7 TheLa~PanelPreliminao,Reporl:UK~ComemusConference Mmeum, London,UK on P/antBia~.hno/ob'y 2.-4/~vember 1994 (19e;4), p. 3, 5cier,ce 10 Me::~l,~ (Ig94) New ..¢ci.144, 4 Museum,london, UK 11 Tbe Lay PanelPre/~inary Repon: UK ~ Comemm Con[erence 8 ll~l.ayPanelPrelimiraryRelx~:UKNat~lCow*~em~ on tVanCB~tec~ 2-4 ~1994 (1994),p.12, Sd~nce on l~antBiotec~ 2-4 Novend~1994 (1994),p. 8, Sciew'_e Maxseum,Londen, UK Museum,London, UK 12 Z-lo~dlxxe~tCozTm~egeOorton~ege~ui~o~d~Ug 9 ~ Lay PanelPreliminary Report: UK NationalConsensus Conference on PlantBiofechno~ 2-4 November1994 (1994), p. 10, Science (1993), HMSO,London, UK

Review preservation Hurdle technology was developed several years ago as a new concept for the production of safe, stable, nutritious, tasty by hurdle technology* and economical . It advocates the intelligent use of combinations of different preservation factors or techniques ('hurdles') in order to achieve multi-target, mild but reliable preservation effects. Attractive applications have been ident- Lothar Leistnerand Leon G.M. Gorris ified in many food areas. The present article briefly intro- duces the concept of hurdle technology, presents potential applications and gives details on a recently concluded study concerned with this topic and to which scientists from vative factors (hurdles) that any mict~3rganisms l~esent 11 European countries have contributed. should not be able to overcomet~-6. These hmdles may be temperatm~, (a,), pH, redox potential, , and soon. It requites a cena~ amonnt of effort from a micromganism to overcome each hurdle. The 'higher" the hurdle, the greater the effort (i.e. the The spoilage and poisoning of foods by micrnorgenisms larger tbe number of orgenisms ueeded to overeome it). is a problem that is not yet under adequate control, Some hurdles, like , can be high for a despite the range of preservation techniques available large number of different types of (e.g. freezing, , pasteurizing and ). In whereas others, like salt content, have a less strong fact, the current Consumer demand for more natural and effect or the effect is limited in the range of types of fresh-like foods, which urges food m~nufactorers to use it effects. only mild preservation techniques (e.g. refrigeration, The fact that a combination of weservafive factors modi~ed-atmosphere packaging and biocoaservation), influences the microbial stability and safety of foods has should make this problem even greater. Thus, for the been known for many centuries. Tbe coueopt is mole or benefit of food manufacturers there is a strong need for less unconsciously used in many traditional foods, es- new or improved mild l~..~-rvation methods that allow pecinlly in the developing countries. It was re-invented for the production c_ fresh-like, but stable and safe some 15 years ago in the meat industry where the con- foods. The concept of hurdle technology is not new but seinus employment of hurdles was found to be highly addresses this need in full '.2. favonrable for the production of shelf-stable sansages2. Hurdle technology (also called combined methods, The concept is now ready to be iatxedueed for u~ with combined processes, combination preservation, combi- a much wider range of food products, including fruits nation techniques or barrier technology) advocates the ve~, t,~-y products, dairy products, fish, deliberate combination of existing and novel preser- and so on. Several novel factors (e.g. vation techniques in order to establish a series of preser- gas packaging, biueooservation, hacterincins,ultraldgh- pressure treatment, edible coatings, etc.) that speciti- *Revised version d an a~icle ~l~ished in 1994 in Vaed/n~m/dde~n- c~lly fucilltate this development have been assessed7. ~chnotog~ 27(21), 15-17 [in Dutch] and AUTECNA4, 17-19 [in Hurdle technology is a crucial concept for the mild Portuguese]. Weservation of foods, as the hm'dles in a stable product concertedly control microbial spoilage and food poison- tear teMner is at the Federal Centre for Meat Research, E.C. Baumannstrasse20, D-95326 Kulmbach, Germany.L,'~ C~M.Gm'fis tag, leaving desiged fennentation processes unaffected. (curespondin8 author) is at the Institm for/~mechnol~.~: Re~,.h Because of their concen~ sometimes synergistic ef- (ATO-DLO), BornseKeeg59, PO Box 17, NL-6700 AA Waseningen,The fect. the individual hurdles may be set at lower intensi- Nethedands(fax: +31-8370-12260; e-mail: L.G~.CORRISt~TOAGRO.NU. ties than would be required if only a single hurdle were

Trends in & Technoto~/FebruaTy 1995 [VoL 6i re.s, ~.~.r sc~e L~ ogZ4-Z2On~Og.SO 41 used as the Weserva~ion tecb~ue. The application of in Fig. la represents a food containing six hat'dles: high ~s co~ ~ ~ven very success~ as en appropd- temperature during processing (F value), low tempera- a~e combin~on of hurdles achieves microb~l stability ture during storage (t value), low water activity (a~), and safety and also stabilizes Ihe sensory, nutritive and acidity (pH) and low redox potential (Ell), as well as eco~n~ ~es of a food~ . preservatives (pres.) in the Im~uct. Some of the micro- organisms present can overcome a number of hurdles r~mp~ of ~hehm~ ef~ but none can jump over all the hurdles used together. A food pgodect is mlcrobiologically stable and safe Thus the food is stable and safe. This example is only a beceme of the presence of a set of hurdles ~ is theorelical case, because all hurdles are depicted as hav- spec~ for t~e pmicu~r product, in terms of the naune ing the same intensity, which is rarely the case in prae- end suengt~ of t~e~r effecL Tog~, t~ese bpadles keep rice. More likely, hurdles are of different intensity, as in spoilage o~ pathogenic n~cmm'g~ms under control the second exemple (see Fig. lb), where a, and preserva- bcca~ Ibee¢ n~moorgmdsms cannot overcome ('jump fives m-e the main hurdles and storage te~, pH over') all of the hm'~es p~.eent. Exmnples of sets of and Eh are minor hurdles. If there are only a few micro- hurdtes are ~lustreted by Figs la-e. The example shown orgadsms l~nt at the smt (see Rg. lc), fewer dilferent

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Fiveexamp~ d the hurdleeffect used in ."lEe individual hurdlesmay be encounteredsimultanemJsly or sequentially,depending on the type of hurdle and the overall processing.Symbols have the ~lowing meaning:F, heating;t, chilling; ~, low wmr activity; pH, aciclif'~a~on;Eh, low redoxpo~ial; pres.,presevatives; V, vitamins;N, nu~ents.See text ~ordetails.

42 Trends in FoodScience & TechnologyFe~umy 1995 Wol. 6] hurdles or hurdles of lower intensity my echieve stable, innovative food preparations based on tropical microbiological stability. On the other hand, if high and subtropical fruits (peach, pineapple, mango, pep~a, numbers of microorganisms are present owing to poor ete.)to.-. hygienic conditions, the usual set of Mudles may not An overview of ~ of hunl~ that have suffice to prevent spoilage or food poisoning (see eitber been studied or already employed, to date, in a Fig. ld). The example shown in Fig. le is a food rich range of food Weducts is given in Table 1. Ina~ in nutrients and vitamins, which may allow for slum- of recently developed food products, in almost infinite term, strong growth of the microorganisms, and as a sbeff life can be obtaln~ An exami~ of tl~s is cammi result their initial number is increased sharply ('booster peas ram'keted in the UK, in which tbe host-stable effect'). In the examples shown in Figs Id and le, ad- hacterio~ nisin is used as m extra har~L~. Nomad. ditional or higher hurdles are needed to assure product heating and pH reduction m~ the only two hurdles stability. employed, bm these do no~ s~o~.ss the l~mw~ of sw- riving egid-tolerant, spree-forming clostrkUa, wh~h are Ex~ of hm~e~ned ~ completely inhibited by nisin. Using hurdle technology, salami-type fermented saus- ages can be produced that are stable at ambient tempera- Homeo~B a~ hallle ~ ture for extended periods of time. The microbial stab- An impomm phenomenon ~ is cn~ with m- ility is achieved by the use of a combination of hmdles gard to hun~ techaologyis the so-called~ of that are hnportant in different stages of the ripening pro- micmo~|.~m~3. Homeostasis is the constanttendency cess, leading to a stable final product. Important hurdles of micrea~anisms to malnteintho stabilityand balance in the early stage of the ripening process of salami (uniformity)of theirintental environment. For instance, are the preservatives salt and nitrite, which inhibit many although the pH values in different foods may be quite of the hacteda present in the meat hatter. However. variable, ~-. ~ms living in them expend con- other bacteria multiply, u~ up oxygen ~_n,:l thereby siderable effort keeping their internal pH values within cause a drop in Ell, which inhibits aerobic organisms very nawow limits'4. In an acid food, for example, they and favours the selection of bac*.eria.The lac- will actively expel ptotom against the pressure of a tic acid bacteria then flourish, causing acidification of passive Woton influx. Another impemm hotucosta~ the product and a decrease in pH. During long ripening mechanism regulates the intenud osmotic pressure of the salami, the various hurdles gradually become (osmohomeostasis). The osmotic strength (which is lower:, nitrite is depleted, the number of lactic acid bac- inversely miated to the a,) of a food is a cngial physicel teria decreases, Eh and pH iucrease. On the other hand, property, which has a great effect on the ability of a,, decreases with time, and thus becomes the main organisms to proliferate. Cells have to maintain a posi- hurdle in long-ripened raw sausage4. ~ aware- tive tm-gar (pressure) by keeping the osmoladty of the ness of the concerted effects of the various hardios used cytoplasm higher than that of the environment' and they in combination has made the production of fermented often achieve this using so-called osmowotective com- sausages less empirical. Similar combinations of hmdles pounds such as praline and betainet3.ts. in other types of fermented foods (e.g. cheese and veg- l~.servafive factors (hurdles) may disturb several or etables) are responsible for the stability and quality of just one of tbe homeosta~ mechanisms of ~n- the products. isms, and as a result the ~ will not nmlti- The hurdle technology approach has also been estab- ply bat instead remain inactive or even diet3. In fact, lished for use with non-fermented foods, for instance in food preservation is ~:hieved by disturbing the homeo- the production of torteilini, an Italian pasta product. In stasis of microorganisms in foods, either tempe~rily or this case, reduced aw and mild heating are the principal pemmnently, and the optimal way to do this is to hurdles employed during processing, in addition to a deliberately disturb several of the homeostatic mech- of ethanol vaponr in the peckage anisms simultaneously ~. This means that any hurdles chilling of the product during storage and retail dis- included in a food should affect the undesired play (Ref. 8 and P. Giavedoni, PhD thesis, University of organisms in several different w~ys, for example by Udine, Italy, 1994). Ethanol was found to be very effec- affecting the cell membrane, DNA, enzymes, pH, tive in inhibiting microbial growth, especially moulds Eh and a. homeostasis systems. This multi-targeted and microcucci. app~h is the essence of butdJe technolo~ 7, Fur- A recent survey of foods traditionally preserved using thermore, this approach is often more effective than hurdle technology, conducted in 10 Latin American single-targeting and enables the use of hurdles of lower countries, identified some 260 different food items intensity, and thereby has less of an effect on woduct derived from fruit, vegetables, fish, dairy products, meat quality. Also, it is possibile that different hurdles in a and ~, which often had a high a. (sometimes as food will not just have an additive effect on stability, high as 0.97) and that were stable at ambient tempera- but might act syne~y t-s-7. Ill wactical ter~ this tuse (25-35°C) for several months9. Based on the could mean that ig is more effective to use a combi- increased knowledge of the principles underlying hurdle nation of differeta preservative factors with low inteusities technology, the Latin American seientists involved in that affect different microbial systems or act synergisti- this study are now applying the concept to design shelf- cally than to use a single preservative factor with a high

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44 T~Is in Food Science & Techno~ February 1995 Nol. 61 intensity. Moreover, in the hurdle t~chnology concept, used impommt hurdl~ am ldgh tmp,m,~, low tom- the objective is to inhibit the growth and pre]iferation of peramm, low ~,, a~idity, low nxlox 1~. ~m- undesired organisms rather than to actually kill them, pctidve mi~ (e.g. lactic acid Imcs~rm) ml thus allowing for the use of hurdles that are not too po~ervmiws (e.& ultd*e, smba*e and sulphitc). However, ex~eme. many other hurdles ase of ~ Ix~ms~ of th,qr po- Another phenomenon of practical relevance is teatial for use in fnod preservation. Receatiy, a gronp of referred to as the autosmilizafion of stable, hurd~ sckati""~s f~n hboratoriss in t I differe= E=repe~ preserved foods. In some ambient-tempetatme-stable co.n~es, sponsored by the ~ U~'s meat products containing clustridin and bacilli (Food-linked Agro-bxlusu~ Rese~h) gogramme, that had survived the heat treatment applied during pro- studied the tntdilioual and novel Madies used in food cessing, it has been observed that some of these spores lxeservation in detail, and compikd a report to doca- are able to germinate to form vege~_tive cells, but that merit their findings7. This report gives an imreductina these appear not to be viable and therefore din off. The to the application of combined lmx:esses in fnod preser- viable spore count thus shows a gradual decrease during vation, presents practical examples of foods pte~rved storage~.~e. The same phe~ has been observed by combined precessas and descdbas v=rinus pt~=~rv- for several bacteria, yeasts and moulds in hurdle- ative factors ('hurdles') that have the potential to be ex- preserved fruit products stored without refrigeration'°. A ploited in food lxeservation. A number of relatiw~ly new general explanation for the phenomenon might be that, hm'dies am discussed in the form of mini-overviews. because, of the elevated temperaua'e, which favours and Among others, the emerging hurdles include ultrahigh probably triggers microbial grow.h, vegetative cells strain every possible repair mechanism to overcen~ the and edible coatings. The report has been written to as- various hurdles presrnt. In doing so, they become meta- sist food woceLqx's, scientists and students who me bolically exhausted; they completely use up their energy interested in the field of huMle le¢Imology to aRply this so~ and die. Thus, because of such autostedlization, gentle p~eservafion system to onr foods, A copy of the hurdle-preserved foods that are microbiologically stable 120-pag~ final report can be obtained free of dwge by become even safer during storage, especially at ambient contacting the cone~g author. temperatures6.

Potential Imnlles Thor= has been b~:reasme mtere= m em design and Up to now, -50 different hurdle~ i" ~ve been ideati~ed of h=me ==hno~y m fnod preserva~m for use in food preservation (Box 1). "lhe most commonly over the past few years. It is expected that this devel- opment will proceed in the he= future, especially as national and international fonds bare now bean estab- lished thin shonld allow for (pre-)compedfive studies in this field. The Commission o~the European Union, for instance, has taken up the aw.a .~f ~ precasses as a Windty theme (area 3.3.2) in the fcxtlgoming AIR2 ['Agriculture and Agro-industry, including fisheries' (also including food tedmology, foresuT, aquacultu~ and rural development)] programme that runs under the Fourth Framework initiative from 1994 to 1998. The combinadon of various hmdles in the pmcasslng and storage of foods has the primary target of obtain- • ing safe foods that are stable with respect to microbial spoilage, using as mild a treatment as possible. How- ever, the concept of hurdle technology may also con- ~ib=e to impmv~g the orga~p~c quality or quaJity of foods as perceived by consumer-, and devel- opments in this respect are also expected in the near future. Rm 1 Leigner,L (1978)in FoodQua//ty and/~'/6on (Downey,W.K., ed.), I~. 553-557,Elsevier 2 Lehtner,L, RDdel,W. and Kdspien,K. (1981} in We*~rAc~: /nfluenceson FoodQual~/(I¢ockland, L.B. and Stewmt.G.F., eds), pp. 855-916,Academic Pless 3 Lei.qner,L (1985)in l~ of Water/n Foods/n~qat/on 1o Qual/tyand 9ab/I/l),(Simatos, D. andMelOn, I.L, eds),pp. 309-329, MarlinesNijhoff Pt~hhe~, gord~ "flleNefit,. "= "~ 4 Leistner,L. (1987)in Wa~erAc~: Theoe/and,~o//r..a//ons to Food

Trendsin FoodScience & TechnologyFebruary 1995 [Vol. 6] 45 (Rock~nd,LB. and 8euch~ L.R., eds),pp. 295-327,Marcel Dd/.ker 11 Lop~z-Malo,A., Patou,E., Welti,I., Co~eP. and Argaiz,A. (1994) $ ~, L (] 992) FoodBes. Int. 25,15|-158 FoodRes. Int. 27, 545-553 6 Leke~n~,L (1995)in New Me~ of FoodPreservat/on 12 Gords,L.G.M. and Bennik,M.H.J. (1994) 7-. Lebensmittdwirlsch. (Go~l, G.W.,ed.), pp. 1-21, Blackie 45(11),65-71 7 Leism~',L. and Gonis, LG.M., eds(! 994) FoodPreserva~on by 13 Gould,G.W. (1988)in ~t/c Mechanismsh~/~t;c,oorganisms Comb/nedProcessm g/JR 15776EN), Commission of the European (Whiflen~, R., Gould, G.W., Banks,I.G. and Board,R.G., eds), pp. Community, Bm.c~ts, Belgium 220--228,Bath University Press, Bath, UK 8 Gi~'edeni, P., R0del,W. and Dmsel,J. (1994)Fleischwir~schaft 74, 14 H~iussinger,D., nd. (ig88) pH Homeostasis:Mechanisms and Contml, 639-642 Academic Press 9 AguileraR,~ic, I.M., Chidfe,J., Tapiade Daza,M.S., Welti Chanes,J. 15 Csonka,L.N. (1989)Microbiol. Rev. 53,121-147 and Par~ Arias, E. (1990)Inven~ar/o deAlimentos de Humedad 16 Leislner,L. and garan-Djurdji~,S. (1970)Fleischwirtscha~ 50, Trad/c/an,~ de/bero,Yn~r/ca,Imtit~o Polit~nico Nacional, 1547-1549 Unid,~l Prd~ior~l In'mdbciplinadade Bio~-nologia,Mexico 17 Begh-Serensen,L(1994)inFoodP~vationbyCombinedProcesg.s 10 Alzanmra,SJvL, Ta~a,M.S.,Argaiz, A.andWelfi, l.(1993)FoodRes. (EUR 15776EN) (Leislner, L. and Go, is, LG.M., eds),pp. 7-24, fat. 26,125--130 Commissionof the EuropeanCommunity, Brussels,Belgium

Review The developmentof the cooking of foods and the mechanisms involved. Many excellent reviews covering this area of chemistry process flavors have already bean publishedt'~. This article will explore those areas in which chemistry can assist the flavorist in creating flavors that are developed by the use of the Charles H. Manleyand SajidAhmedi 'process' of cooking. The flavorhl's approach The flavorist's approach to flavor development is a creative one, bm with a scientific foundation. Let us first The scientific, artistic and regulatory aspects of process review how the flavorist sets about the task of creating flavors are reviewed. The role of the flevorist, as it relates to flavors from scratch. the creation of process flavors for commercial purposes, is The most important step is to first evaluate the target discussed. An overview of the chemisW and analytical tech- flavor in descriptive terms. A commonly used method is niques used as tools for the artistic approach is also given. to describe the flavor's character in terms of its top, middle and base 'notes'. The flavorist draws upon experi- In addition, the development of 'building blocks' for flavor ence gained from past work on different flavor blends. creation is discussed, and comparisons are made between An ingredient that might be suitable for providing a ear- the methods used for the creation of classical flavors versus taln top note may adversely affect the middle and base process flavors. Finally, the regulations relatinp to the safety notes; therefore, a vast amount of expefieuce as well as and labeling of these flavors are considered. a degree of artistry arc needed to effectively combine these notes to produce a well-balanced flavor. Sensory evaluation methods may help define the flavor in terms of its attributes and strength. A small 9~nel of sensory expe~ (5-8 flavorists) is needed to do Man's interest in 'cooked' flavors stetted with the evol- this. Efforts are focused on selecting appropriate de- ution of the human species, and m~m Lq the only repre- scriptive terms (usually 6-8) that relate to the character sen~tive of the ~imal kingdom that has established the and quality of the flavor or taste. ~ctice of conking food before eating it. The use of fire, A more in-depth evaluation may be accomplished by and subsequandy other heat sources, to render a raw using analytical methods such as the extraction of vol- nu~m~d pelamble is one of man's higher intellectual atilas, followed by their separation using gas chroma- achievements. The cre~on of a 'cooked' flavor presents tography end aroma evaluation of the separated compo- the flavofisz with a major challenge. nents (olfactometry). A number of good reviews on Cbendstry, p~ficularly analytical and natural product such methods for evaluating a flavor can be found in a chcnds~'y, allows for the further development of our recent Imbrication 4. By the use of these methods, the basic u~erst~llng of the components that arise during important volatile flavor components can be identified. Once the basic formula of a flavor has been obtained H. ~ and ~ Ahng,~are at TakasagoInternational Corp. using the analytical methods, the flavorist is ready to (USA),Teted~o, Hi, USA (fax:+1-201-288-1692). put artistry to worL The aroma components that have

©1995.EIsevi~ Science Ltd 0924-2244/95/$09.50 Trends in Food Science & Technology February i 995 [Vol. 61