Review

c )vrbr tta, p;rst thrcr: clcc:;~ltrs, tlrc*+xh h;rs bc!cn tremendous

grnwth in new food t)l”cr,ssinl:/t’;“-k,ll:inl: tcchnolo~ies such

,6 dseptir prowssing, nricr<!wa\ e (~nckaging, con!rollcd/mo&.

lied atmosphere packagittg and sous-vide (vacuu.n cooking]

technology in industrialized cou&ies worldwide. This has

resulted in a variety of new forms of convenience :ood txc-

ucts with extended supermarket shell lives. Part I oi this

review discusses the processing/cooking considerations, ,md

advantages and disadvantages ,Issor:iated with acc:ptic/ultm-

high ~cmpurafure WITS processi ng/p,lckat:ing and

tnic ~OWWV lrnc.k,ly:il,8/c:or,kinr:.

The growth of nel1. packaging/processing technologies. for both medium- and Innp-tcim food preservation, is due a number of intcrrclatc<l factors. These arc:

The succc:;:: of any packaging technology ;*s a means of extending the shcl? life at’ food is dependent on the permeability characteristics of the packaging materials enclosing a product. Developments in polymer chem- istly have resulted in the prodt&on of packaging tjlms, such as polyvinylidene chloride (PVDC) and ethylene vinyl alcohol < EVOH). Both of these films hrve excel- lenr water vapour and gas harrier characteristics and can hc laminated to other polymers to give structures with lhe desired strength, heat sealability and permeability characteristics for extending the shelf life of food pr,)d- ucts. Developments in high-speed continuous end thrrmoforming packaging equipment, compatible with *ii; :t:.:_ililltibility characteristics of such films, have also promoted the growth of new packaging technologies.

Increased urbanization In the past. production of food was a major task for

the majority of the population. During the industrial revolution, rural populations decrease&as people moved into cities to be closer to where they work. Most urban- ized nations are dcpendeni on a food supply chain that extends from the farm, which may be thousands ot miles away, to the urban table. Since, due to its biolcrgi- cal natun’, most food is highly perishable, the fol,d processing/packugir.g industry provides an esacntial link in this long ch~lin by ensuring that consumers have a constant supply of a variety of foods that arc shelf- stable, safe and nutritious.

Market needs and wnsumer demands for convenience Over the pitsI 20-30 years thcrc have been many

changes in the lifestyles and food preferences of

Developments in

food packaging

technology.

Part I:

Processing/cooking

considerations

James P. Smith, Hosahalli S. Ramaswamy and Benjamin K. Simpson

consumers. In industrialijzd countries, there has been a fundamental change in Ihe traditional occupations ;1i’ women, one of which wa!. meal preparation. As more than 60% of women in in3ustrializcd countries are in some form of paid emplo,/mcnt. the time previously rvailabie for shopping and food preparation has decreased substantially. Fur hermorc, nros~ households own a microwave oven and a home freezer. The result is that many consumers are prtspared to buy shelf-stable, ‘heat and serve’ WXIX th;l. require a minimum 44 prcpnralion, In fact, the consumer is indirecily asking the food industry to take ova a part of or. in some cases, all of rhr more onerous steps associated with +i~od preparation. In most caszs. packaging facilitates easy handling, opening and portion control, arid may even be. used as a sewing utensil in the case of ‘TV dinners’.

Increasing energy costs Increasing energy costs associated with traditional

methods of food preservation/storage, such as freezing. h;~vc resulted in the growth of less energy-intensive and more economical methods of short- and long-term preservation, such as modified atmosphere packaging (MAP). It has been estimated that MAP is IX-X% less energy-intensive than freezing for rhe extension of the shelf life ol’hakery products’.

As a result of the factors described above. food packaging technology has gone thrclugh a tremendous tran:.fonnatitrn. Packaging now provides increased consumer information, and is used effectively as a marketing tool; it has cvolvcd from its primary (and,

Aseptic/&a-high tempcraturc WHT) packaging tleat sterilization is the healing of’ foods al ;I

sufficiently high temperature tor a sufl‘icicnd~ long time to destroy microbial and enzymatic aclivity. Ah 4 result, sterilized foods have a shelf life of at least 6-12 months. The most common method of sterilizing solid and vi.y,cous food products is ‘in-ccrutainer srvrilizalion (e-g. canning). tlowr\rr, tliSildV~ltltil~CS associated whh irl-c,tltItiIineI .,ICI.ili%ill~‘~‘I I$ iiUWl

pmducl> iIIclu&:: the low raft of heat prnclration lo the ihcrmal ccntre of’ the conIainer, the long processing fimcs lo achivvc the de>i?rd sterility, d:tm:rgc to Ilie nutritional ,1nt1 sensory I:har;Ic:rcrir;tics crl Ihl: prtrdIIc_t, low pri>ciWtivily, illltl trigh energy WSIS. ‘l’ib 0Vt’rL‘olfll’ the limitations of in-ronrainrr slcrililalion. products can bc sterilized at higher temperatures for a shorter tintc period bcfrm filling inm pre-atcrilizcci containus urrder swilc cwtlitions. This forms .the basis of uhra-high temperature (Ul I’!‘) processing and aseptic packaging, in which the Cod and packaging nmcrial WC sbxilizcd scl>ia;lIcly and assemhletl under sterile corrdilions.

Aseptic processing ww devrloped in the ear!? ! YC!:;, anti has hrrn yny-~.:‘~ _.,.,,;ly usea m Europe and Japan for more than two decades. It has rapidly gained popclarity in North America as a thermal processing technique since the use of hydrogen peroxide for the sterilization OF packaging materials was approved in 1981 (Ref. 2). 1 h!: process has long been successfully nsed to sterilize a wide range of liquid products (e.g. milk, fiuit .jJiccs and concentrates, cream, yoghurt, salad dresbirrg.5. egg:. arid kc cream). Kcccnl dc- vzloprncnts in ‘high-barrier’ plaxlic p;rck;rging man&Is and aseptic ptoccssing and Cillitrg tcchrrnlogics have rcsulred in the expansion ot’ the process to rhc stcrilizalion of acidic, low-acid and viscous foods, and of semi-solid foods that contain discrete particles. Cot&~: r:hec%r. hahy totrds. Iomato pastes. fruits and vegetahlcs, ~CIII~S. and rice dcsscr~s can all be processed ahrpliadly.

In the aseptic process for particulate foods, a f&d purnp continuously passes the Iluid food through a scraped/swept-surface heat-exchanger (SHE) or. in the case of non-vixous liquid products, through a simpler shell-and-tube or plate heat-exchanger, which quickly raises the temperature to stcriiizing levels. The product is then passed Ihrough a hnlding tube, when: the stcnlizatiou process is complclcd, and finally Ihnnigb iI second I&It exchanger, IistIally another SSIIE. 10 cc101 the product. Cooled product is transferred lo a slerilc tank that serves as a feed tank for aseptic f?lling. Pre-sterilized containers, usually laminated uarronh e.g. ‘Tetra-Pak’), WC aseplically filled with the cold !IWduct. and contiIinCrs iIre sr:Iled witbin Ilrr+ i~UCl”il~ ,:h:ltnbrt

1UM

i~plic lilling. ami s;ltotdd pi:rniil lhc uplIl;~:Itio~I and ~II:tiltt~‘IlitI1C~: of‘ hcrurrtic 5c~illiIlg. antI Ill<? IIlililllCII~tIlCI~ of slcriltly r)L’ 111~ proclucl, during storapc and himdling. Rigid, semi-rigid and flcxiblc packaging containers haW2 ail been used for ascplic packaging. The sterilization and scaling techniques are somewhat different in each situation. Basic packaghg materials useii in aseptic packaging include metal sheet and foil, giass. plastic films, and paper. Metal has been ~KLYI from the onset of aycr,:ic packaging becxtsc it is ‘L durable hurrier thrtt c:i!: he sterili-rcd and her- mcZii:alfy i;l?,lM. However. rnc.lirl COtltilinCrs :!rc ill%<1 cxpctk+ivc, hvc poor :Id:Iprirbility 10 ncln.cyliIl~lr~i~.:rl sllapc% :iIrd l”ItInol hc II~*iIl<~.ct itI IlticYO\VilVk’ 0V~vlH. Glass ccrntainci-s olli.=r :klv:ltll;q!r:s similar IO metal ctrntaini~rs. hut sul’lbr <rrrni the ;Idditirrn:It disadvantages ot’ fragilily and density. Plaslic materials offer vcrsa- tility, bul camot hc used alone. since they du FOI posx~ss all of the required properties and, consoqucnily, must be laminated to other materials lo xi tic packages with the dGxx1 requirrrtncrrts. Al present, a popular pack- aging material lor ascplis pxkag,in~, is a p:tpcrhoard- --fuil--plastic taminatc, “l’etra-Pak’. This I:ttninatecl structure consists ol’ as many as six layers 43 materials: polypropylene. ‘.Sur!:rl’, I:uZ, poiyelhylene, paperboard, dnu polyethylene as the innermost layer. Other vari- arions that can be. used for thermal sterilization include laminations of ‘Saran’, ethylene vinyl alcohol (EVOH). polyethylene and polystyrene; or a metallized polye::ter, which consists of vinyl ethyl axtmz, nylon, fiBi and potyelhylene. Alurninium foil is the nIost commonly used barrier material; l~ol4propylene or polyolefin (c.g. polycthylcne) are ihc most common heat-scaling and food-contact surfaces. Foil needs to be protected from rucchattical datnagc; protclcrian is usually provided by p:~pcrhoard. All 01’ the cclmpusirc packages described WI :I’ barriers to rnoibtulc, oxygen, light and microbes, znd lrave the strength and heat sealab;lity needed f;>r a successful aseptic package.

Any packaging material must be sterilized prior 10 filling. Method.4 commonly used for the sterilization of packaging ma~zrials or packages involve the use of steam (saturated or superheated), hot air. hydrogen peroxide. ultraviolet light, irradiation or the Iteat generated during the co-extrusion of certain films. Steam is the most widely used technique for ihe sitcriliz:llion (~1’ uIet;ll and glass containers. Supcrhealcd steam, althI>u& desirable because of its higher lempcralure and smaller effluent condensare. may be less cfi‘cctive against microbial spores than saturated steam. Hot (dry) air has similar disadvantages. Hydrogen peroxide, together with heat or ultraviolet radjat.ion iieaimcnl, is one of the common agents used for the sterilization of paper-hasud packaging marerials. Although the exacr meclra~rism of action of IIydrogan peroxide is not known, its cttc:c’t is gen- WIlly :iltrihrilrd ItI c*ilhcr lhc liberation 01’ oxygc13 hy iI% cd’l~cl 01 heal or the iorrnal’ion of hydroxyl radicals by Ilhraviolet r’adialron. ‘lltcrc are restrictions on lhc conccnlration of hydrogen peroxide that

Trends in Food Science xi Technology November 1990.

can be used .rrid on the :unounI of rcvidual 11~drogcn peroxide that can hc tolcratcd in the producl. The heal used fcrr the rnlthing and co-extrusirm of plastic reams during the fabrication of thcrmo-fotmeJ containers is usually high enough to gtcrilizc the packaging .material. Sterility iy maintained by tht: application of a thin, superficial film, which is aseptically removed prior to fabrication’. The use of y-irradiation is also an effective way of pre-sterilizing packaging materials, although relatively lnnp expcrsurc times ar: required,

Aseptic stcrilizalinn has scvcral advanlapcs ~jvcr in- coulaincr slerilizaGon: processing conditions are indc- pcndenl of container size and, thcrcfore. very large containers can be used; the process is highly aummarcd, resulting in higher productivity; the process is more energy efficient and less expensive; packaging costs art: lower in terms of container type (paperboard lanrinarcs versus tin plate or glass): transport and storilpc ~::ls, and distribution costs of UHT milk, for cxamplc. *Ire much 1~1:s than those of pastcurizcd milk, which requires a refrigerated transport networkJ.

The main disadvantages of UHT processing arc: the maintenance of sterile air and surfaces in filling machines, higher skill levels required by operators snd maintenance staff, processing problems associated with the sterilization of large pieces of solid food, the cost and complexity of the processing plant required IO ster- ilize packaging materials. and oxidative spoilage prob- lems that may arise from failure to remove oxygen from the package heac%pace or from the permeation of oxy- gen through the packaging material+.

An excellent review of aseptic processing and pack- aging techniques is provided by Lopez’; historical per. spectives of aseptic packaging technology and of the use of plastic packaging materials are described in other recent puh’,icatinns’ ?.

Packaging microwavable foods Microwave cooking and processing are becoming an

integral part of our food preparation system. The popu- larny of the microwave oven as a consumer appliance slarted in the 1970s and the market has grown exponen- tially since then. Presently. -70% of IJS and .,251% of Canadian households have a microwave oven. Such statistics have provided a major incentive for food compames to re-direct their product development efforts lo account For this growing market. Dynamic changes are taking p&c in the fond industry as it responds to consumer acceptance of the microwave oven and to consumer demands for LI wider selection of quality microwavable food products at a reasonable co&‘,“. Sales ot microwavable foods arc expected to reach US$350 million hy 1991, with the greatest market growth anticipated for cr~n&s, dinners and pot pies (Pig. I ).

Micmwnve heating is a form of eleclrnnic or radio- frequency heating that is related to inductive hearing and capacitalive healing ‘I1 Three basic relaled processes are involved: induction, capacitancs or dielectric heat-

Trends in Food Science &Technology kovcmber 1990

ing, and microwave heating. Induction involves placing a sample within a primary coil activated, by an altemat- ing current. If the sample has appropriate resistance properties. electric currents are induced in the sample, causing it to heat locally. Capacitance or dielectric heat- ing involves placing a poor :lectricaI conductor berwecn charged plates and alternating the charges on the plates at macrowave frequencies of I-3OOMHz. Heating is caused by molecular fricti;m due to dipole polarization of polar molecules forreu to oscillate to maintain orientation wilh the rapidly alternating electric field. Microwave heating is a form of dielectric heating, but refers to heating that takes place in d non-conductor due to polarization effects in the high frequency regitin, between 300 MHz and 3OOGI-b (corresponding to wzveiengths between I m and 1 mm). Microwave heating takes place in a cavity, rather than between electrodes or plates.

Because the microwave region of the electromagnetic spectrum is used extensively for radar and communi- cations, the allowed frequencies for industrial, scientific and medical use (ISM frequencies) are regulated and fixed by international convention. For North America, the most common frequencies are 91.5 and 2450 MHz, aIthou,rh other frequencies are used elsewhere”.

Unique features of microwave energy are its ability to pnetrate a food product and its efficient conversion to heat. which is cubscquently dissipated by conduction. The penetration mechanism oft&s a means of overcoming the slow surface-inward conduction ,mechanism encountered in conventional infrared and hot-air systems.. The degree of microwave penetratio? is attenuated exponentially a$ a function of depth, and is inversely related to the dielectric constant

A&licd rusaiirch and Jmxess &v&pmrnt is seeking

new ilFplio;nirms Ibr microwave technology. Ilowcvrr. cvcn today, the pace of advanccmcnt is not very &;I- matir. Dynamic changes arc! taking place iIS the ti~otl induWy respond?: to d&mds by consw~~cr~ lirr ;I wider sclcction ol‘ quality microw;rvahlr t’ood prc bducts :II ‘1 ronbon;~blc Cost. 7plc rcsp~risr 1~) the IJ~Illillld lir impwvcd quality is not u;lsily met bccausc ol‘ the uniqur:

heating mechanism ;as.+trciatcd with microwaw energy. Common Inicrowavc-l~nctrahle packaging IrlnteriaIs, such as pupcrboard, fhcilit:itc microwave pcnetmtion, so that food heats and cooks directly in the pack;~go. t Iowcvcr. it common complaint about ~nicrow;rvo cooh- ing is its innhility to prrduce browning and crisping in packaged products. To overcome such a majl:r Iimi- tation nr:d to ensure the market growth of microwavable food products, microwave ‘susceptm films are now being dex&qxt. Xtitctive heat control through the use of susccptors wiH he the salvation of many products that have, until now, been difficult LO formulate for the microwave oven because tlavour dcveloprncnt clcpcnded on the Maillard reaction.

A susccptor is a material that is able to absorb and convcrl the efeclrical componcrit of microwave cncrgy into Infrared or radiant cncray. l’hu lllwx c0l111111~11 susceptors are made of ;Iluti~iniunr-ct,atcd p’_r]ycster films or board stock. The aluminium absorbs miLrowavc energy and xts as B xcondary heat source; this canses the li~otl surface to heat locally, much like in a con- vciitionai crvcn, crisping nnd browning the product. ‘I’hc Lcy t’acror in srrscrptor dcsign is the dcgrce of ;;lu~niniun~ deposition, which is usually measured in

ICrmS of optical density: conventioniit aluminized films

range in opticat density from 0.18 to 0.29. Films with

optical densities that are greater tharr 0.35 will cause arming, while those with optical densities below 0.12 arc no: dense enough to cause wave absorption and sip.. nificant heating. The drnwhncks to current rnicrr,w:~v- susceptors arc:

they only absorb the electrical component of the microwave ,:nergy;

tbry arc’ not able tc, control ‘t,ur~away’ heating of crisp, irrcgnlarly sh:~pcd fimds.

Sdc’ond +neration susccptor.s wi LI be brscd on orhcr met& uuc:h as nirkcl. sobalt, iron add stainless steel.

110

?‘lkYC h:lvi: illC iitlV~lllt;t&!LT ilka idulniiiienl Of convcliirlg tiic m;qznctic ccrmpnricnt of micrc,w;rvcs inlo therm;i~ t’c1crg.Y. hiif ltldy require ii n,orC cxpensivG Xputtcr- coating application process. By using a combination 01’ metals ;mtI hy using grids of varying densities, a susce~- tar can hc customized to match the product under con- sideration. Hence. footJ products can be selectively shielded from, or allowed to absorb or transmit, microwave energy. In the case of a conventional three- part dinner (e.g. meat, French ft-its &d veg.etables), the malt can he lcfi unshielded, the French fries ‘suscep.~ torctl’ to improve crisping: ;n~tl ?hc vegetahlcs hc!ib\fiiy shielded Lo rcllcct ni~isl crl’ ihc nlicrow;lvc cnk7-gy. JIi<: crm~hinatiotl Icxls Lo a llnrclucl with the, dcsirt4 3’:1;Irx- icrisliL:s: blc~wnctl 1;lcnr:h fries, clrc)kclI meal illllj crisp Y~~t?lirbl<‘S,

i%w cusccptnr lechnologics <:urrcntly being devci- opcd will allow higher tcmperatttrcs (4SO-500°C) and monc variable (hut controllable) heat Ituxcs (?O-2X0 Cal in-’ min’ ‘). Improved adaptability to odd shapes will be attai..cd through the use of conl’ormablc or shrink- wr;:p sgsceptor films. ‘I’hc de~e?opmrnt of fl:llic-coated susceptvrx in various configurations iri a polymer mc- dium will provide ‘dual-mode heating and controllable heat flux and tmnsmissivity. ahld will have uniform (isotropic) properties. Such developments will Jx major advances, allowing new microwavatx products to com- pete with comlzationally prepared foods. Susceptor technology may also assist in Inlking present or future industrial microwave processes more viable. Such advances arc terrr,x’red hy 1111: fxt that the high tcmpera- turcs that mdy bc generated by susccptors can affect the film carrier, requiting in nxzlting and the tr;rnsfcr of materials to the food product. These potcntiat problems hnvz led to a search for now plasticizers, and fur more stnhlu pickaging WKI uro!;slinkilig mxteriuls that wiII prcvM tha Icaching of packaging compounds into the food during microwave heating”.

References 1

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