CONDENSED AND DRIED MILKS

Module 13. Technology of dried milks

Lesson 37
FACTORS AFFECTING INSTANTIZING

37.1 Introduction

Dry milk manufactured specifically for agglomeration usually gives the best results. The success of the instantizing operation depends upon adequate control of each step during manufacture.

37.2 Factors Affecting

1. Moisture content and particle size should be as uniform as possible.

2. A minimum of fine particles, less than 20µ in diameter is desired with the preferred particle range of 25µ to 50µ.

3. Nonfat dry milk for agglomeration should be low in fat content.

4. Low heat (6 mg or more of WPN) or medium heat nonfat dry milk (less than

6 but more than 1.5 mg WPN) is normally used.

5. High-heat nonfat dry milk will agglomerate satisfactorily, but it shatters much more easily in handling after agglomerating and redrying.

6. The powder distribution into the wetting zone space must be uniform and at a constant rate.

7. Moisture condition must be uniform in all respects to avoid over or under wetting of particles.

8. Over wetted particles dissolve slowly and too little wetting permits excessive shattering during handling.

9. The air movement has to be stabilized to assure optimum particle collision. Excessive movement causes product adherence to the equipment lining.

10. Control of the redrying air temperature and its flow rate is necessary for adequate moisture removal without heat damage to the agglomerated product.

It is the well known fact that agglomeration lowers the density of dry milks. Usually flavor is not affected, but changes in flavour can occur as a result of agglomeration that is detrimental if the process is not carefully controlled.

37.3 Process Parameters

1. The plant is operated such that the powder leaves the primary drying stage with 2-10% higher moisture than wanted in the final product. The cyclone fraction is returned to the atomizing device, where the dry fine particles will collide with the primary particles thus forming agglomerates.

2. The powder leaving the chamber is therefore warm, moist and consists of stable agglomerates. Consequently by gentle after-drying performed in integrated fluid beds and/or a Vibro-Fluidizer the agglomerated product structures are maintained. The cooling should always be done in a fluid bed.

3. The powder obtained by this process can be characterized by

• Agglomerated product structure

• Non-dusty

• Lower bulk density than for powder from the pneumatic plant

• Good flowability

4. The decreased drying air outlet temperature and consequently lower product temperature will result in:

• Improved solubility because of less thermal damage

• Low content of occluded air, because in the critical stage of the drying, with a water content of 30-10%, the blowing-up of the particles is avoided.

37.4 Instant Skim Milk Powder

In order to obtain a skim milk powder with good instant properties, the agglomeration plays the most important part. In order to avoid too quick wetting of the particles, the powder particles are agglomerated thus reducing the specific surface. The specific surface can also be reduced by bigger primary particles, however, with a risk of a high insolubility index. Furthermore, the instant properties, especially the wettability, are improved, if the agglomerates are so compact that the moisture absorption and dissolving process are prolonged enabling a dispersion of the powder agglomerates in the water, after which the final dissolution can take place. The powder should also have a good insolubility index.

The agglomeration is improved by reduction of the pasteurization temperature before evaporation, keeping h igher solids content in the concentrate and bigger primary particles. If recycled amount of fines are higher, introduce fines closer to the wheel or nozzle and i ncrease moisture content in the powder from the drying chamber

Typical operation conditions for the spray dryer when producing a first class instant skim milk powder, depending on type of dryer, will be having drying temperature 200ºC ± 20ºC and s olids content in the concentrate 48-50%.

37.5. Instant Whole Milk Powder

In whole milk powders, some of the fat is present as free fat. Free fat rejects water making it impossible to dissolve these powders properly in cold water. Hence, i n the case of whole milk powder it is required that the water temperature is >40ºC. Homogenization of the whole milk concentrate prior to drying reduces the content of free fat in the final powder. However, to be called ‘instant’ whole milk powder, it must be agglomerated (Figure 37.1) as well as have a surface-active agent (lecithin) applied to improve water affinity. The product then becomes instant – even in cold water.

Fig 37.1 Micro photo of agglomerated whole milk powder

For this purpose lecithin (originating from soy beans) dissolved in pure butter-oil, in order to make a liquid, may be used. Lecithin is superior as to the functional performance, i.e. achieving of instant properties. The butter-oil is chosen also in order to use a natural milk component, as using a vegetable fat, even it is done in many cases, could be considered a falsification. The amount of lecithin and of the total free fat (i.e. original free fat + added butter-oil + lecithin) in the final powder may vary from 0.1-0.3% and 1-2%, respectively. However, variations within these limits result in rather big differences as to the desired properties.

A high amount of total free fat together with high amount of lecithin improves the wettability, but on the other hand it is affecting the flowability and may seriously affect the dispersibility. At lecithin levels >0.5% it is possible to detect the characteristic soy flavour. The structure of the powder and the degree of agglomeration are of importance too, as poorly agglomerated powders require higher amount of wetting agent than well agglomerated products.

Two-stage process

1. In the two-stage process the basic powder is collected for intermediate storage. It is important to prevent any damage to the powder by mechanical treatment. The intermediate storage is therefore preferably accomplished in bins or similar containers of 1-2 m³.

2. The basic powder is then transferred from the bins into the supply silo and is metered into the first Vibro-Fluidizer by means of a dosing screw. The powder is heated and at the same time any fines are blown off.

3. The lecithin dosing equipment consists of two vessels, dosing pump, powder trap with two-fluid nozzle, and control panel. The first vessel serves for the preparation of wetting agent, and the second one as supply vessel, from where the wetting agent is metered to the two-fluid nozzle for spraying on to the powder.

4. The flows, temperatures and pressures of the wetting agent and of the atomizing air are recorded. Interlocking in the control panel ensures that the flow of powder will stop automatically, if for some reason no lecithin is applied. Consequently no powder will leave the plant without a proper lecithin coating. The second Vibro-Fluidizer, also supplied with warm air, ensures a gentle but proper mixing of the powder to obtain a uniform distribution of the lecithin mix over the particle surface.

5. The powder leaving the lecithination unit is packed into retail packages. The filling machine is placed preferably directly below the lecithination unit with a hopper for short intermediate storage to avoid any unnecessary transport.

Continuous Process

The powder production and lecithination can be made in one continuous process . In this case the powder trap with lecithin nozzle is placed between the integrated fluid bed and the Vibro-Fluidizer. The product quality can be compared with the one achieved by the split process operation described above.

Advantages of Split Process

1. Retail packing of milk powder is never a fully continuous operation, since there is always a natural break between the powder production and the packing. During this break the powder must be stored in bulk for one to several days, preferably in bins to avoid damage.

2. For quality reasons it is better to store unlecithinated powder. The lecithination process therefore fits best as a part of the packing line forming one continuous operation.

3. The intermediate storage of the powder after production makes it possible to analyze the product to classify it and to calculate the composition and quantity of wetting agent in order to achieve the desired properties.

4. Fines created during storage and transport can be blown off in the first Vibro-Fluidizer of the lecithination unit.

Today, however, most instant whole milk is produced in plants equipped with a lecithin dosing equipment, placed between the integrated fluid bed and the Vibro-Fluidizer, in one processing step. The final powder is conveyed to silos by lenient low speed vacuum conveying systems - being very gentle to the agglomerated product - before packing either in retail packs or 25 kg bagging lines. The conveying lines may be equipped with pre-gassing by N₂/CO₂ for prolonging the shelf life.

37.3 Packaging

37.3.1 Packaging of non fat dry milk

• A suitable container for dry milk should be impervious to moisture, light, gases and insects; should be durable for handling, resistant to corrosion, of low cost; and be relatively easy to fill, seal, handle, and empty. The retail package should have a reclosable opening.

• Nonfat dry milk for industrial use and storage may be packaged in barrels, drums, and bags or for retail purpose in metal cans, glass jars, or cartons. A polyethylene liner having 3 mm thickness inside a 6 ply Kraft paper bag is recommended. The non fat dry milk in bags having a tape over the top seal is helping to prevent insect infestation of the product after packaging.

• Nonfat dry milk in commercial trade is commonly packaged in a 2 mil polyethylene bag inside a 4-ply Kraft paper bag. The outside layer is usually plain, but a wrinkled type is also available. Freezing, high temperature, and low humidity during storage of bags cause them to become brittle and thus damage more readily in handling.

• Manual filling of the bags is most commonly completed by means of a simple device attached to the sifter. Automatic bagging equipment is readily available to dispense the correct weight of powder in one bag before shifting the product flow into the next bag. Bags are sewn automatically or a manually operated sewing machine suspended and counterbalanced within easy reach of the filling area. After closing, the bags are usually stacked on pallets. The bag overhang from the pallet should not be more than 5 cm.

37.3.2 Retail carton

• Cartons of fiberboard, foil, and plastics have largely supplanted glass and metal as retail containers.

• Because of the hygroscopic nature of dry milk, the packaging materials must provide a good vapor barrier.

• Packaging of nonfat dry milk is quite a routine practice and the principal concerns involve keeping machine downtime to a minimum maintaining the correct net weight within narrow limits, and providing a good seal.

• Coding of each package provides a means of identification for quality control.

37.3.3 Packaging dry whole milk

• Average production conditions and normal market periods for export and retail sales necessitate gas packaging of dry whole milk to delay oxidative changes.

• The rancid flavor deterioration of dry whole milk due to oxidation necessitates inhibitory measures.

• One of these consists of packaging the product with low oxygen content.

• The general procedure is to immediately remove oxygen by subjecting the product to vacuum within 24 hr of drying with final packaging within a few days.

• If gas flushing is not done, the powder may be packaged into multilayer paper bags with a polyethylene inner layer. Whole milk powder, however, is often packaged in tins or in plastic containers to minimize oxygen uptake.
  

37.3.4 Gas packaging

• The so-called gas flushing, essentially displacing air by N2 or a mixture of N2 and CO2, is to remove a considerable part of the oxygen and thereby to improve the stability toward autoxidation; it can be done once or twice.

• To obtain a low level of headspace oxygen in dry whole milk, a double gassing technique is applied. The customary procedure is the collection of filled cans on trays to be conveyed into the vacuum chamber. The air is removed rapidly (60 sec.) with the gauge indicator decreasing to 73.6 cm of vacuum. After a 2 to 5 min hold, the pressure is restored with nitrogen to 0.035 to 0.07 kg/cm2 above atmospheric pressure. Nitrogen may be replaced with a mixture of nitrogen and carbon dioxide, the latter being restricted to 5 to 20%.

• After removal from the chamber, the containers are sealed by soldering the 1 to 2 mm hole in the lid or crimping on the lid. The containers are held for oxygen desorption. When an oxygen equilibrium has been attained in the headspace, usually within a week but at the most ten days, the cans are punctured and the vacuum treatment, - pressure restored with nitrogen, and sealing step's are repeated.

• Gas packaging of dry whole milk should not be delayed after drying. Otherwise quality deteriorates during the holding period. Warm powder directly from the drier tends to have a more rapid rate of oxygen desorption under vacuum. If the production is not large, dry whole milk may be placed into metal drums and air exhausted. By holding the product under partial vacuum for oxygen desorption the first gassing step in the package may be eliminated, and yet the final maximum of 2% oxygen can be attained.

• In the recent times, commercial plants employ continuous packaging machines which completes the work of clinching the embossed bottom on the filled, inverted tins, create vacuum in the head space, flush with the inert gas and finally seaming the tins in one run. Multilayer packages are also being used for the purpose.
  

a) Oxygen Limits

A good commercial operation using continuous gassing equipment can reduce the oxygen level to ~ 2.5 % with a single gassing. This may be satisfactory for many storage conditions, but not 32°C or above. A maximum limit is 3.0 to 3.5 % oxygen in the headspace of the can for a noticeable delay of oxidation.

b) Oxygen Removal by Reaction

The procedure consists of the addition of 5% hydrogen to the nitrogen used to restore atmospheric pressure after de-aeration. A packet containing a catalyst is added to the container after filling with the dry milk. The catalyst may be palladium or platinum. Each of these causes the oxygen to react with the hydrogen forming water which cannot pass back through the pouch wall into the dry product, thus effectively removing the oxygen available for oxidative reactions.