two advantages claimed for the unit are that the workheads can be changed to suit
each individual product and that by using the appropriate feeding/metering equipment,
the liquid/solid ratio of flow can be precisely controlled. However, a similar unit known
as Flashblend can also be used to wet and disperse powders into liquids rapidly but the
mode of operation is different.
The use of an In-Line mixer alone has its limitations, because the delivery of milk
powder through a funnel into a recirculating circuit inevitably leads to ‘arching’.
186 Tamime and Robinson’s Yoghurt
However, the use of a Flashmix mixer overcomes this difficulty because the liquid and
solid ingredients are fed simultaneously into a specially designed hopper before being
sucked immediately into the upper rotor/stator. This workhead converts the milk powder/
liquid phase into a slurry which is then dispersed as the result of the high-speed shearing
effect of the bottom or second workhead. It is obvious that each mixer is designed for a
particular pui-pose and a combination of these two types of mixer in the recombining
process brings the advantages of both units, that is, the mixing process involves three
workheads rather than one or two, so ensuring complete dissolution of the powder with
the minimum incorporation of air. Some degree of homogenisation of the mix can be
obtained by using different types of stator head or screen on the high-speed mixer, so that,
for example, a disintegrating effect is achieved using large circular holes or slots, a fine
screen produces an emulsificationlhomogenisation effect and a screen with square holes
imparts a high shearing effect.
This type of in-line powder mixer was developed by the SemiBulk Systems Inc. in the
United States. An overall illustration is shown in Fig. 3.22. The system has the following
features: (a) an air-pallet/ejector mixer section conveys, wets and dispenses the powder
into the liquid; since the design generates its own vacuum to draw in the dairy powders,
the mixer allows total separation of dry handling from wet processing, and also, by
introducing the powder within the liquid stream, powder plugging is avoided, (b) the inline ejectorimixer conveys and mixes the powder on a ‘skidded system’ without using
mechanical equipment (e.g. conveyors, rotary valves, receivers and in-tank mixers); this
system can be fully automated including CIP, and can operate on batch recycle, single
pass or continuous modes and (c) the air cone hopper is designed for easy discharge of
powders that can cause delivery problems; details of the construction and principles of
operation have been given (Anon., 1996b) and the use of low-pressure air or other gases
eliminates the bridging effect of the powder in the hopper and facilitates discharge.
Incidentally, this system of mixing can be easily used to dissolve sugar into the milk
In-tank mixing unit
Efficient mixing of powder in a tank relies entirely on the agitation system provided. The
familiar flow pattern which occurs during liquid mixing is illustrated in Fig. 3.23. These
patterns are largely influenced by:
shape and size of the agitator system (paddle, turbine, propeller, scraped surface,
anchor, etc.);
position of the agitator, i.e. top or bottom entering, perpendicular or sloped, and/or
centrally mounted or not;
speed of rotation of the agitator;
shape of the processing vessel, while more specifically the efficiency of mixing is
related to speed of rotation of the agitator, velocity difference between the bulk fluid
and the agitator; the creation of a vortex, incorporation of air into the bulk fluid and
any shearing effects.
All these factors are relevant to the dispersal of powder into the bulk fluid and hence an
equipment manufacturer has various options in terms of design. Recently the practical
considerations for reconstituting dairy powders to high solids content in-tank have been
reported by Fitzpatrick et al. (2001) and Fitzpatrick and Cuthbert (2004).
Fig. 3.22 Vacucam"'M continuous in-line powder mixing system.
Reproduced b y courtesy of Semi-Bulk Systems Inc, Missouri, USA.
188 Tamime and Robinson’s Yoghurt
Vortex formation
No vortex
Circular pattern
Fig. 3.23 Liquid mixing flow patterns. Note: the paddles are perpendicular, top entering and
centrally mounted.
After Tamime and Greig (1979).
Reprinted with permission of Daily Industries International
Multipurpose processing tank
This type of tank (i.e. the batch pasteuriser) can be utilised during all stages of yoghurt
making (see Fig. 3.6), since the agitation system consists of a high-speed motor which is
operated during the preparation and processing of the milk, a slow-speed motor for
mixing in the starter and later for cooling the coagulum, and the drive shaft of the slow
speed motor can be fitted with a one- or two-propeller agitators and is usually top
entering and sloped (see also Anon., 2003).
Simple mixing tank
Different types of high-speed mixer (Silverson and Greaves) could be used in simple
tanks that resemble a batch pasteuriser, but do not have a properly mounted agitation
system. Thus in yoghurt production, two of these tanks will be installed in parallel for
preparation of the milk base, so that while one tank is being emptied, the other tank is
normally being filled up; a continuous flow of yoghurt milk to the incubation tanks can
be achieved in this way. In practice, a tank is filled with water or milk warmed to around
40-50°C and the milk powder is emptied from the sacks. Recombination is achieved
using a high-shear mixer/homogeniser and the mixers can be mounted permanently in
each tank or, alternatively, can be removed from one tank to the other with the aid of a
hydraulic lift (see Fig. 3.24).
An older type of high-speed in-tank mixer is the Ystral mixer described by Dalhuisen
(1972). The powder mixing procedure is: (a) powder is emptied into the special chute, (b)
the high-speed action of the mixing head creates a vacuum at the tip of the powder
deliveiy pipe, thus transferring the powder down the pipe from the chute, and (c) powder/
liquid mixing takes place in the absence of air; there is little risk of the powder forming
Crepaco ‘Multiverter’
This is a specially designed tank that provides rapid and complete dispersion of the dried
ingredients into the liquid slui-ry. The tank has a 15” or 35” cone bottom which facilitates
easy and rapid unloading and it is fitted with a high-speed motor which drives a special
centrifugal agitator. This unique agitator incorporates a ‘squirrel cage’ design resulting in
a dual blending action, combining an overall swirl with a deep-draw vortex that quickly
and effectively disperses the milk powder into the aqueous phase with a minimum of
foam. Although the tank is specifically designed to emulsify two or more immiscible
products, the blending action is especially effective in dispersing any fatty constituents in
the yoghurt milk. Furthermore, the tank can also be fitted with a CIP system.
Processing plants and equipment
Fig. 3.24 Examples of high-speed mobile mixers (A) Silverson and (B) Greaves.
Reproduced by courtesy of Silverson Machines, Chesham, UK and Joshua Greaves
and Sons. Bury, UK.
Crepaco ‘Liquiverter’
This high-speed mixedblender is capable of both dispersing the diy ingredients and
incorporating fat into the liquid phase. The impeller/agitator is centrally mounted from
the bottom of the square tank and the action of the Liquiverter pulls the added milk
powder through the liquid vortex at the centre and forces the mixture up the walls in
continuous circulation.
Large-scale recombination plant
Two systems could be used during the large-scale production of a milk base (Anon.,
2003; see also Aneja, 1990). In the first system, the fat is dosed into the mixing tank (see
Fig. 3.25).
Potable grade water is heated in a PHE to facilitate easy rehydration of the SMP and
is metered into one of the storage tanks (see Fig. 3.25 (7)).The circulation pump (5) is
started when the tank is half full and water flows through a bypass line from the mixing
tank to a high-speed powder blending unit (4). The feed rate of skimmed milk powder
(SMP) through the blending system is up to 45 kgmin-’. A vacuum is created by an
interplay between the circulation pump (5) and the booster pump (6) which causes the
blender to draw the ingredients into the eye of the centrifugal impeller. The agitator in
the mixing tank is started at the same time as the circulation pump. Water continues to
Fig. 3.25 Iiccombination in a largc-scalc plant whcrc thc rat is addcd in thc mixing tanks.
I , Tank containing melted Fat (e.g. cream o r anhydrous milk Fat (AMF); 2, insulated pipe for delivery of Fat; 3, weighing funnel for fat; 4, funnel
with high-spccd blcndcr (SCCFig. 3.20); 5, circulating pump; 6, boostcr pump; 7, mixing tank; 8, dischargc pump; 9, liltcrs; 10, PHB; 11, vacuum
dc-acrator (optional); 12, homogcniscr; 13, storagc tanks.
Reproduced by courtesy of Tetra Pak AiB, Lund, Sweden.
Processing plants and equipment
flow into the tank while mixing is in progress until the specified quantity has been
When all the SMP has been added, the agitator and the circulation loop are stopped
and the contents of the tank are left until the SMP has dissolved completely. At a water
temperature of 35-45 "C this will take about 20 min. At the end of this period the agitator
is restarted. In the meantime, the blender is reconnected for the next batch to be
recombined. AMF is now added from the fat storage tank (1). The quantity is measured in
the weighing funnel (3). The agitator, specially designed for optimum fat dispersion, runs
for several minutes and finely disperses the fat in the skimmed milk. The piping for the
warm fat fraction is normally insulated to prevent the temperature of the fat from falling
below the melting point (see also Kaya, 2000).
When all the ingredients have been mixed and added to one tank, the process is
repeated in the next tank. The skimmed milk/fat mixture is drawn from the full mixing
tank by pump (8) which forwards the mixture through duplex filters (9). After being
preheated in the PHE (lo), the product is pumped to the homogeniser (12) where the
dispersion of fat globules is completed. During recombination, air might be incorporated
into the milk base, and a vacuum de-aerator vessel (1 1) can be installed in the line before
the homogeniser to eliminate this; such a unit can reduce the air content from 1.3-1.8% to
0.1-0.2% which can improve the texture and consistency of the yoghurt (Rage et al.,
1987). The product is preheated to 7-8 "C above homogenisation temperature before
being flashed in the de-aerator, where the vacuum is adjusted so that the outgoing product
has the correct homogenisation temperature, typically 65 "C. The homogenised milk is
pasteurised and chilled in the PHE (10) and is then pumped to the storage tanks (13) or
direct to packaging. However, for yoghurt production the milk is heated to higher
temperature as described in Fig. 3.13.
Alternatively, in-line fat mixing (Fig. 3.26) can be used in which the recombination of
the powder is similar to that described in Fig. 3.25 (Anon., 2003). In this system, the
process could be described as follows. When a mixing tank has been filled and the contents
have been given time for complete hydration of the SMP, the reconstituted skimmed milk
is pumped through duplex filters (6) to a balance tank (7) (see Fig. 3.26). This ensures a
constant flow rate to the process. A centrifugal pump (8) feeds the skimmed milk through a
preheating section of the PHE (9). Although the addition of fat can suppress foaming in the
skimmed milk, in this instance, a de-aerator vessel (10) is required. The milk base is
preheated and homogenised in the manner described in Fig. 3.25, but then the milk flows
through an in-line injector (13) where liquid fat from the fat-melting tank (11) is
continuously metered into the flow by a positive displacement proportioning pump (12).
Blending is completed in an in-line mixer (14) downstream of the injector. Immediately
after mixing, the recombined milk continues to a high-capacity homogeniser (1 5) and then
returns to the PHE (9) for further processing as described in Fig. 3.13.
When dealing with the recombination of milk powder, two conditions in the
reconstituted milk have to be monitored. First, not all the particles of milk may dissolve
during the recombining process, perhaps through the use of poor quality powders,
inefficient mixing equipment andlor the presence of scorched particles. Any undissolved
particles must be removed using in-line stainless steel mesh, or a stainless steel mesh and
nylon filter called the duplex, or centrifugal, clarifiers. Clarifiers are excellent for the
removal of any fine or undissolved particles and any extraneous matter but, for
convenience, filters are more commonly used. Normally two interchangeable filters are
installed in a milk reconstitution line, especially in large dairies, so that in the case of
clogging, the flow of milk can be easily diverted while one of the filters is being cleaned.
Fig. 3.26 Largc-scalc rccornbination plant with in-linc Cat mixing.
1, Funncl with high-spccd mixcr (SCCFig. 3.20); 2, pump l'or circulation; 3, boostcr pump; 4, mixing tanks; 5 , dischargc pump; 6, liltcrs; 7,
balance tank; 8, feed pump; 9, PHE; 10, vacuum de-aerator; 1 I , tank containing melted fat (e.g. cream or AMF); 12, positive displacement pump;
13, l'at injcctor; 14, in-linc mixcr; 15, homogcniscr.
Iicproduccd by courtcsy ol' Tctra Pak Am, Lund, Swcdcn.
Next Page
Processing plants and equipment
The removal of such particles is essential, since their presence in the milk can damage the
orifices in a homogeniser andlor increase soiling in heat exchangers. Second, reconstituted powders require up to 15 min to achieve complete hydration, otherwise
sedimentation becomes evident. The hydration effect may not be important during the
manufacture of yoghurt, since the time elapsing between recombination and the end of
heat treatment of the milk can be as long as 15 min.
Evaporation of milk
Concentration of standardised milk base can be achieved by use of an evaporator, in
which the average amount of water removed is 10-25 g 100 g-' and the total solids is
increased by 1.5-3 .O g 100 g-', corresponding to the recommended foi-tification with
milk powder (Anon., 2003). In order to remove the desired amount of water and avoid
damage to the milk constituents at high temperatures, the process of evaporation is
normally carried out under vacuum.
Single-effect evaporators can be used directly in a yoghurt processing line. The milk
base is pumped from the balance tank to the condenser where it is preheated and then
enters the plate section of the evaporator for fui-ther heating. After reaching the preset
temperature, the milk flows to the separator section and water vapour is removed from
the milk; the cycle is repeated until the desired concentration of total solids in the milk
base has been reached. Heat recoveiy during the evaporation process is very efficient and
is achieved using a thermocompressor, that is, factory steam is mixed with the vapour
produced from the evaporator.
Another type of single-effect evaporator that could be used to concentrate the milk
base is supplied by Tetra Pak MB.The sequence of operations is as follows. The
standardised milk base is preheated to 70 "C in the regeneration section of the PHE using
the condensate from the evaporator (see Fig. 3.13). Subsequently the milk is heated to
85-90°C in the heating section of the PHE and the preheated milk enters the vacuum
chamber where the inlet is shaped as an expansion tube to prevent burning of the milk.
The milk is recirculated four to five times until the desired degree of concentration is
achieved. The recirculation cycle is controlled by the capacity of the vacuum chamber,
evacuation pump and the float controller; during each recycle, about 3-4 g 100 g-' of
water is removed. The capacity of such evaporators is up to 8000 1h-', but for larger
plants, different types of evaporators are used with capacities up to 30 000 1h-'.
In general these evaporators offer the advantages of minimum requirement for space,
efficient heat recovery and immediacy of use. Furthermore, yoghurt made from milk
concentrated in this way exhibits an excellent organoleptic quality.
Mernbrane concentration of milk
An alternative method of fortification of the milk base is by concentration of the milk
(whole and/or skim) by membrane filtration (i.e. UF and RO). The basic differences
between the UF and the RO systems are first that the operational pressures are much
higher in the case of RO, and second that the RO membrane is less permeable than the UF
membrane; the pore size for RO is <40nm and for UF is >200nm (see Fig. 3.27).
The milk constituents that pass through a membrane are referred to as the permeate,
and the material that does not pass through the membrane (i.e. concentrated fraction) is
known as the retentate. The different components present in milk can be divided into
three main groups based on the molecular weight, that is, large molecules (proteins and
fats), medium (lactose and salts) and small (water). The RO membrane allows only the
small molecules (water) to pass through the membrane and the retentate consists of a
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