updateMicrobialherbs & spices

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Throughout history, herbs and spiceshave been an integral part of foodsand cooking. It is believed that trading

in spices was going on in Asia and the FarEast over 4000 years ago, and it is acceptedthat the Ancient Egyptians used herbs inembalming procedures because of theirpreservation characteristics. Over 3000years ago medicines derived from herbswere thought to be used in the Far East. Inthe middle ages, spices became more widelyavailable in Europe as expensive importedcommodities only really available to the veryrich. In more modern times, herbs andspices have become more widely availableto all, and with the increase in consumptionof ethnic foods in the western cuisine, mostpeople now eat a considerable variety ofherbs and spices as key ingredients withintheir foods.With increased consumption, our interestin these products grows and they are of par-ticular interest to the microbiologist. Astheir early uses show, many have antimicro-bial properties, being used as preservativesor medicines. Indeed their widespread earlyuse in Asia and the Far East was probablymore to do with their ability to preservefoods than to impart flavour, the latter beinga secondary benefit. We still see a greatinterest in the antimicrobial nature of theseingredients. Each year sees numerousresearch papers published considering theuse of herb and spice oils as ‘natural antimi-crobials’, their abilities to slow and stop thegrowth of a range of micro-organisms havebeen well proven. However, we also haveto deal with some of the negative aspects ofthe microbiology of herbs and spices whenwe use them as food ingredients. They arenatural ingredients growing in a natural envi-

ronment, and themethods andenvironment of

growth,harvest,and dis-tribu-tiongivethem anatural

Dr Roy Betts, Head of Microbiology,Campden BRI, Chipping Campden,Gloucestershire, GL55 6LD, UK.Produced as a service to the food industry by Thermo Fisher Scientific.

microflora which could include micro-organ-isms considered to be human pathogens.We need to be aware of this and takeappropriate action to minimise any risks ofillness arising from their use.

What are herbs and spices?

A wide variety of plant materials are cate-gorised as herbs or spices. Culinary herbstend to be considered as the leafy greenparts of a plant, they differ from other plantsused in foods in that they are used in smallamounts to impart a flavour rather than sub-stance to a food. Spices are differentiatedfrom herbs in they are not produced fromthe green parts of the plant but from otherstructures such as seeds, fruits, roots oreven the bark of a variety of plants.

Micro-organisms of concern

As noted previously, because herbs andspices are produced from natural materialsand many may come from distant parts ofthe world, they will, in their native state, becontaminated with a wide variety of micro-organisms. On occasion, production meth-ods may not be fully hygienically controlled.Drying of spices can be done in an openenvironment in the sun, resulting in har-vested materials becoming further contami-nated as they are open to contact frominsects, reptiles and other animals. It shouldbe considered that untreated ‘native’ herbsand spices could contain high levels of bac-teria and fungi, some of which may be con-sidered pathogenic to humans. Spore formers from the Genera Bacillusand Clostridium, will certainly be able tosurvive any normal drying process, whilstvegetative enteric pathogens such as salmo-nella and Escherichia coli must be consid-ered a risk. Numerous studies haveconsidered the contamination levels ofherbs and spices. Aerobic Plate Counts (at30°C) can be greater that 106 per gram, withcountable levels of B. cereus, C. perfringens,enterobacteriaceae, yeasts and moulds allbeing noted. Psychotrophic organisms canbe found, probably originating from materi-als harvested from more temperate cli-mates, and the presence of mycotoxinsmust be considered in materials with a highmould count.In a report to the UK Advisory Committeeon the Microbiological Safety of Foods in2008, Sagoo et al reported that the salmo-

nella contamination rate of dried herbs andspices could be between 0.6% and 14%,with UK samples being contaminated at arate of 1%. In more recent surveys con-ducted in the USA, the FDA sampled 2,844imported dry spice shipments from 2007through 2009 and found about 7% con-tained salmonella, twice the rate of otherFDA regulated food products. The agencyfound the highest prevalence of salmonellain leaf-based seasonings like basil andoregano.

These issues of contamination mean thatusers of herbs and spices must be particu-larly careful with their risk assessments. Any herbs and spices that are to be usedin/on ready to eat foods must themselvesbe ready to eat and contain no pathogenicmicro-organisms. If they are to be used infoods that are going to be cooked beforeeating, then the presence of sporeformershas to be considered, particularly if thecooked food has a long shelf life. Survivingspores may well become spoilage or safetyrisks within those foods.Of course most herbs and spices are driedand their stability is dependent on their lowwater activity. This will ensure that micro-organisms cannot grow. However any viableorganisms will survive within the material,potentially for long periods of time. Usersmust be aware of this; if such dried materialsare rehydrated, for example by being addedto higher water activity foods, then anymicro-organisms could begin to grow,resulting in increased risks of spoilage or ofthe presence of higher levels of food poison-ing organisms. We must also be aware thatany organisms present in low water activityenvironments will become more resistant toa variety of antimicrobial treatments, includ-ing heat. As an example a reported heatresistance of salmonella contained in amicrobiological broth (with a water activityof about 0.99) at 62°C would mean it wouldtake 24 seconds to reduce the number oforganisms by a factor of 10 (a one log

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reduction). A similar organism contained ina wheat flour at a water activity of 0.5 take100 minutes at 62°C to get a similar onelog reduction, whilst at a water activity of0.4 it would take 876 minutes to get a simi-lar reduction in numbers. Food producershave to be aware that whilst dried herbsand spices are microbiologically very stableproducts, they could contain a hiddenmicrobiological flora, that is also very stable.

Issues and outbreaks

A number of outbreaks of food poisoninghave been attributed to herbs and spices bya number of authors. In a review publishedin 2013, Van Doren et al considered food-borne illness outbreaks occurring between1973 and 2010. They identified 14 illnessoutbreaks attributed to the consumption ofcontaminated spices during this period.These outbreaks were reported by coun-tries including Canada, Denmark, Englandand Wales, France, Germany, NewZealand, Norway, Serbia, and the UnitedStates. Together, they resulted in 1946reported human illnesses, 128 hospitalisa-tions and two deaths. These authorsreported that salmonella was identified asthe causative agent in 71% (10/14) of out-breaks, accounting for 87% of reported ill-nesses. Bacillus spp. were identified as thecausative agent in 29% (4/14) of outbreaks,accounting for 13% of illnesses. Interestingly these authors also estimatedthat 70% of illnesses could be attributed tofoods in which spices had been added to thefood after the final microbial reduction stephad been applied, highlighting the risks asso-ciated with contaminated herbs and spices. Looking particularly at the USA, the USFood and Drug Administration consideredfood poisoning outbreaks occurring in theUSA between 1973 and 2010. They identi-fied 14 outbreaks that involved spices, thesecausing around 2000 individual illnesses. Ten of the outbreaks noted by the FDAinvolved Salmonella enterica subtypes.Pepper spices – black, white, red, andunspecified – were implicated in nine of theoutbreaks. Most of the spices they identifiedwere imported.Food producers must be constantly vigilantto new risks associated with all ingredientsincluding herbs and spices. There appears tobe a recent increase in interest in the pro-duction of flavoured oils. These may containa range of herbs or spices and are oftenseen as a premium product. One potential risk posed by such productsis their potential to allow the growth of C.botulinum. This organism is an anaerobeand will not grow well in the presence ofoxygen. However under anaerobic condi-tions, for example in oils, growth can occurand then there is the potential for theorganism to produce a neurotoxin thatcould have very serious health implications.Any food producer considering production

of herb or spice flavoured oils, particularly iffresh herbs/spices are used, must be awareof the risks of C. botulinum growth in suchproducts and take appropriate action tocontrol and eliminate that risk.

Processing

The widespread acceptance of the potentialfor herbs and spices to be contaminatedwith human pathogens makes the process-ing of such ingredients a very important con-sideration for the industry. Processing canreduce or eliminate the presence ofpathogens if applied correctly; however, itdoes pose some problems. Fresh leafy herbscan be difficult to process. Any form of heatprocessing could destroy the properties offresh herbs and washing may be the onlyform of ‘process’ that can be applied.Washing is routinely used on ready-to-eatsalad produce. It can, if correctly applied,reduce the level of micro-organisms on suchmaterials, it cannot be guaranteed toremove pathogens, however it will helpimprove the microbiological quality of theseproducts. Drying also cannot be guaranteedto remove pathogens, it can prevent growthbut may allow survival. A number of yearsago dried spices were treated with gaseousethylene oxide. This had a surface sterilisingeffect and was effective at reducing micro-organisms on spices. The ability to use ethyl-ene oxide varies country by country, insome locations it is illegal to utilise this gasand local legislation must be consultedbefore considering its use.Heat processing can be difficult to use dueto the increased heat resistance of micro-organisms in dried materials. Even very hightemperature may not be sufficient to guar-antee elimination of organisms. However, inrecent years a number of equipment manu-facturers have developed systems that allowa mild steam treatment of dried materials,before removing the moisture and redryingthe material. This procedure applies heat toa partially ‘wet’ product. This is much moreeffective at killing organisms and has thepotential of considerably reducing microbialnumbers whilst maintaining the sensoryproperties and providing a dry end product.Large scale commercial producers of driedherbs and spices have done considerableresearch in order to develop productionmethods that ensure that their productshave low levels of contamination, and a neg-ligible risk of pathogen contamination.

Test methods

Microbiological criteria are often applied toherbs and spices. Laboratories testing theseingredients face two major issues. The firstis that they are often in powder or dehy-drated form. This means that a particularweight of sample contains a large amount ofmaterial. This can result in a standard one in10 dilution becoming more of a slurry than a

suspension, and thus difficult for laboratoriesto handle effectively. The second issue istheir antimicrobial nature. As noted earlier,many herbs and spices have a potent antimi-crobial effect and their addition to microbio-logical diluents and enrichment broths mayinhibit the growth of, or kill contaminatingorganisms. This could mean that the pres-ence of key contaminating organisms couldbe underestimated if routine or standardmethods are used. There are various sug-gested ways to minimise such problems.The first thing that a laboratory should dobefore testing herbs and spices is to checkthat the methods they intend to use cancope with these materials. Various standardor reference methods can suggest specificpreparation methods to be used when test-ing herbs and spices. These methods areusually intended to attempt to overcomethe issue of their antimicrobial nature, givinga greater chance of accurately assessingnumbers of critical micro-organisms in suchsamples. Users of rapid methods shouldconsult the kit insert or contact the methodmanufacturer to check that the samplepreparation procedure is optimised for herband spice samples and, if validated, that themethod validation actually includes herbsand spices in the scope of the validation.

Conclusions

Herbs and spices have been used in foodsfor millennia. Originally, it is possible thatone of the main reasons was to use theirantimicrobial properties to help preservethe food, but it is certain that their charac-teristics soon made their inclusion in foodsfor reasons of flavour. They were one of the first ‘food’ com-modities used in worldwide trade, due totheir high value. Today they are still majorfood ingredients used the world over. Due to their methods of production, theyare often contaminated with a wide varietyof micro-organisms, some may be consid-ered human pathogens. The users of herbsand spices tend to understand this potentialproblem and do their risk assessments withgreat care. We can see that herbs and spices havebeen recorded as the cause of a number ofoutbreaks of food poisoning however, con-sidering the amount of foods that containherbs and spices, the number of recordedoutbreaks is small. It could be argued that this is because ofthe very small amounts of these ingredientsthat are used in a food, but it is also verylikely that most food manufacturers under-stand the risks and source their herbs andspices from known high quality sources,who ensure that suitable interventions areused to render herbs and spices safe. nb food.testing.admin@thermofisher.com

References are available from the author on request

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