CHAPTER 2

ENERGY IN ESTABLISHED MILK COLLECTION AND PROCESSING

2.1 General

The consumption of energy in food processing plants is usually characterized by specific consumption and peak-hour requirements, separately for each type of service. In milk plants data are usually presented for steam, refrigeration, well water, compressed air and electric power.

There are no standards for service requirements in the dairy industry applicable to all plant capacities or to specific products. Such requirements depend not only on the overall capacity and the type of the plant but also on the source of energy utilized.

An artisanal cheese-maker heats milk on simple wood-fired stoves in which not more than about 10 percent of the energy of the wood is used for milk heating and 90 percent is lost. In modern gas-fired steam raising plants 90 percent of the gas energy can be transferred into steam and finally over 80 percent of the energy in gas will increase the temperature of milk or, in condensing and drying processes, will cause evaporation of water. In modern milk plants plate heat exchangers with regeneration can reduce the nominal heat requirements to one fifth or even one tenth; modern multistage milk condensing plants require not more than 20 to 40 percent of the nominal heat requirement per kg of evaporated water. Thus, taking the example of milk pasteurization, the furnace fuel energy required in a small-scale artisanal plant per unit of product can be about 40 times that in a large-scale modern milk plant.

On the other hand the specific consumption of electric power may be nil or negligible in artisanal plants and relatively high in modern factories. Even within a given class of milk processing plants, the service requirements may differ substantially. Fuel consumption for drying processes depends very much on the inlet air temperatures which are kept usually on the high side when oil or gas heaters are used for air heating in spray driers but on the low side with steam heaters. Water consumption for chilling and condensing depends very much on the initial well water temperature and the humidity of the ambient air. It in turn affects the refrigeration requirements. The energy requirement figures quoted in this study are therefore of indicatory nature only.

2.2 Collection

2.2.1 Modern industries

The prevailing pattern of milk collection systems includes milk chilling directly after milking and tanker collection from the farm gate using tanker-mounted meters for measuring milk quantity. Several cooling systems are in use. The milk may be cooled in a bulk tank by a chilled water jacket containing an icebank or by a direct expansion refrigeration system. It may be precooled in a plate heat exchanger using mains water and chilled water or glycol, and then stored in bulk with or without direct expansion refrigeration to counter subsequent heat gains. The use of chilled water generally increases the total energy required in kWh/t ¹ but decreases the demand in kW and so is usually preferred for larger installations. Water requirements for cleaning are small and can be estimated at about 0.15 m³/t of chilled milk or less. For bulk tanks cold in-place cleaning is often used and is particularly suitable for icebank chilling systems.

The total energy requirements for milk chilling prior to transportation to milk plants can be estimated at 25 to 30 kWh/t net or 85 to 105 kWh/t gross respectively. This total requirement refers to electric power and includes the refrigeration unit, milk agitator, power requirement for water supply and heating or pumping in cold cleaning-in-place (CIP) systems.

Apart from the well-known fact that immediate milk chilling is one of the main factors in manufacturing quality products, it also necessitates a low energy input in milk processing in the plant. Chilling, either directly after milking or during processing if the milk is unchilled, requires as mentioned above, an estimated 25 to 30 kWh/t energy net input forming part of the milk processing operation.

¹ t= metric ton (this measurement is used throughout the publication)

2.2.2 Small-scale industries

The main feature of small-scale milk collection systems is the collection centre where milk is delivered by producers in small vessels in quantities depending on farm yield. This may vary from about 0.5 l to as much as 100 l and as a rule, the milk arrives at little below milking temperature.

In the most simple systems there is only quantity measuring and bulking into cans which are washed in the central plant. There is thus no energy consumption for milk treatment in such centres. A milk chilling system by means of refrigeration machinery installed in the milk collection centre is usually the next development step in small-scale milk collection systems, sometimes accompanied by simple milk separation equipment. In centres where cans are washed in the central plant, the energy consumption is similar to that in modern systems though available data show that it is about one third higher because of lower capacity utilization and higher heat losses. It can be estimated at 35 to 40 kWh/t net and 105 to 135 kWh/t gross, respectively.

In many countries long distances from production areas to milk plants make two-stage milk collection systems necessary. In such systems the producer delivers the milk to a simple collecting point from where, with no initial handling, the milk is transported in cans to the second stage collection centres equipped with chilling machinery and can washing facilities. Such centres are usually equipped with steam boilers and can washers and sometimes with milk tanks from which the chilled product is pumped to road tankers.

The net electric power requirement for chilling in such centres, together with stirring and pumping, can be estimated at about 40 kWh/t. In addition there is the energy requirement for cleaning which includes water supply and heating. It may vary very widely but for the majority of centres belonging to this sub-group, the specific requirement of steam could be estimated at about 10 kg/t which is equivalent to about 7 kWh energy in steam per l t of milk. Total water requirements may rise to 0.3 m³/t. The total net specific energy requirement could be thus estimated at about 47 kWh/t of collected milk. The corresponding figures for gross requirement can be estimated at about 15 kWh/t in furnace fuel and about 140 kWh/t in electric generator engine fuel which brings the total to about 155 kWh/t. These figures are no doubt surprisingly high but the early protection of fresh milk quality is a necessary precaution if quality products are to be manufactured in the milk plant.

It should be mentioned that in some countries, increasing the temperature of milk or cream to about 60°C and keeping it at that temperature during transportation to the milk processing centre or the customer was traditionally a way of preserving milk quality. This system still prevails in some African countries such as Mali. Although objections could be raised concerning the suitability of such milk for the manufacture of some products, this system should not be disregarded when considering cheap methods of milk marketing since it does not require high capital inputs. The respective energy requirement, even if relatively high, can be covered by locally available cheap fuels.

2.3 Processing

2.3.1 Modern industries

Modern large-scale milk processing plants prevail in industrialized countries. The daily processing capacity per plant usually exceeds 100 t of milk intake but even smaller plants with a daily intake of about 30 t are generally equipped with modern machinery. Their specific energy requirements are similar to those of large-scale factories though usually slightly higher since they do not benefit from the effects of scale.

To provide a background for the later consideration of small-scale operations, data from about 30 European plants have been analyzed. From these, typical energy requirements have been estimated for nine groups of products and are presented in Table 1 and Figs. 1 to 6. In calculating these figures certain basic assumptions were made, viz.

1. Specific water requirements were estimated at 3 m³t of processed milk, independent of plant capacity or processing programme.

   This consumption is expressed as an equivalent electric power requirement of 6.4 kWh/m³ and is included in the total electric power consumption. The relatively low energy requirement for compressed air consumption is also included.

2. All heat requirements for large-scale milk processing were calculated as steam condensed and the coefficient of 0.66 kWh effective heat from 1 kg standard steam was used in all calculations. For all cleaning processes the total heat requirement was increased by about one third and the equivalent electric power consumption by one tenth.

3. Refrigeration plays a very significant role in the overall energy requirements of a modern milk plant, often constituting above 40 percent of the total electric power consumption. In the specific energy consumption estimates, data concerning refrigeration are presented separately: (a) as total energy equivalent (heat removed from the product during processing and from the air in cold stores) and (b) electric power consumption by refrigeration machinery.

It has been necessary to ignore factors such as purchasing water from municipal mains, producing electric power from the plant's own generator, covering part of the heat requirements by direct fuel heating and running machines of different efficiency. All estimates have been made in “net requirements” and “gross requirements”. The first include figures representing the increase of enthalpy in the heated milk or the energy obtained by steam condensing in the processing machines or energy produced by electric motors moving the processing machines. The “gross requirements” represent the amount of energy in furnace fuels and engine fuels used to produce the equivalents of the respective “net requirements”.

For all milk plants with modern equipment (both large and small-scale) the efficiency coefficient for steam-raising plants was taken to be about 0.8 and about 0.5 for all other plants with steam boilers. The efficiency of milk heating with no use of steam as the intermediate medium was estimated at 0.1. In order to estimate the “gross requirement” was equivalent for electric power consumption, the “net requirement” was multiplied by the factor of 3.5 which illustrates the energy input/output ration in medium capacity electric generators. It should be borne in mind, however, that the application of this factor reflects better the “gross requirement” of electric power for small-scale plants than for modern industry which usually draw power from more effective systems.

All consumption figures are expressed in kWh (1 kWh= 3.6 × 10³ kJ= 860 kcal).

FIG 1. SPECIFIC NET STEAM REQUIREMENT

FIG.2. SPECIFIC NET REFRIGERATION REQUIREMENT

FIG.3. SPECIFIC NET HEAT REQUIREMENT

FIG.4. SPECIFIC NET ELECTRIC ENERGY REQUIREMENT

FIG 5.SPECIFIC GROSS HEAT REQUIREMENT (FURNACE FUEL)

FIG 6. SPECIFIC GROSS ELECTRIC ENERGY REQUIREMENT ( GENERATOR FUEL )

Figs. 1 to 6 have been compiled to facilitate estimates of energy requirements for processing programmes including any or all of the nine product groups specified in Table 1. An example for a plant processing a total of 160 t of milk per day into four products is given in Table 2.

The trend of development indicates that in most modern plants the total energy requirement is increasing, mainly due to the increase in electric power consumption, whereas the specific heat consumption remains substantially unchanged. Such trends are the result of increasing mechanization and automation which requires more equipment in processing lines and particularly in stores. This tendency can be exemplified by comparing methods of equipping milk tanks with valves. In a plant with manually operated valves a milk tank is equipped with 5 to 7 valves whereas automatic operation of a similar tank needs 12 to 15 pneumatic valves, requiring compressed air for their operation.

2.3.2 Small-scale industries

Data concerning energy usage in small-scale milk processing plants having a capacity from 1.0 to several tons per day have been collected in various developing countries. These plants can be considered in three groups.

(1) Shops with neither electric power nor steam. These include artisanal processing shops equipped with the simplest locally made equipment, comprising cans, vats, pots, tables, moulds, simple cheese presses, etc. Such milk processing stations are often found in the Near and Middle East where yoghurt and cheese are manufactured, the former basically in urban and the latter in rural areas. In these shops energy is used only for heating to bring the temperature of milk to the level required for fermentation (yoghurt) or renetting (cheese). Wood, coal, gas or liquid fossil fuels are used, usually in simple stoves with an energy conversion efficiency between 10 and 15 percent.

Cheese manufacture of a similar kind can also be found in some Latin American countries. In the Far East sweetened condensed milk is produced under primitive conditions in rural areas where milk with sugar is kept at boiling temperature, or close to it, with constant manual stirring to evaporate part of the water.

It is difficult to calculate precisely the energy requirements for such processes particularly as little attention is given to cleaning and general hygiene problems. For yoghurt and cheese the net requirement could be estimated at about 60 kWh/t of processed milk (heat only) and the gross requirement at about 400 to 600 kWh/t. For sweetened condensed milk the corresponding figures would be about 580 kWh/t and 3 900 to 5 800 kWh/t respectively. It should be mentioned, however, that in this group of plants daily processing is rarely greater than 50 to 100 1 of milk.

(2) Stations with electric power but without steam. These plants are usually equipped with some modern facilities. The simplest are farm dairies manufacturing unpasteurized packaged liquid milk. This type of milk processing can be seen in a number of countries all over the world, usually for small daily quantities of less than 0.3 t. In such farms fresh milk is packaged immediately after milking in bottles or single service plastic or paper cartons, stored overnight under refrigeration and sold the next day directly to customers. The energy requirement of this type of small-scale milk processing is usually limited to electric power since electric heaters are used even for water heating. The net requirement could be estimated at about 35 kWh/t of packaged milk. The gross requirement is about 120 kWh/t.

(3) Plants with simple mechanization using electric power and steam. These plants operate a similar system of packaged liquid milk production but include pasteurization directly after milking. They are mainly found on larger farms with daily processing capacities exceeding 0.3 t. Such plants are equipped with low pressure steam boilers, pumps, simple filling and sealing machines and pasteurizers usually of batch type, though plate heat exchangers with regeneration sections are slowly being introduced. The energy requirement in these plants is divided into furnace fuels for steam raising and electric power for water supply, refrigeration and mechanical operation of machines. The requirement for heating could be estimated at about 50 kWh/t net and about 100 kWh/t gross (batch pasteurization system) and for electric power at about 25 kWh/t net and 90 kWh/t gross which brings the total to 75 kWh/t and 190 kWh/t, respectively. These figures may be taken as reflecting the energy consumption in cheese and yoghurt manufacture in plants in this group.

The small-scale milk processing industry also includes milk plants equipped with the relative modern standard dairy machines, steam boilers, piped water and electric power supply. Their capacity may be in the order of 3 to 10 t/day or even more and their manufacturing programme may vary from a single product, such as liquid milk or cheese, to a combination of products, very often changing with the season. Manufacture of dried and condensed milks is, however, very expensive in small plants because of high capital inputs required for modern equipment. These products are, therefore, rarely included in the processing programmes of modern plants with low daily throughputs. The net energy needs of the modernly equipped small-scale plant differ from those of modern large-scale plants as they are less in heat requirements but substantially more in consumption of electric power for refrigeration. The majority of plants in small-scale dairying receives unchilled milk. As a result their heat exchangers have a regeneration effect seldom higher than 60 percent which increases the specific net steam requirement by 7 kg/t or about 5 kWh/t for every product manufactured. The requirement for refrigeration is however almost doubled, as the temperature sequence in their exchangers will be, for example, 30-57-75-48-38-4°C (with a water chilling section 48°–38°C) as compared to 4-61-75-18-4°C with chilled milk intake.

This substantial increase in electric power requirement for refrigeration is partly compensated by the reduced consumption in other sections of the plants since they are much less mechanized. The overall requirement figures for steam and total electric power from Table 1 are applicable also to this group of plants.

2.4 Summary

The indicatory figures calculated and collected in previous parts of this chapter illustrate the wide range of energy requirements in milk collection and processing systems and their dependence on the major factors affecting the specific energy consumption. Of particular interest are comparisons between modern large-scale dairy industries and small-scale collection and processing systems. A selection of figures concerning products manufactured in these groups is presented in Table 3 and Fig. 7.

It can be seen that energy requirements for milk collection and processing are the complex result of many factors.

In milk collection with chilling the effect of high milk intake results in slightly lower energy requirements, net and gross, for modern collection systems.

In liquid milk processing the high quality standards established in modern dairy industries and the high degree of mechanization lead to higher net and gross energy demands. A similar situation can be seen in cheese and yoghurt manufacture although it is less evident in gross energy requirements. In the simplest milk processing systems applicances used for milk heating are of such low efficiency that a great majority of the energy requirement of the furnace fuels is wasted resulting in a higher gross energy requirement even though more manual labour is needed and the final product is often of low quality.

Manufacture of condensed milk is an example of a high energy requirement for a product demanding a substantial heat input. Such inputs are dependent on the nature of the product and the process. The differences between modern and simple plants are, therefore, much more pronounced in gross requirements where the impact of the efficiency of heat producing equipment becomes so dramatically evident. Objections could be raised to comparing manually bottled unpasteurized liquid milk processed without sanitary control with bottled pasteurized liquid milk processed in modern plants and meeting all the highest quality standard demands. Similar objections could be raised to comparing condensed milk manufactured under primitive conditions and sold in 20 kg gasoline tins with high quality products of world brands sold in 400 g tins.

¹ Unpasteurized
² Figure indicating the heat absorbed by milk components during direct heatingon simple stoves

FIG 7. SPECIFIC NET (---) AND GROSS (—) ENERGY REQUIREMENT (kWh/t)

On the other hand such comparisons are justified bearing in mind that traditional ways of milk processing in developing countries cannot be replaced at one time by modern methods. Neither the producer nor the consumer could afford such a change since it would result in an enormous increase in costs. Furthermore, the required capital input needed for such rapid modernization is not available - exceptions confirm the rule only. It, therefore, seems justified to compare the order of magnitude of specific energy requirements in different milk collection and processing systems for products of a similar nature though of different quality standard.

Apart from the quantities of energy required for milk collecting and processing, of paramount significance are the physical parameters of energy carrying media.

Heating of milk in vats placed on simple furnace stoves is an example of a heat transfer process where the temperature difference between the heating medium (burning gases) and milk is very high, very often a few hundred degrees. Heating by means of steam is an example of medium temperature difference between condensing steam and milk. Steam is the most common thermal energy carrier in the dairy industry. High pressure steam of 7 to 20 bar is used in condensing and drying plants and in some types of sterilizers. Low pressure steam (1.5 to 3.0 bar) is used in heat exchangers for heating processes up to required temperatures and for CIP. Steam is mostly used in those processes to elevate the temperature of water which, in turn, exchanges its heat with milk. Hence, except for condensing, drying and sterilization, the most common heating medium in heat exchangers for milk is hot water.

The inlet temperature is usually a few degrees above the required milk temperature which means, for the majority of processes with hot water utilization, that the highest water inlet temperature is below 100°C. This fact is paramount when considering further alternative energy sources for milk collection and processing. Except for sterilization condensing and drying, milk processing following relatively modern technologies is feasible with hot water as the heat carrying medium, provided the equipment is adapted accordingly.

Refrigeration in modern milk processing is based on ammonia or Freon condensing units whose effective operation requires electric power for compressors, fans, agitators and pumps.

In most cases, particularly in large-scale ammonia plants, water is required for compressed gas condensing and compressor chilling. Ammonia refrigeration systems are usually run at condensing temperatures of 35°C to 45°C and evaporating at -10°C. In cold stores the refrigerant evaporators are placed in the store area where heat is removed from the ambient air. For milk cooling in heat exchangers chilled water is used.

In modern systems refrigeration in milk collecting and processing requires electric power and usually also water. Absorption refrigeration has so far not gained any significance in the dairy industry.

Electric power is rarely used for heating purposes. Its main application is in driving motors by means of which all remaining processes including water supply and refrigeration are performed. In milk processing, as in the majority of other industries, alternating current (AC) at 110V or 220V is used for single-phase appliances and 220/360V or 380/640V for three-phase appliances, mainly electric motors above 5 kW.

Direct current (DC) is mainly confined to low-voltage battery-driven fork lift trucks or other wheeled vehicles.

For all practical purposes there is little alternative to electric power in any processing industry, including dairying, if mechanical energy is required. A man can develop for a short while a power equivalent to about 0.3 kW but working continuously he can only develop about 0.075 kW; obviously manual handling of processes is confined to low energy requirements.

Two basic conclusions concerning energy requirements result from the analysis presented above:

1. There is little room for economizing on energy consumption in milk processing industries, as long as a consumption-oriented society demands products of present standards and as long as it will be able to afford them. Modern milk plants might become interested in using alternative economic energy sources provided the supply from such sources can meet at least a substantial proportion of the demand. Harnessing new energy sources for large-scale industrial application, including the dairy industry, depends entirely on future development.

2. Small-scale milk collection and processing systems are likely to be sufficiently flexible to adapt technologies and equipment suitable for utilizing cheap energy from unconventional sources.