Spray-Drying of Food Flavors - ACS Symposium Series (ACS

Chapter 7

Spray-Drying

of Food Flavors

Gary A. Reineccius

Downloaded by UNIV LAVAL on July 5, 2014 | http://pubs.acs.org Publication Date: May 31, 1988 | doi: 10.1021/bk-1988-0370.ch007

Department of Food Science and Nutrition, University of Minnesota, St. Paul, MN 55108

Spray drying is the most commonly used technique for the production of dry flavorings. In spray drying, an aqueous infeed material (water, carrier, and flavor) is atomized into a stream of hot air. The atomized particles dry very rapidly, trapping volatile flavor constituents inside the droplets. The powder is recovered via cyclone collectors. Flavor retention is quite satisfactory if dryer operating parameters are properly chosen. Flavor retention is maximized by using a high infeed solids level, high viscosity infeed, optimum inlet (160-210 C) and high exit (>100 C) air temperatures and high molecular weight flavor molecules. The shelf-life of oxidizable flavor compounds is strongly influenced by the flavor carrier. Spray drying i s the major process employed to produce dry f l a vorings. The popularity of this process i s p a r t i a l l y h i s t o r i c , i.e., i t was the f i r s t process used i n the flavor industry to produce an "encapsulated" f l a v o r i n g . However, the merits of the process have ensured i t s continued dominance i n the flavor area. These merits include a v a i l a b i l i t y of equipment, low process cost, wide choice of c a r r i e r s o l i d s , good retention of v o l a t i l e s , and good s t a b i l i t y of the finished flavoring. The i n i t i a l step i n spray drying of a flavor i s the selection of a suitable c a r r i e r material. One can divide the major flavor c a r r i e r s into three classes (and blends thereof): hydrolyzed starches, emulsifying starches, and gums (essentially gum arabic). The hydrolyzed starches are inexpensive, bland i n flavor, very soluble (up to 75%), and exhibit low v i s c o s i t y i n solution. The major shortcomings of these products are a v i r t u a l lack of emulsifying capacity and marginal retention of v o l a t i l e s . The emulsifying starches have been p a r t i a l l y hydrolyzed and then derivatized to impart l i p o p h i l i c properties. The l i p o p h i l i c

c

0097-6156/88/0370-0055$06.00/0 1988 American Chemical Society

In Flavor Encapsulation; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


56

FLAVOR

ENCAPSULATION

group added to the starch backbone comes from the reaction with 1-octenyl succinic anhydride (1, 2). These starches provide excellent v o l a t i l e retention during spray drying and emulsification properties (3, 4). Their p r i n c i p l e drawbacks include o f f flavors, cost, and poor protection of the flavoring to oxidation. Gum arabic has been the standard of excellence as a flavor encapsulating material. I t i s an excellent emulsifier, bland i n flavor, and provides very good retention of v o l a t i l e s during the drying process (4). The major shortcomings of gum arabic have related to high cost and limited a v a i l a b i l i t y . Once a c a r r i e r choice has been made, the c a r r i e r i s hydrated (sometimes with heating) to the optimum solids l e v e l . While most work 05) has shown that increasing the solids level of the dryer infeed matrix not only greatly improves flavor retention during drying but has an added advantage of increasing dryer throughput, there i s an optimum infeed solids level for each c a r r i e r material (_5). This i s the solids l e v e l at which maximum s o l u b i l i t y of the c a r r i e r i s achieved. The hydrated c a r r i e r i s then mixed with the chosen flavoring material, a coarse emulsion formed v i a high sheer mixing and then i t i s homogenized prior to going into the spray dryer. While there i s considerable v a r i a t i o n from company to company on the l e v e l of homogenization given the infeed material, i t has been suggested that there i s a s i g n i f i c a n t advantage to e f f i c i e n t homogenization (6). There are many d i f f e r e n t types of spray dryers used i n the flavor industry. They d i f f e r i n size, shape and type of atomization. Atomization i s t y p i c a l l y accomplished by either a s i n g l e - f l u i d high-pressure spray nozzle or centrifugal wheel. While the two-fluid nozzle i s used i n some applications, i t i s not commonly used in the flavor industry. Centrifugal wheel atomizers have an advantage of handling very viscous and abrasive infeed materials while the pressure spray systems o f f e r greater f l e x i b i l i t y i n producing larger p a r t i c l e size powder. Approximately 80% of the industry u t i l i z e s centrifugal wheel atomization. V i r t u a l l y a l l spray dryers used i n the flavor industry are cocurrent i n design, i . e . , product enters the dryer flowing i n the same d i r e c t i o n as the drying a i r . This results i n very rapid drying and does not subject the flavoring to as much heat as would a counter current system. In the cocurrent dryer, the flavoring never exceeds the exit a i r temperature of the dryer. The atomized product i s cooled by water evaporation and the powder temperature i s normally 35 to 40 C below the dryer e x i t temperature. Drying chamber shape predominantly i s either conical or f l a t bottomed. The flat-bottomed dryers remove the powder as i t f a l l s to the floor of the dryer by use of a rotating pneumatic powder discharger that functions as a vacuum cleaner. These dryers subj e c t the product to s i g n i f i c a n t l y more heat than do the conebottomed dryers. While for many types of dry flavorings this additional heat i s i n s i g n i f i c a n t , thermally l a b i l e materials (e.g., natural flavorings - tomato, cheese, and numerous f r u i t juice based products) may suffer from the additional heat.



7. REINECCIUS

Spray-Drying of Food Flavors

The dry product i s primarily collected i n cyclone c o l l e c t o r s (a few bag houses s t i l l remain), sieved, and f i n a l l y packaged i n moisture barrier containers. The e x i t a i r from the dryer often has to be treated to meet l o c a l p o l l u t i o n control laws. While many of the older dryers use gas incineration, as energy costs have increased these incineration systems have become quite costly to operate. New dryer i n s t a l l a t i o n s use scrubbing systems (e.g., aqueous/chemical sprays) to remove entrained s o l i d s and gaseous v o l a t i l e flavors. At f i r s t glance one might be surprised that v o l a t i l e flavor compounds are retained during spray drying. The major v o l a t i l e constituent i n the infeed matrix i s water. During this drying process, at least 90% of the water i s evaporated but yet the more v o l a t i l e flavor constituents (e.g., d i a c e t y l , ethylacetate, ethyl butyrate) are nearly completely retained when optimum drying conditions are followed. One would expect the flavor constituents to be lost to a large extent during the drying process. The reason for the surprisingly good retention of v o l a t i l e s has been the subject of substantial research (7-35). The accepted explanation for this phenomenon relates to the fact that as an atomized droplet of infeed material contacts the hot dryer a i r , i t starts to dry on the outside. The surface of the drying droplet decreases very rapidly i n moisture content. When this surface reaches a moisture content ranging from 7 to 23 percent, i t i s no longer permeable to most flavor compounds but remains quite permeable to the r e l a t i v e l y smaller, soluble water molecules. Therefore, this dry ( a < 0.90) surface acts as a semipermeable membrane permitting the continued loss (or d i f fusion) of water but e f f i c i e n t l y retaining (or stopping d i f fusion) flavor molecules. A study by Menting et a l . (20, 21) showed that between 40 and 100% moisture, d i f f u s i o n c o e f f i c i e n t s of water and organic flavorants varied by less than a factor of 10. However, once the c a r r i e r material attained _< 7% moisture, the d i f f u s i o n constants of water to organic flavorant d i f f e r e d very greatly. As an example, the d i f f u s i o n c o e f f i c i e n t of acetone was 300 times less than that of water. The vastly reduced d i f f u s i o n c o e f f i c i e n t of acetone e f f e c t i v e l y prohibits i t s movement through the dry matrix and i t cannot reach the surface to undergo evaporation. In order then to determine what influences flavor retention during drying, one must focus attention on the very early stages of dehydration. In fact, i t has been shown that the major f r a c t i o n of t o t a l v o l a t i l e s l o s t during nozzle-atomized spray drying occurs within ten centimeters of the pressure nozzle (17, 33, 35). The process parameters which have been stated as influencing the retention of v o l a t i l e flavor compounds during spray drying are (36): w

1. 2. 3. 4. 5.

Solids content of the infeed material. Molecular weight and vapor pressure of flavor compounds. Type and molecular weight of the c a r r i e r used. Concentration of the flavor components. Viscosity of the dryer infeed material.


57

58

FLAVOR


6. 7. 8. 9. 10.

ENCAPSULATION

Drying a i r v e l o c i t y . Dryer i n l e t and e x i t a i r temperatures. Percent humidity of the dryer i n l e t a i r . P a r t i c l e size of the atomized droplet. Dryer feed temperature.

The primary factor determining the retention of v o l a t i l e s during drying i s infeed solids content (14, 20, _21, _23, 24, 37). High infeed solids dryer feeds increase retentions during drying by reducing the time necessary to form a semipermeable membrane at the drying p a r t i c l e surface. The very strong dependence of flavor retention on infeed solids content i s readily apparent from Figure 1. A study by Leahy et a l . (4) has shown that infeed solids content i s even more important i n determining flavor retention during drying than i s the type of c a r r i e r . While previous data has suggested that one should use the highest infeed solids possible, recent work has shown that there i s an optimum infeed solids content for the drying of flavoring materials 05). An optimum solids level exists since one generally uses a constant r a t i o of flavoring materials to c a r r i e r s o l i d s . At some solids content, s o l u b i l i t y i s exceeded by adding more c a r r i e r . While i t may be possible to pump and atomize this higher solids matrix, the undissolved c a r r i e r does not provide any e f f e c t i v e encapsulating e f f e c t and poorer flavor retention i s noted during the drying process (Fig. 2). I t i s apparent that each c a r r i e r material has i t s own optimum infeed solids for flavor retention which i s based on s o l u b i l i t y . The fact that both molecular weight and vapor pressure of the flavor compounds have an influence on their retention during spray drying i s both obvious and well documented i n the l i t e r a ture (4, 8, H), 24). Molecular weight i s a reasonable representation of molecular size which actually i s the primary factor determining d i f f u s i o n . For flavor molecules of increasing molecular size, d i f f u s i o n rate slows and the flavor molecules do not reach the p a r t i c l e surface as r e a d i l y . A second factor promoting the retention of larger flavor molecules i s that the drying surface becomes impermeable more quickly during drying. Diffusion i s e f f e c t i v e l y stopped at a higher moisture content. Both of these factors favor the retention of larger molecular weight (molecular size) flavorants. Vapor pressure or v o l a t i l i t y plays a secondary role i n determining flavor retention due to i t s influence i n c o n t r o l l i n g flavor loss u n t i l the drying droplet surface becomes semipermeable. The end result i s that small, very v o l a t i l e flavor compounds are l o s t to a greater extent than the larger less v o l a t i l e flavor compounds (Fig 3). While the infeed solids content of the infeed material has an unquestionably greater influence on the retention of v o l a t i l e flavors than does the type of c a r r i e r used, c a r r i e r type does influence flavor retention during spray drying (4, _16, _31> 37, 38). This influence can be i n d i r e c t i n the sense that some c a r r i e r materials become very viscous at r e l a t i v e l y low solids contents. Low solids means poor flavor retention. The effect of type of c a r r i e r on flavor retention can also be d i r e c t . Carriers v



REINECCIUS


0

5

10

15

20 25 30 35 TOTAL SOLIDS (X)

40

45 50

Fig. 1. Effect on solids l e v e l on the retention of v o l a t i l e s during spray drying. (Reproduced with permission from r e f . 37. Copyright 1969 American Dairy Science Association.)

90i

;z—

80· ~ 70·

• ETHYL ACETATE

ζ 60· O

g

LU

50-

• 2-HEPTANONE

tu 40· ce

• METHYL ANTHRAN1LATE

30· 20 10· 035

40

45 50 55 INFEED SOLIDS (X)

60

Fig. 2. Optimization of solids levels for the retention of v o l a t i l e s during spray drying. (Reproduced with permission from r e f . 5. Copyright 1982 Allured Publishing Corporation.)



60

FLAVOR

ENCAPSULATION

which are good emulsifiers and/or good f i l m formers t y p i c a l l y y i e l d better flavor retention than do c a r r i e r s which lack these properties. Higher flavor loads generally result i n poorer flavor retention (_7, _23, _37 ). Due to this phenomenon, most products to be spray dried are dried at a solids to flavor r a t i o of 4 to 1. One of the few exceptions to the 4:1 r a t i o l i m i t i s the patent of Brenner (39). Brenner advocates the use of a p l a s t i c i z i n g c a r r i e r component which reportedly permits the e f f e c t i v e encapsulation of flavoring at up to 1:4 ratios (carrier : f l a v o r ) . While this patent has been i n existence for about 11 years, this author knows of no commercial products using this high loading. Infeed v i s c o s i t y exerts an e f f e c t on flavor retention during drying by influencing c i r c u l a t i o n currents within the drying droplet. If v i s c o s i t y i s low, internal mixing may occur during drying which delays formation of the semipermeable surface. This delay permits greater flavor losses. Therefore, at the same infeed solids content, one would expect greater flavor retention for a high v i s c o s i t y infeed vs. a low v i s c o s i t y infeed material. High dryer a i r velocity (relative to the atomized p a r t i c l e ) i s known to improve the retention of flavorants. This e f f e c t i s due to a more rapid heat and mass transfer associated with the drying process. This factor i s largely controlled by dryer design and cannot be changed to a s i g n i f i c a n t degree as a dryer operating v a r i a b l e . The influence of dryer i n l e t and e x i t a i r temperatures has received considerable study (2, 8, 9, 23, _37, 40, 41). It i s desirable that a high enough i n l e t a i r temperature be used to allow rapid formation of a semipermeable membrane on the droplet surface but yet not so high as to cause heat damage to the dry product or "ballooning of the drying droplet. Inlet a i r temperatures of 160-210 C are reported as giving optimum flavor retention during drying (2, 8, 23, 37), Inlet a i r temperatures above 210 C have been found to decrease flavor retention for some types of c a r r i e r s . This decrease i n flavor retention i s due to ballooning during drying. Ballooning occurs when s u f f i c i e n t l y high i n l e t a i r temperatures are used that steam i s formed i n the i n t e r i o r of the drying droplet. Steam formation causes the droplet to puff-up (or balloon) thereby producing a thin-walled hollow p a r t i c l e . This p a r t i c l e w i l l not retain flavor compounds as well as the non-ballooned counterpart. The ballooning temperature i s primarily a function of c a r r i e r material and dryer design. Spray dried flavorings have been successfully produced using i n l e t a i r temperatures from 280-350 C (36, 41). The influence of dryer e x i t a i r temperature on flavor retention i s not as well documented. Reineccius and Coulter (37) have shown that flavor retention increases with increasing exit a i r temperatures. This i s presumably due to the higher exit a i r temperatures (at a fixed i n l e t a i r temperature) giving the dryer a i r a lower humidity. Low humidity results i n more rapid drying and, therefore, better flavor retention. There are other concerns for using high exit a i r temperatures, however. High exit a i r temperatures may result i n heat damage to some flavoring materials 11


REINECCIUS


(e.g. cheese, tomato, and natural f r u i t juices) and also result i n decreased dryer throughout. Therefore, in practice, dryer e x i t a i r temperatures usually range from 80-90 C. Dryer a i r humidity can be controlled by dehumidifying the i n l e t a i r . This would favor rapid drying and flavor retention. Dehumidification t y p i c a l l y i s cost prohibitive and, therefore, i s seldom done. Infeed temperature has also been studied by numerous workers (8, 23, 24, 27_ 28, 33). Sivetz and Foote (24) have noted that c h i l l i n g the dryer infeed (30% coffee solids extract) before drying markedly improved the flavor of the spray dried coffee. Cooling the infeed material would increase the feed v i s c o s i t y which, in turn, would a f f e c t c i r c u l a t i o n currents within the atomized droplets and size of these droplets, along with the vapor pressure and d i f f u s i v i t y of the flavor compounds. Thijssen's work tends to disagree with these findings (8, 23, 27, 28). Thijssen stated that dryer infeed temperature should be elevated such that higher infeed solids ( i . e . greater s o l u b i l i t y ) may be used. The higher infeed solids would result in better flavor retention. The role of p a r t i c l e size of the atomized droplets in determining flavor retention i s also c o n t r o v e r s i a l . Several workers have reported that larger p a r t i c l e sizes result in improved flavor retentions C7, 9, ^3, 24, 42, 43). To the contrary, Reineccius and Coulter (37) could find no e f f e c t of p a r t i c l e size on retention. This controversy has been p a r t i a l l y cleared up by work showing that p a r t i c l e size i s not s i g n i f i c a n t i f high infeed solids are used (8). Reineccius and Coulter (_37) did their study at a high infeed solids content. While there may not be a r e l a tionship between flavor retention and p a r t i c l e size, i t often i s desirable to produce large p a r t i c l e s to f a c i l i t a t e rehydration. Small p a r t i c l e s tend to disperse very poorly, e s p e c i a l l y i n cold water and instead form lumps on the l i q u i d surface. Large part i c l e s can be obtained through judicious choice of dryer operating conditions (e.g., high infeed v i s c o s i t y and solids, low-pressure l a r g e - o r i f i c e i f using a pressure spray atomizer or low wheel speed i f using a c e n t r i f u g a l atomizer) or the use of agglomeration techniques. The f i n a l subject of this review w i l l consider the s h e l f - l i f e of spray dried flavorings. This discussion w i l l be b r i e f since there are several other papers on this subject as part of this symposium. A large portion of the dry flavorings produced include some c i t r u s o i l s . These c i t r u s o i l s are prone to oxidation during storage (45). In the past, the c i t r u s o i l s have been s t a b i l i z e d v i a the use of antioxidants (BHA). However, today's market i s demanding preservative-free flavorings. This creates a very s i g n i f i c a n t s h e l f - l i f e problem. As i s shown i n other work presented at this symposium (41_, 44), the s h e l f - l i f e of dried orange o i l (no antioxidant) may be only a few weeks at room temperature. The industry requires at least a year s h e l f - l i f e . In order to improve upon s h e l f - l i f e , the encapsulated f l a voring must be protected from oxidation. This brings up con-


9


FLAVOR


62

88

102

116 130 MOLECULAR WT.

144

ENCAPSULATION

150

Fig. 3. Effect of molecular weight of v o l a t i l e s on their retention during drying.

ο χ ο

DE DE DE -θ- DE -*- DE —

• en Ε

10

20 30 40 50 60 STORAGE TIME (DAYS)

4 10 20 25 36

70 80

Fig. 4. S h e l f - l i f e of orange o i l s (as measured by limonene epoxide formation) as influenced by flavor c a r r i e r . (Reproduced with permission from r e f . 46. Copyright 1986 Institute of Food Technology.)



7. REINECCIUS


sideration for the presence of antioxidants (natural), trace metals (e.g., copper and i r o n ) , entrained a i r and oxygen barrier properties of the f i n a l spray dried p a r t i c l e . Anandaraman (45, 46) has shown that there i s a very strong protective e f f e c t of higher dextrose equivalent (DE) starches (corn syrup solids) against oxidative deterioration ( F i g . 4 ) . The higher DE products have a greater number of free reducing groups. I t i s possible that some of the s t a b i l i t y observed by Anandaraman (45, 46) i s due to the high reducing environment provided by the encapsulating matrix. One might consider the matrix i t s e l f to be acting as an antioxidant. One must be very cautious about using flavor c a r r i e r s which contain trace minerals that are pro-oxidants. There i s no question that copper and iron w i l l catalyze the oxidation of c i t r u s o i l s . One w i l l find a s i g n i f i c a n t variation i n trace mineral content of commercial flavor c a r r i e r s . The role of entrained a i r ( i . e . , a i r included or trapped within the p a r t i c l e ) i n determining s h e l f - l i f e of spray dried flavorings has not been studied. I t would appear l o g i c a l that one should minimize the entrained a i r since any a i r contact w i l l promote oxidation. Probably the major determinant of s h e l f - l i f e of spray dried flavorings i s the porosity of the dried p a r t i c l e to oxygen. While there i s no direct data to support this statement, we have found vastly d i f f e r e n t s h e l f - l i v e s for products which contain e s s e n t i a l l y s i m i l i a r trace metal l e v e l s , surface o i l s , and absolute densities. We can find no other explanation f o r the d i f ferences i n s h e l f - l i f e other than matrix porosity. This area needs to be further studied i n order to confirm this hypothesis and then take advantage of i t to improve the s h e l f - l i f e of spray dried flavorings. In closing, spray drying has been the t r a d i t i o n a l means of producing encapsulated flavorings. I f adequate care i s used i n selection of spray dryer operating conditions, a very high quality product can be obtained at r e l a t i v e l y low cost. Recent marked trends ( i . e . , "natural") as well as competitive processes for encapsulation are putting demands on the spray drying process for improvement of s h e l f - l i f e c h a r a c t e r i s t i c s . Work currently i s i n progress to enhance the s h e l f - l i f e of spray dried flavorings.

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Furda, I.; Malizia, P.D. U.S. Patent 3,973,049. 1976.

2. Marott, N.G.; Boettger, R.M.; Nappen, B.H.; Szymanski, C.D. U.S. Patent 3,455,838. 1969. 3.

King, W.; Trubiano, P.; Perry, P. Food Prod Dev, 1976, 10(10),54.

4.

Leahy, M.M.; Anandaraman, S.; Bangs, W.E.; Reineccius, G.A. Perfumer Flavorist, 1983, 8,49.


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64 FLAVOR ENCAPSULATION

5. Reineccius, G.A.; Bangs, W.E. Perfumer Flavorist, 1982, 9,27. 6.

Reineccius, G.A.; Anandaraman, S.; Bangs, W.E. Perfumer Flavorist, 1982, 7(4),1.

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7. REINECCIUS Spray-Drying of Food Flavors 65

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Technology, the Netherlands.

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RECEIVED December 29, 1987


Spray-Drying of Food Flavors - ACS Symposium Series (ACS

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