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Edible water vapor barriers: properties and promise
Revie~'¢
Edible water vapor barriers: properties and promise Claire Koelsch Demand for edible barriers to retard water vapor transmission has resulled in a plethora of research in this emerging area. This review addresses the development of edible barriers and what is known about their physicochemical nature, and identifies key areas for new research and industrial applications. The development of edible water vapor barriers has focused upon barriers containing lipid (which are less permeable) or protein (which are more permeable). While the application of edible barriers to processed foods has been limited, the potential of edible barrier technology for both the extension of the shelf life of processed foods and the creation of innovative foods is promising.
Recent interest in edible barriers for food packaging has grown due to the following factors: • environmental legislation: •
expanding distribution channels:
• consumer expectations for a variety of fresh foods: • need for extended shelf liie' foods: • opportunities for new foods with edible barriers. The development of water vapor barriers has focused upon barriers containing primarily cellulose, lipid and protein. In general, barriers containing lipid are less permeable to water vapor than barriers containing proreins L'~. However, protein-containing barriers do have applications as edible barriers for certain products, such as fresh fruits and vegetables, that have high rates of moisture loss. Lipid-based edible b a r d e r s The support matrix A non-lipid support matrix is required to reduce the brittleness/fragility associated with barriers containing lipid and to allow application of the edible barrier onto the food. The most commonly used support matrices are morttied celluloses of hydroxypropylmethyl cellulose .~, ethyl
cellulose"~'~and methyl cellulose (Ref, 6 and Kamper, S., MSc thesis, University of Wisconsin, Madison, WI, USA, 1983), which are dissolved in ethanol-water-plasticizer solutions. Other support matrices include chitosan [poly-13(l--+4)-glucosaminel7 and whey protein isolate (WPI) s in solvent-plasticizer solutions. Selective structural engineering of cellulose has been used to create synthetic cellulose structures'~."~.However. extensive research into the water vapor permeation rates of different cellulose derivatives has not been reported in the literalure. Theoretically, methyl cellulose would result in a more dense (less permeable) support matrix, because of its high solubility in ethanol. Ethanol aids the formation of a uniform solution of cellulose and reduces the time required for barrier preparation and drying. Plasticizers of various types have been incorporated into edible barriers as a processing aid, to facilitate barrier application onto foods and to increase the machinability of the edible barrier. Food-grade polyethylene glycol is the plasticizer most commonly reported in the literature. Plasticizers reduce the brittleness of the film by interfering with the hydrogen bonding between the lipid and the cellulose entities. Thus, high polyethylene glycol concentrations result in barriers with high water vapor permeabilities (Kamper, S., MSc thesis). Details of the preparation of support matrices are found in Refs 3-10. There are essentially two types of lipid barriers, laminates (in which the lipid is a distinct layer within the -'dible barrier) and emulsions (in which the lipid is uniformly dispersed throughout the edible barrier). A support matrix (which comprises 50-100%. v/v of the resultant edible barrier) either provides a means to supporl the lipid (in the case of a laminate barrier) or disperse the lipid tin the case of an emulsion barrier). Lipid-based emulsion and laminate barriers are discussed below.
Preparation of lipid-based barriers Lipid is added to an emulsion barrier when the cellulose (or WPI or chitosan) within the support matrix is dissolved. Once a homogeneous mixture of the lipid and support matrix has been achieved, the barrier is cast upon an impenetrable (glass or metal) level plate. The water and ethanol are evaporated from the mixture at an elevated temperature (60-90°C). Once the barrier has dried/cooled to a moisture content of 2-5%, it is peeled from the plate and stored in an environment with a controlled relative humidity and temperature before testing. The formation of a laminate barrier involves a twostep process. The molten lipid is either painted, sprayed or poured to form a distinct layer on the dry support matrix (Refs 3-5, 11, and Kamper, S., MSc thesis). The laminate is then dried/cooled, the film peeled off the plate and stored in a humidity- and temperaturecontrolled environment before testing.
Structure/function of the lipid within edible lipid-based emulsion and laminate barriers Water vapor permeabilily depends on the type, Claire Koelschis ~.ilh KraftGeneralFoods,801 W,'~ukeganRoad,Glenview, location, volume fraction and polymorphic phase of It 60025,USA. lipid in an edible harrier.
76
, q',,,~ u,~,,,~.,s,,~,,,~,,t~d.'..'4 -.','44,,~4'SO:U.
Trends in Food Science & Technology March 1994 [Vol. 5[
da~a indicate that, and for 5430I thePublished same concentration of lipid plasticizer, emulsion barriers have 5 lower water vapor permeability than laminate barriers (Ref. 6 and Kamper, S., MSc thesis). This is due in part to -4o the high degree of inherent tortuousity (the increased path length for water ~vapor diffusion due to an interlock-' ing network of lipid within the barrier) "? 30~ within emulsion barriers, and in part E to the tendency of laminate barriers :~ to crack on their surface (Koelsch, C., = -If PhD thesis. University of Minnesota, 20 MN. USA. 1992). Water vapor dif~_ fusion through emulsion barriers fol15 lows a more tortuous path than ~through laminate barriers because the lipid and support matrix entities are 10 interlocked within emulsion barriers, whereas the lipid entity within lami5nate barriers interconnects with the support matrix at only one interface, aCapric Myrislit Palmitk Palmitic/ Slearic Stearic/ Arachidic Behenic Laminate barriers do. however, offer stea.'ic arachidic the advantage of being easier to apply Fatty acid within methyl cellulose matrix due to the distinct natures of the support matrix and lipid. For example, Fig. 1 during the lamination of lipid onto a support matrix, the temperatures of the Water vapor pem~ea.bilit', ,1~,a iunctlon of the ~alt,. acid used {at a concentration of ]0%~ two entities can easily be controlled in emulsion barriers with a methyl cellulose .~upport matrix. separately: however, during the application of an emulsion barrier, the temperature of the range of 0.53-1.00 g,mil.m-2.d-I.mmHg I). Epifluoreseent emulsion mixture must be above the lipid melt tem- microscopy coupled with digitized image analysis of perature but below the temperature for ethanol/water films produced using seven different volume fractions ( 9 - 4 6 % ) of stearic acid within a cellulose support volatilization. As shown in Fig. 1, the minimum water vapor per- matrix yielded quantifiable results to describe the mormeability in a cellulose-based emulsion barrier is phology of stearic acid within a cellulose support matrix achieved when stearic acid is used as the lipid entity ''". (Koelsch, C.. PhD thesis). Results suggested that the The low permeability through barriers containing stearic interlocking network of stearic acid chains v,'ithin the acid can be explained by the fact that stearic acid forms support matrix reaches a maximum (i.e. maximum loran interlocking network: the effect of this network is tuousity) at -30% stearic acid. Similar results were seen significaat enough that these barriers retard moisture for palmitic acid (in volume fractions of (I-305;r) within transfer more effectively than those based on, say, a WPI barrier ]~. Permeability as a function of the w~lmyristic acid, which has shorter chain lengths, or ume fraction of patmitic acid or stearic acid within a behenic acid. which has such long chains that they hin- matrix based on methyl cellulose, and of beeswax. der the formation of a tightly interlocking network ~2. In stearyl alcohol or palmitic acid within a matrix based on essence, stearic acid provides the optimum chain length WPI is shown in Fig. 2. This theory of an interlocking without hindering the formation of an interlocking net- network of fatty acid chains within emulsion films is a plausible explanation for why the water vapor perwork. Kamper and Fennema ~ and Koelsch I PhD thesis) have meability of an emulsion barrier is approximately two addressed the impact of lipid concentration on the water limes less than thai of a laminate barrier of the same vapor permeability through an edible barrier. Water compositionL Mathematical models to explain and prevapor permeability decreases with the addition of fatty dict the water vapor permeation as a function of the volacid from 28.98 g-mil-m -'-d t.mmHg * for 9% stearic ume fraction and morphology of the fatty acid within a acid to I.{10g.miFm--'.d-~.mmHg-t for 30% stearic acid methyl cellulose support matrix have been published" Iwhere film thickness is measured in rail: I mil = and are discussed below under the heading "Futme 0.001 in = 2.54x 10 ~ m: pressures are given as mm research opportunities'. [(ester and Fennema u related c~, 13 and [~' polymormercury: 1 m m H g ~ 133 Pal. When the stearic acid concentration is in the range of 3 0 - 4 6 % , the water phic states of the fatty acid (lipid) component to the vapor permeability remains relatively stable (iu the water vapor pelmeability. Results showed thal barrier:~
!
Trends in Food Science & Terhnolog,.' March ]c)c)4 [VoI. 51
77
packaging industry are less soluble in Wilier than these edible barriers. A complete listing of edible barriers containing lipid entities :md the relative humidity and temperature ut which the permeability was measured has been pub!i,.hed~'.
i~:~ 5 0 ~ -
"~ 30
•
Protein-based edible barriers
Protein barriers, while more permeable m water vapor than barriers containing lipid, do have promise in certain food applications. The following discussion con,t.. * siders edible barriers containing either animal-derived (gelatin) or vegetable-derived (zein, gluten. WPI) protein. Unlike the lipid barriers, which contain a support • i ' l l [3 matrix, the protein itself acts as both the support matrix o o t'o and the water vapor barrier component of the film. 20 30 40 50 Although animal proteins have been used historically to Volume fraction 1%) t. . . . . . . . . . . . . . . . . . . . . . : form edible barriers (e.g. sausage casings), the majority of recent research has focused on vegetable proteins. Fig. 2 This shift in favor of vegetable proteins may be due to Water vapor permeability as a function of the volume of palmitic economic factors as well as the broad range of edible acid and stearic acid within a methyl cellulose support nlatrix 7 and barriers that can be derived from the various vegetable beesv,'ax, palmitic acid and stearyl alcohol within a whey protein proteins. Most of the research on the ingredient isolate IWPI) support matrix ~a. (: I, palmilic acid : methyl cellulose structure/function and water vapor permeability of barmatrix; (,~I, stearie acid : methyl cellulose malrix; riers made from vegetable proteins has occurred within (IIL beeswax : WPI matrix; (o), palmitic acid : WPI matrix; the last five years,
~,o
;n: i
and (o), stearyl alcohol : WPI matrix.
containing the e~ polymorphic phase were more permeuble to water vapor than barriers containing the 13' polymorphic phase: this correlates with the higher solubility of water in the a phase. However, the barrier containing the 13 polymorphic lipid did not exhibit the lowest water vapor permeability, although theoretically it would be expected to as the 13 phase has the highest acyl chain packing density and lowest molecular mobility. This discrepancy has not been resolved, largely because the existence ofc~, [3 and [3' states within the barrier has not been confirmed: this is mentioned below, under the heading "Future research opportunities'. Selected lipid barriers representative of the full range of barriers that contain lipids are shown in Table 1. Relative hura.idity (RH) values are included in Table 1 since water vapor permeability through the barrier increases as the relative humidity to which the barrier is exposed increases. For example, as shown in Table I. water vapor permeability through the barrier containing 35% stearic acid measured at a relative humidity gradient of 12-56% is higher than that through the barrier containing less stearic acid (30%), which has been exposed to 0% relative humidity on one side of the barrier and 85% on the other side of the barrier. Generally, front the data reported in the litcrature, barrier hydration tvia the use of higher relative humidity values on each side of the barrier) results in higher water vapor permeability values. The reasoning for this is the same as that for synthetic polymer structures and is a restdt of the increased sorption of the permeant (in this case water) by the barrier at higher relative humidity values. The difference between these edible barriers and synthetic polymers is that synthetic barriers in use in the 78
Collagen
Collagen, a natural polyamide, is the most prevalent edible barrier and is commonly used for sausage casings. Its derivative, gelatin, has been used as a key ingredient in edible barriers t4.u. Mazza and Qi ]~ formed gelatin barriers with the addition of gums and CaCI, to induce crosslinking but the water vapor permeability of these barriers was not published. Glicksman ~4 determined that the water vapor permeability of gelatin barriers is largely a function of the relative humidity gradient due to the hygroscopic nature of gelatin. Water vapor permeabilities were in the range of 26.5-74.6g'mil'm--"d-l.mmHg t (~5-20 times greater than Ihose of barriers containing fatty acids). Zein
Prolamins such as zein are dissolved in 11.9% saline and alcohol solutions or ethyl alcohol since they are insoluble in wa!er (Ref. 2 and Mendoza, M., MSc lhesis, University of Massachussetts, Amherst, MA, USA, 1975: Trezza, T. and Verguno, P., unpublished). As with lipid barriers, when a plasticizer (e.g. glycerol) is used In decrease the brittleness of the barrier and allow its application to a food the water vapor permeability increases. As shown in Table 2, the addition of 3% glycerol increased the water v.apor permeability of a specific zein barrier by 10% (Mendozu. M.. MSc thesis): similarly, the addition of 20% glycerol to another type of zein barrier increased permeability by 3()% (Trezza, T. aud Vergano. P., unpublished). Thus, the degree to which a plasticizer impacts the water vapor permeability depends on the concentration of the plasticizer as well as the structure of the barrier itself. Highly plasticized commercial zein coatings exhibit a water vapor Trends in Food Science & Technology March 1994 IVol. 5]
permeability two 0rder~; of magniIude larger than lh(tse developed in laboratory environments:.
Table 1. Summaryo( the water vapor permeabil[tlesof selected edible barriers c0nlaining lipid in or on a cellulosic support malrix
Gluten
Gonlard et al. L" assessed the structure/function of ingredients within gluten edible barriers manufactured from 5-12.5% gluten, 2 0 - 4 5 % ethanol and 2(IC/e glycerol. The lowest water vapor permeability (25.84 g'mil'm--"d-~.mmHg ~) was achieved when the barrier was made with I(1% gluten and 45% ethanol and at a pH of 4.5. The barrier with the highest water vapor permeability was made as above except at a pH of 2.It. Gennadios and Weller-" developed a barrier composed o f gluten and soy protein (2 : 1). The pH and level of plasticizer were nol reported. The resultant permeability was ten times that reported by Gontard et al. I'' Use of high-glutenin gluten as a support matrix wilhin an edible barrier has particular promise due to its inherent fibrous structure, its high degree of extensibility and the presence of e~-helices (which restrict interactions with other molecules such as water). Research on the water vapor permeability of high-glutenin gluten hats not been reported. WPI McHugh et al. s explored (he impact of WPI concentration (7. 9 and 12% in solution before drying), solution pH (5.0-101 and plasticizer type (glycerol, sorbitol. polyethylene glycol) on the water vapor permeability through barriers composed of WPI, lipid and plasticizer. The results showed that the optimum concentration for development of WPl-based barriers is 10% (concentration in solution before drying), largely due to the fact that strong gelling occurs at concentrations exceeding 10%. Strong gels inhibited barrier formation at pH values greater than 9.0 when heterogeneous barriers were formed. WPI barriers using 50% sorbitol as the plaslicizer had significantly lower water vapor permeabilities than those containing either 50% glycerol or 50% polyethylene glycol. Water vapor permeability as a function of the percentage glycerol increased linearly with increased glycerol concentration. The lowest water vapor permeability of 14.99 g.mil.m-:.d ~.mmHg * was achieved with 10% WPI at a pH of 7.0 and 37.5% sorbitol ~. Oxygen permeability was -11 g.mil.m -~-d-~-mmHg J for these parameters-'". Subsequent work 13 addressed substituting WPUI6% sorbitol for the methyl cellulose support matrix commonly used in lipid emulsion barriers. While the flints are predominantly protein based, lipid entilies have been added to complement the barrier properties. Transmission electron microscopy showed that the lipid tin this case stearyl alcohol) was a discrete entity within the WPI. This differs front the configuration of lipid within the methyl cellulose support matrix, in which the lipid forms an interlocking network within the support matrix (Koelsch, C., PhD thesis). This d i f f e r e n c e in the morphology of the lipid within the two support matrices could explain the difference in the water vapor pet meability of the two types of barriers shown in Fig. 2: Trends in Food Science & Technology March 1994 [Vol. 51
Permeability
Barrier
(g'mil'm-Z'd-~'mnlHg -')
Emulsinn hpid barr~er~ S/eark acidl l~",a ~:,learic at ill qICY',:1' Stearic ,[ci(I/[]dlmltl~ acid el :1, total of /()" ,b Oleic acid ¢~(r',,~ Laminate bpid baml,rs Stearic. acld/palmlht acid it : I. t{}tal of ](1",,~ and bee~,}x' Slearic a(id/llalmlh~ acid II : I. l(}t,d of I0",,~"
0.53
RH (%)
Thickness Imill
72-56
1.9
(t. 17
0-8"3
1.3
(/.24 19.Jl
(/ 85 () 85
1.3 1.5
0.30
(}-if7
22
0.46
0-83
4.4
'D,ila lakt,n Iron1Rt,t h Data taken from Rt'l
' Dab1hlkt,n Ir~nlRel I I RH. Relatl;ehumKhlv
the water vapor permeability through a palm(tic acid/ methyl cellulose matrix was 2.93g.mil-m -'-d-J.mmHg ~. while the permeability through a palm(tic acid/WPl matrix was 4.65g-mil-m -'-d-~.mmHg ~ at a volume fraction of - 2 9 % palm(tic acid. A select listing of the reported water vapor permeability properties of edible barriers containing protein is shnwn in Table 2. The greater water vapor permeability values for the zein-based barriers as compared with the WPIand gluten-based barriers may be a result of lhe increased level of barrier hydration {higher RH values) in the zein barriers. More research is needed to establist'~ firmly how water vapor permeation is affected by the relative humidity.
Selecting and developing an edible water vapor barrier As with synthetic barriers, some edible barriers are applicable in certain situations and inapplicable in others. For example, it would be more appropriate to use an edible barrier with a low water permeability for a food with a high water activity Ca) and long shelf life than a barrier with a high water permeability. Predicting the required water vapor permeability that an effective edible barrier must exhibit saves both time and energy in the development or selection of an appropriate edible barrier. A procedure has been developed to determine ".,,'hat water vapor permeability an edible barrier must have to be the predominant factor in retarding moisture loss from a food COaled with an edible barrier +"~7. This procedure, which has been detailed by Hone et al. ~:. involves lhe empirical determination ol" the rate of water vapor loss ¢i.e. moisture migration) from the food. the application of classical diffusion equations ~-'' and the concepts of Taoukis et aL:" The final re~,ult is a 7q
physicochemical nature of components within an edible barrier is needed to allow predictions of water vapor permeability and to understand further how the properties of an edible barrier can be optimized in a manner similar to that commonly used to develop synthetic polymers.
Table 2, Summary of the water vapor permeabilities of edible protein barriers Permeability (g.mil.m-2.d-'.mmHg -~)
Barrier Zein barriers Zem' Zein/20% glycerol'
Zein13%glvcerolb
226.12
318.89 1462.00
WPI barriers' WPII28% palmitica( id/I6% sorbitol WPI/28%beeswax/I6% sorhitol WPI/28%stearylalcohol/I6% sorbitol WPI/16%sorbilol
RH
Thickness
(%)
(mill
50-100 50-I 00
4,65 4,q4 I 1.21 12.78
0-If)[} 0-100 0-1 O0 O-100
Gluten barriers 10% Gluten/20"/.glvcerol,pFl of 4"
25.84
0-100
10';;, Gluten/20% glycerol pH o/2" Gluten/soy protein isolate"
42.36 248.83
0-I00 l |
~Data taken/rum Tmzza, T. antl 'Vergano. P,. unpublished t'Dala taken from Mendoza, M.. MSc lhesi~ ' Data taken fronl Ref.13 "Data laken from Rei. I t) 'Dala taken from Rei 2 RH, Relalive humidity WPI. Whey protein i,,olate
prediction of the water vapor permeability required of an edible barrier to retard water vapor loss from a food For example, if the water vapor permeability through a food is 4.8× It) ' kg.m ~.h ~.mmHg ~, then a 1 mil thick edible barrier would need a permeability of less than 0.28g'mil'm2'd ~.mmHg-~ to effectively extend the shelf life of the particular food 22. This is in the permeability range of most of the lipid barriers shown in Table 1.
Future research opportunities Basic research
The inajority of the research into edible barriers has explored the barrier component within the edible barrier. Research into the n~odilication, or use. of alternafives 1o cellulosics and proteins currently used as the support matrices, and also into the selection or development of plasticizer is needed, Specifically. research into the influence of the support matrix on the morphological character of lipid within the edible barrier is a critical next step in understanding edible barrier ingredient functionality. Little is known aboul the oxygen permeability, organoleptic properties, tensile strength, sealability, glass transition temperature ar,d grease resistante of developed edible barriers. Predictive methods Attempts to predict water vapor permeability through a barrier based on the water vapor permeabilities through the distinct phases li,e. the support matrix (the continuous phase) and the barrier contpDneots (the dispersed phasel] have been moderately successhd in specilic applications 12. Extensive research into tile
80
Applications of edible barrier technology W i t h the p l e t h o r a o f e d i b l e
5.4 6.73
barriers.
5.9
some of which are shown in Tables 1 and 2, the creative application of these barriers within new processed foods is I.q imminent. While oxygen permeabililies I.O have not been reported extensively in 0.3 the literature, edible oxygen barriers do have particular promise for the packaging of oxygen-sensitive foods. The major hurdle to the application of edible barrier technology in the processed food industry is the lack of industrial-scale processes to produce edible barriers. This is a necessary step towards the increased use of edible barriers and realization of the important role they will play in future food product and package developmere. Potential industrial applications of edible barriers include: • retaining the flavor and maintaining water activity differences between foods within heterogeneous food systems (such as enrobing differently spiced crackers within a cracker package, components within a frozen pizza and the crust of frozen pies): • allowing controlled flavor release by enrobing sensitive flavors with a temperature-sensitive barrier that will release the flavor upon healing: • extending the shell" life of controlled/modifiedatmosphere packaging (CAP/MAP) foods by enrDbing fruits and vegetables with different respiration rates with different edible barriers; • reducing the rate of degradation reactions in complex tk~od systems by selectively enrobing lipids, proteins and water to decrease the rates of lipid oxidation, nonenzymatic browning and microbial growth.
References r 2 3 4 S 6 7 8 9
Kester. I. and Feunema O. ,I')8[,~ Foud Technul 4OEI2L 47-q5 Gennadio~, A. and Weller, C. ql 9q(I) Food Technol 44llt}~, 6]-67 KampL,r, s. alld reflnenld, C). (19~411. FoodSci. 4'L ]478 ]4~I Greener, I and Fel~nenla. (). i1~tBnl I. Foot/Sc.t. 54l{~L I ~n$. I $')q Gr,'ener. I. and Fennunla. O. 098911. FoudStL 54{hl. 1400-1406 Kueb.dl, (-. ancl Lahuza, I, H99JI Lehensnl. Vegx Tecllnol. 25, 404-411 Ghaoulh, A.. Arul. I and Ptmnanlpalam. R. qIq~H, 1. Food Process. Pre~er~. 13. ~9 {b8 McHugh, T. and Krochta, ]. I] ,tn4~ Food Tel hnol. 4tML 97.. I tH Nara~,an.R. ~1nwh ilt (~qluhM¢~ [hh/,ith;n: Rc~t'art h ,it]f/R('l~.lr(g hi
Trends in F,lod Science & Technology March 1994 lVo 51
I0 II 12 13 14
¢ellulo+itsdn,l~,+ki H ,rod Phdlips G (). eek+ lip t 1(I l IB EI.t,~lt,t G,irv+D. t l()8')l in ( ellulos+t, l+'tdte,lt,+m Rt,,t,,m h ,rod Ruu,tr+I,m (t,lhdcJ~ltsqln,lg,lkL tt and Phllhpy.G (). ca+, up 12 + i {I, [I,~,~,t,r Kester+I. ,ina ft'tlnu'nl,t f)~](J~l)k/F(H,¢IS(I S4r(,l I ~++~ I ~(t Koelsth, C.M, Sdllru.\ ,lfld [ahuzd, Tp I. Ftx)rl Dig
e/ll, pp. 47 lift) (?R( pit'.. 15 M d l z , L G , , m d Q i , ll.+lgtHIl~gr~( I,,,ICht,m Y+ 21+,; 21+,h 16 Singh,RP ~1t)82~ftu)d h,( hn,,l 2.87 t~l 17 Hung.Y.C.. K,els(b ('.'d. ,rod Lahu,,a. TP 'Ill'Jill I f , ' , d P,,t,',,
Prt+,t,r~ -~h 4-~ +,2
18 c r,mk I l',-q xl,t+h+.,rlatJt,urD,tttl,,,,:,2ndcdn, tJxlunl [ nl~t,l,ll~ ]+tt.... tq (,+,
+
Review The increasing economic pressures currenllv being placed upon animal producers demand more-efficient utilization o[ low-grade feedstufls. In addition, consumer awareness and new
Enzymes in the animal-feed industry
legislation require thai any increase in animal production cannot be achieved via growth-promoting drugs ur c,ther chemical suhslances. One increasingly popular approach to this problem is to supplement animal diets with hydrolytic enzymes in an attempt to aid the digestion and absorption of
Gerard A. Walsh, RonanF. Power and Denis R. Headon
poorly available nutrients, or to remove anlinutritional factors from the diel. Concerns raised by this practice include the ability of such enzymes to survive processing temperatures and even the animals' digestive tract,
E n z y m e s are routinely e m p l o y e d in a myriad o f industrial applications. Traditional applic;tlions include their use in f.)od processing as well as in the brewing, baking and leather-bating industries. Seleclcd protecdylic activities are also incorporated into detergenl preparations. Such applications are described extensively in the literature and have gained widespread scienlilic and industrial acceptance. An increased understunding o f e n z y me s and their properties m~w s u g g e s t s several travel applications for these biological catalysts. One such application which, to date, has gained only limited atlenlion from the general scientific c o m m u n i t y , is the use o f selective e n z y m e activities in the aoimul-feed industry. Virluaily all e n z y m e s e m p l o y e d in the animal-feed industry are hydrolases, which are used directly as l~'ed additives to achieve any+ ,,~r ;111 o f lhe follov.'iug ol~jectivesl 5: This ,lrticlt. ~,1~ llrst ptlhli~hud II1 lft.nd, it, I¢~tltt,~hno/og~ \ o l I I pp 424-4 fl). G.A. Walsh and R.L Power arc at ,,'qlk.fh [uropean BiO>tlt'l](t2~( i.,nlrt, rht, Naliol,dl I.,'nixetsit~ Ol Irtqant]. G,tk~a~. Irt,i,uTd D.R, Headon 1, at Ihc Depatlnwnl of Biothenli,tn. The N,ilion+llthli,.t'rslt'~ol Ireland. C+,dX%,l'*fft'Llnd
Trends in Food Sdence & Technolug~ MarCh Iqq4 l\'nl+ "~t
• supplementation o f the e n d o g e n o u s digestive aclivilies o f the host animal, including proteases and amylases;
• remowd o f anlinutritional l:actors such as 13-glucans :.rod phylic acid from problematic feedstuft~'~: • Io render certuin nutrients more-readily available for absorption and Io enhance the energy value o f cheaper teed ingredients. A n t i n u l r i t i 0 n a l factors in f e e d The bulk o f animal-feed constiluents are vegelable in nature. As a consequence, many feedstuffs contain compunenls that may be considered antinutritional. Such antinutritional l';ictors (ANFs) interfere with the digestibilily, absorption or utilization o f nutrients and. hence, adver~,ely affect animal performance. Most A N F s are produced by plant:-, as natural protection against attack by microbes, insecls and birds ~s. Several sludies have shov,n lhat lhe protective effects o f ANFr, are relaled directly to their ability to perturb metabolic processes ill m i c r o o r g a n i s m s :uld i n s e d s " s. Because o f similarilies in d ig e s liv e proces,,es. A N F s can also be expected to dislurb the digestive processes o f f'trm animals. In general. A N F s ;Ire anlinutritional because organisms ingesting them lack lhe appropriate e n z y me s in their gastrointestin,d tracls to render them ineffective. A N F s ,.','ark in a variety o l wu_vs, from the c o m p l e x i n g o f important
, ~,,.,4 ~.,,,,,, .,:,.~,.. ::,+,,,:4 ~_'~.,~s~,-,',,
81
Edible water vapor barriers: properties and promise Claire Koelsch Demand for edible barriers to retard water vapor transmission has resulled in a plethora of research in this emerging area. This review addresses the development of edible barriers and what is known about their physicochemical nature, and identifies key areas for new research and industrial applications. The development of edible water vapor barriers has focused upon barriers containing lipid (which are less permeable) or protein (which are more permeable). While the application of edible barriers to processed foods has been limited, the potential of edible barrier technology for both the extension of the shelf life of processed foods and the creation of innovative foods is promising.
Recent interest in edible barriers for food packaging has grown due to the following factors: • environmental legislation: •
expanding distribution channels:
• consumer expectations for a variety of fresh foods: • need for extended shelf liie' foods: • opportunities for new foods with edible barriers. The development of water vapor barriers has focused upon barriers containing primarily cellulose, lipid and protein. In general, barriers containing lipid are less permeable to water vapor than barriers containing proreins L'~. However, protein-containing barriers do have applications as edible barriers for certain products, such as fresh fruits and vegetables, that have high rates of moisture loss. Lipid-based edible b a r d e r s The support matrix A non-lipid support matrix is required to reduce the brittleness/fragility associated with barriers containing lipid and to allow application of the edible barrier onto the food. The most commonly used support matrices are morttied celluloses of hydroxypropylmethyl cellulose .~, ethyl
cellulose"~'~and methyl cellulose (Ref, 6 and Kamper, S., MSc thesis, University of Wisconsin, Madison, WI, USA, 1983), which are dissolved in ethanol-water-plasticizer solutions. Other support matrices include chitosan [poly-13(l--+4)-glucosaminel7 and whey protein isolate (WPI) s in solvent-plasticizer solutions. Selective structural engineering of cellulose has been used to create synthetic cellulose structures'~."~.However. extensive research into the water vapor permeation rates of different cellulose derivatives has not been reported in the literalure. Theoretically, methyl cellulose would result in a more dense (less permeable) support matrix, because of its high solubility in ethanol. Ethanol aids the formation of a uniform solution of cellulose and reduces the time required for barrier preparation and drying. Plasticizers of various types have been incorporated into edible barriers as a processing aid, to facilitate barrier application onto foods and to increase the machinability of the edible barrier. Food-grade polyethylene glycol is the plasticizer most commonly reported in the literature. Plasticizers reduce the brittleness of the film by interfering with the hydrogen bonding between the lipid and the cellulose entities. Thus, high polyethylene glycol concentrations result in barriers with high water vapor permeabilities (Kamper, S., MSc thesis). Details of the preparation of support matrices are found in Refs 3-10. There are essentially two types of lipid barriers, laminates (in which the lipid is a distinct layer within the -'dible barrier) and emulsions (in which the lipid is uniformly dispersed throughout the edible barrier). A support matrix (which comprises 50-100%. v/v of the resultant edible barrier) either provides a means to supporl the lipid (in the case of a laminate barrier) or disperse the lipid tin the case of an emulsion barrier). Lipid-based emulsion and laminate barriers are discussed below.
Preparation of lipid-based barriers Lipid is added to an emulsion barrier when the cellulose (or WPI or chitosan) within the support matrix is dissolved. Once a homogeneous mixture of the lipid and support matrix has been achieved, the barrier is cast upon an impenetrable (glass or metal) level plate. The water and ethanol are evaporated from the mixture at an elevated temperature (60-90°C). Once the barrier has dried/cooled to a moisture content of 2-5%, it is peeled from the plate and stored in an environment with a controlled relative humidity and temperature before testing. The formation of a laminate barrier involves a twostep process. The molten lipid is either painted, sprayed or poured to form a distinct layer on the dry support matrix (Refs 3-5, 11, and Kamper, S., MSc thesis). The laminate is then dried/cooled, the film peeled off the plate and stored in a humidity- and temperaturecontrolled environment before testing.
Structure/function of the lipid within edible lipid-based emulsion and laminate barriers Water vapor permeabilily depends on the type, Claire Koelschis ~.ilh KraftGeneralFoods,801 W,'~ukeganRoad,Glenview, location, volume fraction and polymorphic phase of It 60025,USA. lipid in an edible harrier.
76
, q',,,~ u,~,,,~.,s,,~,,,~,,t~d.'..'4 -.','44,,~4'SO:U.
Trends in Food Science & Technology March 1994 [Vol. 5[
da~a indicate that, and for 5430I thePublished same concentration of lipid plasticizer, emulsion barriers have 5 lower water vapor permeability than laminate barriers (Ref. 6 and Kamper, S., MSc thesis). This is due in part to -4o the high degree of inherent tortuousity (the increased path length for water ~vapor diffusion due to an interlock-' ing network of lipid within the barrier) "? 30~ within emulsion barriers, and in part E to the tendency of laminate barriers :~ to crack on their surface (Koelsch, C., = -If PhD thesis. University of Minnesota, 20 MN. USA. 1992). Water vapor dif~_ fusion through emulsion barriers fol15 lows a more tortuous path than ~through laminate barriers because the lipid and support matrix entities are 10 interlocked within emulsion barriers, whereas the lipid entity within lami5nate barriers interconnects with the support matrix at only one interface, aCapric Myrislit Palmitk Palmitic/ Slearic Stearic/ Arachidic Behenic Laminate barriers do. however, offer stea.'ic arachidic the advantage of being easier to apply Fatty acid within methyl cellulose matrix due to the distinct natures of the support matrix and lipid. For example, Fig. 1 during the lamination of lipid onto a support matrix, the temperatures of the Water vapor pem~ea.bilit', ,1~,a iunctlon of the ~alt,. acid used {at a concentration of ]0%~ two entities can easily be controlled in emulsion barriers with a methyl cellulose .~upport matrix. separately: however, during the application of an emulsion barrier, the temperature of the range of 0.53-1.00 g,mil.m-2.d-I.mmHg I). Epifluoreseent emulsion mixture must be above the lipid melt tem- microscopy coupled with digitized image analysis of perature but below the temperature for ethanol/water films produced using seven different volume fractions ( 9 - 4 6 % ) of stearic acid within a cellulose support volatilization. As shown in Fig. 1, the minimum water vapor per- matrix yielded quantifiable results to describe the mormeability in a cellulose-based emulsion barrier is phology of stearic acid within a cellulose support matrix achieved when stearic acid is used as the lipid entity ''". (Koelsch, C.. PhD thesis). Results suggested that the The low permeability through barriers containing stearic interlocking network of stearic acid chains v,'ithin the acid can be explained by the fact that stearic acid forms support matrix reaches a maximum (i.e. maximum loran interlocking network: the effect of this network is tuousity) at -30% stearic acid. Similar results were seen significaat enough that these barriers retard moisture for palmitic acid (in volume fractions of (I-305;r) within transfer more effectively than those based on, say, a WPI barrier ]~. Permeability as a function of the w~lmyristic acid, which has shorter chain lengths, or ume fraction of patmitic acid or stearic acid within a behenic acid. which has such long chains that they hin- matrix based on methyl cellulose, and of beeswax. der the formation of a tightly interlocking network ~2. In stearyl alcohol or palmitic acid within a matrix based on essence, stearic acid provides the optimum chain length WPI is shown in Fig. 2. This theory of an interlocking without hindering the formation of an interlocking net- network of fatty acid chains within emulsion films is a plausible explanation for why the water vapor perwork. Kamper and Fennema ~ and Koelsch I PhD thesis) have meability of an emulsion barrier is approximately two addressed the impact of lipid concentration on the water limes less than thai of a laminate barrier of the same vapor permeability through an edible barrier. Water compositionL Mathematical models to explain and prevapor permeability decreases with the addition of fatty dict the water vapor permeation as a function of the volacid from 28.98 g-mil-m -'-d t.mmHg * for 9% stearic ume fraction and morphology of the fatty acid within a acid to I.{10g.miFm--'.d-~.mmHg-t for 30% stearic acid methyl cellulose support matrix have been published" Iwhere film thickness is measured in rail: I mil = and are discussed below under the heading "Futme 0.001 in = 2.54x 10 ~ m: pressures are given as mm research opportunities'. [(ester and Fennema u related c~, 13 and [~' polymormercury: 1 m m H g ~ 133 Pal. When the stearic acid concentration is in the range of 3 0 - 4 6 % , the water phic states of the fatty acid (lipid) component to the vapor permeability remains relatively stable (iu the water vapor pelmeability. Results showed thal barrier:~
!
Trends in Food Science & Terhnolog,.' March ]c)c)4 [VoI. 51
77
packaging industry are less soluble in Wilier than these edible barriers. A complete listing of edible barriers containing lipid entities :md the relative humidity and temperature ut which the permeability was measured has been pub!i,.hed~'.
i~:~ 5 0 ~ -
"~ 30
•
Protein-based edible barriers
Protein barriers, while more permeable m water vapor than barriers containing lipid, do have promise in certain food applications. The following discussion con,t.. * siders edible barriers containing either animal-derived (gelatin) or vegetable-derived (zein, gluten. WPI) protein. Unlike the lipid barriers, which contain a support • i ' l l [3 matrix, the protein itself acts as both the support matrix o o t'o and the water vapor barrier component of the film. 20 30 40 50 Although animal proteins have been used historically to Volume fraction 1%) t. . . . . . . . . . . . . . . . . . . . . . : form edible barriers (e.g. sausage casings), the majority of recent research has focused on vegetable proteins. Fig. 2 This shift in favor of vegetable proteins may be due to Water vapor permeability as a function of the volume of palmitic economic factors as well as the broad range of edible acid and stearic acid within a methyl cellulose support nlatrix 7 and barriers that can be derived from the various vegetable beesv,'ax, palmitic acid and stearyl alcohol within a whey protein proteins. Most of the research on the ingredient isolate IWPI) support matrix ~a. (: I, palmilic acid : methyl cellulose structure/function and water vapor permeability of barmatrix; (,~I, stearie acid : methyl cellulose malrix; riers made from vegetable proteins has occurred within (IIL beeswax : WPI matrix; (o), palmitic acid : WPI matrix; the last five years,
~,o
;n: i
and (o), stearyl alcohol : WPI matrix.
containing the e~ polymorphic phase were more permeuble to water vapor than barriers containing the 13' polymorphic phase: this correlates with the higher solubility of water in the a phase. However, the barrier containing the 13 polymorphic lipid did not exhibit the lowest water vapor permeability, although theoretically it would be expected to as the 13 phase has the highest acyl chain packing density and lowest molecular mobility. This discrepancy has not been resolved, largely because the existence ofc~, [3 and [3' states within the barrier has not been confirmed: this is mentioned below, under the heading "Future research opportunities'. Selected lipid barriers representative of the full range of barriers that contain lipids are shown in Table 1. Relative hura.idity (RH) values are included in Table 1 since water vapor permeability through the barrier increases as the relative humidity to which the barrier is exposed increases. For example, as shown in Table I. water vapor permeability through the barrier containing 35% stearic acid measured at a relative humidity gradient of 12-56% is higher than that through the barrier containing less stearic acid (30%), which has been exposed to 0% relative humidity on one side of the barrier and 85% on the other side of the barrier. Generally, front the data reported in the litcrature, barrier hydration tvia the use of higher relative humidity values on each side of the barrier) results in higher water vapor permeability values. The reasoning for this is the same as that for synthetic polymer structures and is a restdt of the increased sorption of the permeant (in this case water) by the barrier at higher relative humidity values. The difference between these edible barriers and synthetic polymers is that synthetic barriers in use in the 78
Collagen
Collagen, a natural polyamide, is the most prevalent edible barrier and is commonly used for sausage casings. Its derivative, gelatin, has been used as a key ingredient in edible barriers t4.u. Mazza and Qi ]~ formed gelatin barriers with the addition of gums and CaCI, to induce crosslinking but the water vapor permeability of these barriers was not published. Glicksman ~4 determined that the water vapor permeability of gelatin barriers is largely a function of the relative humidity gradient due to the hygroscopic nature of gelatin. Water vapor permeabilities were in the range of 26.5-74.6g'mil'm--"d-l.mmHg t (~5-20 times greater than Ihose of barriers containing fatty acids). Zein
Prolamins such as zein are dissolved in 11.9% saline and alcohol solutions or ethyl alcohol since they are insoluble in wa!er (Ref. 2 and Mendoza, M., MSc lhesis, University of Massachussetts, Amherst, MA, USA, 1975: Trezza, T. and Verguno, P., unpublished). As with lipid barriers, when a plasticizer (e.g. glycerol) is used In decrease the brittleness of the barrier and allow its application to a food the water vapor permeability increases. As shown in Table 2, the addition of 3% glycerol increased the water v.apor permeability of a specific zein barrier by 10% (Mendozu. M.. MSc thesis): similarly, the addition of 20% glycerol to another type of zein barrier increased permeability by 3()% (Trezza, T. aud Vergano. P., unpublished). Thus, the degree to which a plasticizer impacts the water vapor permeability depends on the concentration of the plasticizer as well as the structure of the barrier itself. Highly plasticized commercial zein coatings exhibit a water vapor Trends in Food Science & Technology March 1994 IVol. 5]
permeability two 0rder~; of magniIude larger than lh(tse developed in laboratory environments:.
Table 1. Summaryo( the water vapor permeabil[tlesof selected edible barriers c0nlaining lipid in or on a cellulosic support malrix
Gluten
Gonlard et al. L" assessed the structure/function of ingredients within gluten edible barriers manufactured from 5-12.5% gluten, 2 0 - 4 5 % ethanol and 2(IC/e glycerol. The lowest water vapor permeability (25.84 g'mil'm--"d-~.mmHg ~) was achieved when the barrier was made with I(1% gluten and 45% ethanol and at a pH of 4.5. The barrier with the highest water vapor permeability was made as above except at a pH of 2.It. Gennadios and Weller-" developed a barrier composed o f gluten and soy protein (2 : 1). The pH and level of plasticizer were nol reported. The resultant permeability was ten times that reported by Gontard et al. I'' Use of high-glutenin gluten as a support matrix wilhin an edible barrier has particular promise due to its inherent fibrous structure, its high degree of extensibility and the presence of e~-helices (which restrict interactions with other molecules such as water). Research on the water vapor permeability of high-glutenin gluten hats not been reported. WPI McHugh et al. s explored (he impact of WPI concentration (7. 9 and 12% in solution before drying), solution pH (5.0-101 and plasticizer type (glycerol, sorbitol. polyethylene glycol) on the water vapor permeability through barriers composed of WPI, lipid and plasticizer. The results showed that the optimum concentration for development of WPl-based barriers is 10% (concentration in solution before drying), largely due to the fact that strong gelling occurs at concentrations exceeding 10%. Strong gels inhibited barrier formation at pH values greater than 9.0 when heterogeneous barriers were formed. WPI barriers using 50% sorbitol as the plaslicizer had significantly lower water vapor permeabilities than those containing either 50% glycerol or 50% polyethylene glycol. Water vapor permeability as a function of the percentage glycerol increased linearly with increased glycerol concentration. The lowest water vapor permeability of 14.99 g.mil.m-:.d ~.mmHg * was achieved with 10% WPI at a pH of 7.0 and 37.5% sorbitol ~. Oxygen permeability was -11 g.mil.m -~-d-~-mmHg J for these parameters-'". Subsequent work 13 addressed substituting WPUI6% sorbitol for the methyl cellulose support matrix commonly used in lipid emulsion barriers. While the flints are predominantly protein based, lipid entilies have been added to complement the barrier properties. Transmission electron microscopy showed that the lipid tin this case stearyl alcohol) was a discrete entity within the WPI. This differs front the configuration of lipid within the methyl cellulose support matrix, in which the lipid forms an interlocking network within the support matrix (Koelsch, C., PhD thesis). This d i f f e r e n c e in the morphology of the lipid within the two support matrices could explain the difference in the water vapor pet meability of the two types of barriers shown in Fig. 2: Trends in Food Science & Technology March 1994 [Vol. 51
Permeability
Barrier
(g'mil'm-Z'd-~'mnlHg -')
Emulsinn hpid barr~er~ S/eark acidl l~",a ~:,learic at ill qICY',:1' Stearic ,[ci(I/[]dlmltl~ acid el :1, total of /()" ,b Oleic acid ¢~(r',,~ Laminate bpid baml,rs Stearic. acld/palmlht acid it : I. t{}tal of ](1",,~ and bee~,}x' Slearic a(id/llalmlh~ acid II : I. l(}t,d of I0",,~"
0.53
RH (%)
Thickness Imill
72-56
1.9
(t. 17
0-8"3
1.3
(/.24 19.Jl
(/ 85 () 85
1.3 1.5
0.30
(}-if7
22
0.46
0-83
4.4
'D,ila lakt,n Iron1Rt,t h Data taken from Rt'l
' Dab1hlkt,n Ir~nlRel I I RH. Relatl;ehumKhlv
the water vapor permeability through a palm(tic acid/ methyl cellulose matrix was 2.93g.mil-m -'-d-J.mmHg ~. while the permeability through a palm(tic acid/WPl matrix was 4.65g-mil-m -'-d-~.mmHg ~ at a volume fraction of - 2 9 % palm(tic acid. A select listing of the reported water vapor permeability properties of edible barriers containing protein is shnwn in Table 2. The greater water vapor permeability values for the zein-based barriers as compared with the WPIand gluten-based barriers may be a result of lhe increased level of barrier hydration {higher RH values) in the zein barriers. More research is needed to establist'~ firmly how water vapor permeation is affected by the relative humidity.
Selecting and developing an edible water vapor barrier As with synthetic barriers, some edible barriers are applicable in certain situations and inapplicable in others. For example, it would be more appropriate to use an edible barrier with a low water permeability for a food with a high water activity Ca) and long shelf life than a barrier with a high water permeability. Predicting the required water vapor permeability that an effective edible barrier must exhibit saves both time and energy in the development or selection of an appropriate edible barrier. A procedure has been developed to determine ".,,'hat water vapor permeability an edible barrier must have to be the predominant factor in retarding moisture loss from a food COaled with an edible barrier +"~7. This procedure, which has been detailed by Hone et al. ~:. involves lhe empirical determination ol" the rate of water vapor loss ¢i.e. moisture migration) from the food. the application of classical diffusion equations ~-'' and the concepts of Taoukis et aL:" The final re~,ult is a 7q
physicochemical nature of components within an edible barrier is needed to allow predictions of water vapor permeability and to understand further how the properties of an edible barrier can be optimized in a manner similar to that commonly used to develop synthetic polymers.
Table 2, Summary of the water vapor permeabilities of edible protein barriers Permeability (g.mil.m-2.d-'.mmHg -~)
Barrier Zein barriers Zem' Zein/20% glycerol'
Zein13%glvcerolb
226.12
318.89 1462.00
WPI barriers' WPII28% palmitica( id/I6% sorbitol WPI/28%beeswax/I6% sorhitol WPI/28%stearylalcohol/I6% sorbitol WPI/16%sorbilol
RH
Thickness
(%)
(mill
50-100 50-I 00
4,65 4,q4 I 1.21 12.78
0-If)[} 0-100 0-1 O0 O-100
Gluten barriers 10% Gluten/20"/.glvcerol,pFl of 4"
25.84
0-100
10';;, Gluten/20% glycerol pH o/2" Gluten/soy protein isolate"
42.36 248.83
0-I00 l |
~Data taken/rum Tmzza, T. antl 'Vergano. P,. unpublished t'Dala taken from Mendoza, M.. MSc lhesi~ ' Data taken fronl Ref.13 "Data laken from Rei. I t) 'Dala taken from Rei 2 RH, Relalive humidity WPI. Whey protein i,,olate
prediction of the water vapor permeability required of an edible barrier to retard water vapor loss from a food For example, if the water vapor permeability through a food is 4.8× It) ' kg.m ~.h ~.mmHg ~, then a 1 mil thick edible barrier would need a permeability of less than 0.28g'mil'm2'd ~.mmHg-~ to effectively extend the shelf life of the particular food 22. This is in the permeability range of most of the lipid barriers shown in Table 1.
Future research opportunities Basic research
The inajority of the research into edible barriers has explored the barrier component within the edible barrier. Research into the n~odilication, or use. of alternafives 1o cellulosics and proteins currently used as the support matrices, and also into the selection or development of plasticizer is needed, Specifically. research into the influence of the support matrix on the morphological character of lipid within the edible barrier is a critical next step in understanding edible barrier ingredient functionality. Little is known aboul the oxygen permeability, organoleptic properties, tensile strength, sealability, glass transition temperature ar,d grease resistante of developed edible barriers. Predictive methods Attempts to predict water vapor permeability through a barrier based on the water vapor permeabilities through the distinct phases li,e. the support matrix (the continuous phase) and the barrier contpDneots (the dispersed phasel] have been moderately successhd in specilic applications 12. Extensive research into tile
80
Applications of edible barrier technology W i t h the p l e t h o r a o f e d i b l e
5.4 6.73
barriers.
5.9
some of which are shown in Tables 1 and 2, the creative application of these barriers within new processed foods is I.q imminent. While oxygen permeabililies I.O have not been reported extensively in 0.3 the literature, edible oxygen barriers do have particular promise for the packaging of oxygen-sensitive foods. The major hurdle to the application of edible barrier technology in the processed food industry is the lack of industrial-scale processes to produce edible barriers. This is a necessary step towards the increased use of edible barriers and realization of the important role they will play in future food product and package developmere. Potential industrial applications of edible barriers include: • retaining the flavor and maintaining water activity differences between foods within heterogeneous food systems (such as enrobing differently spiced crackers within a cracker package, components within a frozen pizza and the crust of frozen pies): • allowing controlled flavor release by enrobing sensitive flavors with a temperature-sensitive barrier that will release the flavor upon healing: • extending the shell" life of controlled/modifiedatmosphere packaging (CAP/MAP) foods by enrDbing fruits and vegetables with different respiration rates with different edible barriers; • reducing the rate of degradation reactions in complex tk~od systems by selectively enrobing lipids, proteins and water to decrease the rates of lipid oxidation, nonenzymatic browning and microbial growth.
References r 2 3 4 S 6 7 8 9
Kester. I. and Feunema O. ,I')8[,~ Foud Technul 4OEI2L 47-q5 Gennadio~, A. and Weller, C. ql 9q(I) Food Technol 44llt}~, 6]-67 KampL,r, s. alld reflnenld, C). (19~411. FoodSci. 4'L ]478 ]4~I Greener, I and Fel~nenla. (). i1~tBnl I. Foot/Sc.t. 54l{~L I ~n$. I $')q Gr,'ener. I. and Fennunla. O. 098911. FoudStL 54{hl. 1400-1406 Kueb.dl, (-. ancl Lahuza, I, H99JI Lehensnl. Vegx Tecllnol. 25, 404-411 Ghaoulh, A.. Arul. I and Ptmnanlpalam. R. qIq~H, 1. Food Process. Pre~er~. 13. ~9 {b8 McHugh, T. and Krochta, ]. I] ,tn4~ Food Tel hnol. 4tML 97.. I tH Nara~,an.R. ~1nwh ilt (~qluhM¢~ [hh/,ith;n: Rc~t'art h ,it]f/R('l~.lr(g hi
Trends in F,lod Science & Technology March 1994 lVo 51
I0 II 12 13 14
¢ellulo+itsdn,l~,+ki H ,rod Phdlips G (). eek+ lip t 1(I l IB EI.t,~lt,t G,irv+D. t l()8')l in ( ellulos+t, l+'tdte,lt,+m Rt,,t,,m h ,rod Ruu,tr+I,m (t,lhdcJ~ltsqln,lg,lkL tt and Phllhpy.G (). ca+, up 12 + i {I, [I,~,~,t,r Kester+I. ,ina ft'tlnu'nl,t f)~](J~l)k/F(H,¢IS(I S4r(,l I ~++~ I ~(t Koelsth, C.M, Sdllru.\ ,lfld [ahuzd, Tp I. Ftx)rl Dig
e/ll, pp. 47 lift) (?R( pit'.. 15 M d l z , L G , , m d Q i , ll.+lgtHIl~gr~( I,,,ICht,m Y+ 21+,; 21+,h 16 Singh,RP ~1t)82~ftu)d h,( hn,,l 2.87 t~l 17 Hung.Y.C.. K,els(b ('.'d. ,rod Lahu,,a. TP 'Ill'Jill I f , ' , d P,,t,',,
Prt+,t,r~ -~h 4-~ +,2
18 c r,mk I l',-q xl,t+h+.,rlatJt,urD,tttl,,,,:,2ndcdn, tJxlunl [ nl~t,l,ll~ ]+tt.... tq (,
+
Review The increasing economic pressures currenllv being placed upon animal producers demand more-efficient utilization o[ low-grade feedstufls. In addition, consumer awareness and new
Enzymes in the animal-feed industry
legislation require thai any increase in animal production cannot be achieved via growth-promoting drugs ur c,ther chemical suhslances. One increasingly popular approach to this problem is to supplement animal diets with hydrolytic enzymes in an attempt to aid the digestion and absorption of
Gerard A. Walsh, RonanF. Power and Denis R. Headon
poorly available nutrients, or to remove anlinutritional factors from the diel. Concerns raised by this practice include the ability of such enzymes to survive processing temperatures and even the animals' digestive tract,
E n z y m e s are routinely e m p l o y e d in a myriad o f industrial applications. Traditional applic;tlions include their use in f.)od processing as well as in the brewing, baking and leather-bating industries. Seleclcd protecdylic activities are also incorporated into detergenl preparations. Such applications are described extensively in the literature and have gained widespread scienlilic and industrial acceptance. An increased understunding o f e n z y me s and their properties m~w s u g g e s t s several travel applications for these biological catalysts. One such application which, to date, has gained only limited atlenlion from the general scientific c o m m u n i t y , is the use o f selective e n z y m e activities in the aoimul-feed industry. Virluaily all e n z y m e s e m p l o y e d in the animal-feed industry are hydrolases, which are used directly as l~'ed additives to achieve any+ ,,~r ;111 o f lhe follov.'iug ol~jectivesl 5: This ,lrticlt. ~,1~ llrst ptlhli~hud II1 lft.nd, it, I¢~tltt,~hno/og~ \ o l I I pp 424-4 fl). G.A. Walsh and R.L Power arc at ,,'qlk.fh [uropean BiO>tlt'l](t2~( i.,nlrt, rht, Naliol,dl I.,'nixetsit~ Ol Irtqant]. G,tk~a~. Irt,i,uTd D.R, Headon 1, at Ihc Depatlnwnl of Biothenli,tn. The N,ilion+llthli,.t'rslt'~ol Ireland. C+,dX%,l'*fft'Llnd
Trends in Food Sdence & Technolug~ MarCh Iqq4 l\'nl+ "~t
• supplementation o f the e n d o g e n o u s digestive aclivilies o f the host animal, including proteases and amylases;
• remowd o f anlinutritional l:actors such as 13-glucans :.rod phylic acid from problematic feedstuft~'~: • Io render certuin nutrients more-readily available for absorption and Io enhance the energy value o f cheaper teed ingredients. A n t i n u l r i t i 0 n a l factors in f e e d The bulk o f animal-feed constiluents are vegelable in nature. As a consequence, many feedstuffs contain compunenls that may be considered antinutritional. Such antinutritional l';ictors (ANFs) interfere with the digestibilily, absorption or utilization o f nutrients and. hence, adver~,ely affect animal performance. Most A N F s are produced by plant:-, as natural protection against attack by microbes, insecls and birds ~s. Several sludies have shov,n lhat lhe protective effects o f ANFr, are relaled directly to their ability to perturb metabolic processes ill m i c r o o r g a n i s m s :uld i n s e d s " s. Because o f similarilies in d ig e s liv e proces,,es. A N F s can also be expected to dislurb the digestive processes o f f'trm animals. In general. A N F s ;Ire anlinutritional because organisms ingesting them lack lhe appropriate e n z y me s in their gastrointestin,d tracls to render them ineffective. A N F s ,.','ark in a variety o l wu_vs, from the c o m p l e x i n g o f important
, ~,,.,4 ~.,,,,,, .,:,.~,.. ::,+,,,:4 ~_'~.,~s~,-,',,
81