Drying Systems in the Cheese Industry

IDF Symposium on Cheese
Prague, Czech Republic March 21-25, 2004
Opening Paper: "Spray Drying in the Cheese Industry"
by J. Písecký, Niro A/S, Denmark

The paper is dealing with the application of spray drying in, and in the relation to the cheese industry. In the introduction the technology of producing the cheese powder is briefly described and also the production of the skim milk powder suitable for later production of cheese. Also the technology of whey processing to various dry products is briefly discussed. The main subject of the paper is the description of a new process, called TIXOTHERM™, developed recently by Niro, for the processing of permeate, produced as a by-product from the ultrafiltration of whey, into a nonhygroscopic powder. After evaporation to 60% TS the permeate concentrate is subjected to a three step process consisting of concentration to 86% in the ROSINAIRE™ paddle dryer, holding, stabilization and curing in a screw conveyor with two augers, and finally drying and cooling in a combined back-mix/plug-flow fluid bed. In comparison with the traditional processes the TIXOTHERM™ provides great savings of energy (about 30%) and building costs (up to 75%).

The main subject of my paper will be to inform you about a new process for production of permeate powder. However, let me just start with a general survey of the application of spray drying in the cheese industry. You can find spray dryers in almost every cheese factory, however, these dryers are mostly used for the processing of the by-product, i.e. whey and only very few for making cheese powder.

In order to spray dry cheese it is necessary to bring it into a liquid form. This is done by a normal melting process where the cheese rind, if any, is removed, the cheese is disintegrated, and during heating and agitation melting salts such as phosphates and citrates are added together with water.

Fig. 1 - Single-stage spray dryer with cooling bed. See Spray Drying & Agglomeration for more information.

The aim is to obtain solids content of about 35% and a temperature of 75°C in order to get a feed which is not too viscous for atomization. The drying is conducted in a conventional spray dryer with cooling fluid bed (fig.1) with air inlet temperature of 180-190°C. The cooling of the powder is done in a Vibro-Fluidizer® supplied with ambient air in the first section and cold dehumidified air in the last section. The integrated belt dryer FMD (fig.2) has proved especially advantageous for cheese powder, which is discharged in an agglomerated form. Further, the cyclone fraction is practically zero and does thus not present any operational problems. Cheese powder is mainly used in dips, dressings, biscuits, chips and direct as flavoring on hot dishes like spaghetti and soups. As a certain amount of aroma is lost during the drying, it is recommended to use aged cheese. In order to avoid problems with the odor from the exhaust air this must be filtered through activated carbon filters.

Fig. 2 - Integrated belt dryer FILTERMAT® Spray Dryer FMD. For more inforamtion see: FILTERMAT® - combined spray dryer and belt dryer Spray Drying & Agglomeration

Another interesting application of spray drying, which has a connection to cheese industry, is production of skim milk powder suitable to be reprocessed to cheese. During the last years great efforts have been made to produce skim milk powder for later cheese production. The reason for this is to counteract seasonal variations in supplies of raw milk for cheese production. However, it also opens the possibility of sending powder to countries with no milk production of their own for making special soft cheese with a short shelf-life, or of producing powder for stock, if prices for cheese are low, with a later reconstitution and cheese-making in mind, because the keeping quality of the powder is better compared with that of cheese.

It is not my intention to go too much into details about the drying technique, but it should be mentioned that unless raw milk of top quality, with respect to hygiene, is available a suitable powder for making cheese of good quality cannot be obtained. Skim milk powder can be classified as follows:

Low heat powder: not less than 6 WPNI mg/g powder
Medium heat powder: above 1.5, but below 6 WPNI mg/g powder
High heat powder: not more than 1.5 WPNI mg/g powder

Fig.3 - Two-stage spray dryer with rotary atomizer. See Spray Drying & Agglomeration for more information.

The main target in this production is to obtain a true low-heat skim milk powder i.e. the powder in which the WPNI (Whey Protein Nitrogen Index) is as high as possible which means to apply low pasteurization temperature and gentle treatment during the whole process. Single-pass evaporators are recommended, and two-stage drying (fig.3) is to be preferred. It provides a gentler drying, because the particle temperature is much lower, especially during the critical drying phase from 20 to 10% moisture, which results in a powder with no protein denaturation.

The main application of spray drying in the cheese industry is the further processing of whey. Whey has always been regarded as a troublesome waste product and has been treated accordingly. According to the volume and weight whey is in fact the largest amount of material coming out at the production of cheese. One of the problems the cheese industry is facing is a continuously increasing attention to the pollution problems, which made traditional outlets of whey such as return to the milk suppliers for cattle and pig feeding, dispersing in fields as fertilizer or simply dumping in rivers, practically impossible. It has therefore been necessary to find alternative types of products made from whey which would be more attractive for the consumers, together with alternative low-cost technologies which would be more attractive for the producers.

Whey, considered as a raw material for further processing is requiring the same treatment and care as given to milk. The recommended procedure is cooling down below 10EC just after it is drained from the cheese vats to slow the bacterial activity. It is also strongly recommended to remove the so called cheese dust by clarification and excess of fat by centrifugation, as residues of both will affect further processing. Lack of treatment resulting in developed acidity degrades the quality of final products and causes difficulties during drying. The composition of sweet and acid whey can vary very much, but average values are about 6% total solids, 75-76% lactose in TS and 13-14% whey protein in TS.

Whey, both sweet and acid, can be dehydrated as such and also used as a raw material for a number of products. Modern processes such as demineralization, ultrafiltration and enzyme hydrolyzation have further expanded the product spectrum to modified whey powders, i.e. demineralized and hydrolyzed products, whey protein powders and dried permeate. Furthermore whey can be used as a carrier for fat during the production of fat filled whey powders. Most of these products are difficult to dry requiring special technologies and special equipment. Various types of spray dryers used for drying of whey products are shown on fig.4. from the most simple single-stage dryer (SDP) to two-stage and possibly three-stage drying systems (SDI, CDI, MSD™, TFD) and finally to FMD. The decision which type of dryer to apply for a given product depends on product difficulty and final product quality requirement. In other words the more difficult product and the higher quality requirement the more sophisticated dryer must be used.

Spray Dryer - Pneumatic Transport		Two-Stage Dryer
Filtermat Dryer		Compact Dryer
Multi-Stage dryer		Tall Form Dryer
Fig.4 - Various types of spray dryers used for drying of whey products. For more information, visit http://www.niroinc.com/drying_dairy_food/spray_drying_dryers.asp.

The main lines for processing of whey are:

• concentration and spray drying for producing whey powder, the quality of which could be, according to the type of process and equipment, everything between dusty hygroscopic powder and granulated non-caking powder. Alternatively the fat-enriched whey powder can be produced or demineralized whey powder whereby, as drying is concerned, similar technology and equipment is used.
• production of lactose whereby there remains a problem of the mother liquor which is the by-product and which needs also further processing to the powder.
• treating the whey by ultrafiltration for the production of whey proteins and transforming them into whey protein concentrate powder whereby there follows a by-product which is the permeate. Production of whey protein powder is a good alternative to processing whey since the product is much sought and has high value on the market. It is used mainly as a component for babyfood and also as a protein fortification in various foodformulae. Standard products on the market contain 35, 60 and 80% protein. 80% is the most used and valuable product, which originates from an ultrafiltration plant as an approximately 20-25% total solids concentrate which can be further concentrated in the evaporator up to almost 40%.

Fig. 5 - Tall-form dryer for protein concentrates. For more information, visit Niro A/S, Denmark.

Due to the high protein content the powder tends to be very light and fluffy with a high content of occluded air. To minimize this, pressure nozzle atomization is preferred. The most recommendable plant is a two-stage tall-form dryer (fig.5). Two stage drying is used to protect the proteins against denaturation. A TFD plant has the advantage of a low cyclone fraction giving acceptable powder stack loss levels even without a bag filter. Other dryer types such as SDI and CDI operating with pressure nozzles can be used as well but for these plants a bag filter is necessary.

Each of these possibilities can, depending on local conditions and volume of available whey and considering at the same time the outlet possibilities for the products and production costs, be justified. It is not the intention of this paper to describe the technologies of all these processes. These are very well known.

Considering the value of the product and its utilization possibilities it will probably be the whey protein powder which is most attractive. The other product from ultrafiltration, which is permeate usually contains about 4.5% lactose. Traditionally it can be further converted to powder by means of the so called Straight-through process and Wet process.

The straight-through process (fig.6) applies in sequence the processes evaporation, precrystallization, spray drying and finally fluid bed drying. The evaporation is done in multiple effect TVR or MVR evaporators, whereby the permeate is concentrated to 50-60% total solids.

Special attention must be paid to Ca3(PO4)2 complex, as this may precipitate on the tubes during evaporation jeopardizing continuous operation. This problem is possible to overcome by ion-exchange or heat-precipitation. The concentrate can be dried directly, but it will be very hygroscopic and therefore a precrystallization is recommended. The concentrate is flash-cooled to about 30°C and transferred to crystallization tanks. Depending on the requirements to the product quality this precrystallization takes 6 to 24 hours - the longer precrystallization the less hygroscopic is the product. Many types of drying chambers can be used for spray drying, but nowadays the most convenient type is the COMPACT DRYER™ with integrated annular shaped fluid bed. The atomization in this dryer is conducted by atomizer wheel. The water removing process is then finalized in vibrating fluid bed.

Fig. 6 - Straight-through spray dryer with MVR-evaporator, crystallizers and vibrating fluid bed for permeate powder. For more information, visit http://www.niroinc.com/evaporators_crystallizers/evaporation_technology.asp and http://www.niroinc.com/evaporators_crystallizers/TVR_MVR_systems.asp.

Next — TIXOTHERM™ process for processing of permeate