updateMicrobialyeasts& moulds

10 International Food Hygiene — Volume 24 Number 4

It is quite probable that fungi cause morelosses within the world food supply chainthan any other group of micro-organisms.

As such they should be of major interest togrowers, processers, suppliers and retailersof foods.Fungi tend to be divided into two groups,

the yeasts and the moulds. Yeasts are char-acterised by their single celled nature. Theyreproduce by a process known as budding,where ‘daughter’ cells bud off the parent cellforming new single cells. Yeasts tend togrow within food and drink matrices inplanktonic form and they tend to fermentsugars growing well under anaerobic condi-tions. Moulds on the other hand tend togrow on the surface of objects in the formof a visible ‘mycelium’ made up of manycells. Moulds exist in this multicellular for-mat, usually known as a hyphal mat, andtend to grow on the surface of food anddrinks and not within them as yeasts do.They are aerobic organisms and cannotgrow well under conditions in which oxygenis limited.Yeasts and moulds form an important

group of organisms of great importance tothe food industry. These groups are very dif-ferent from the bacteria that are commonlyassociated with food, being eukaryoticorganisms similar to cells found in plants andanimals. The yeasts and moulds are widelydispersed, found in a variety of locations andare virtually ubiquitous in any environment.

Yeasts

Yeasts are very important within foodmicrobiology as they can create both posi-tive and negative effects. Where would webe without bread, beer and wine, all created

by the fermentative actionof yeasts? Conversely,the ability of someyeasts to surviveand grow at lowpH, low wateractivity, and inthe presenceof some com-mon chemicalpreservatives,makes them

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.

potent food spoilage organisms responsiblefor large economic losses of some foodproducts.The origin of the positive effects of yeasts

in food production probably came about byan accidental contamination of some rawmaterials with environmental yeasts. Mead,a fermented honey drink, is the oldest alco-holic beverage known to man and isbelieved to have been discovered during theStone Age. A chance occurrence of honeycomb

becoming wet from rain and then airborneyeasts fermenting the mixture is thought tohave led to its discovery. Leavened breadfirst appeared in Egypt about 5,000 yearsago, when flat bread dough became conta-minated with wild yeasts which would haveproduced carbon dioxide, and ‘raised’ thebread. No doubt an ‘accidental’ contamina-tion of various fruit juices would havecaused the production of wines.

Contamination of raw materials with wildenvironmental yeasts is still used to producesome foods such as specialist sourdoughbreads and lambic beers. However, mostfood production that uses yeasts will nowutilise specialist strains obtained from cul-ture collections that are cultured and delib-erately inoculated into their growthsubstrate to create the food required.Today in food production, yeasts are more

usually linked with food spoilage. Yeasts areslow growing organisms when compared tobacteria. If yeasts and bacteria were placedin the same optimum environment and bothcould grow, it is most likely that the fastergrowing bacteria would quickly outgrow andoutcompete the slower growing yeast,becoming the dominant flora. However, ifwe move outside the ‘optimum’ growthconditions of most bacteria, into environ-ments that are acidic, or of low water activ-ity (high in sugar), then the yeasts have theadvantage and would rapidly overtake thegrowth of bacteria. It is in these specialistniches in foods that the spoilage yeastsbecome a problem.

Yeasts are generally associated with thefermentation of sugars such as glucose andsucrose, but they are able to utilise a varietyof other compounds, such as alcohols,organic acids, hydrocarbons and aromaticcompounds. Some yeasts are also capableof utilising certain acid-based preservativessuch as benzoic acid, propionic acid and sor-bic acid, and this can make them a majorissue in foods and drinks that rely on thesepreservatives for stability.Other general environmental factors that

influence growth are temperature and theconcentration of oxygen. The temperaturerange for yeast growth is about 0-47°C,with yeasts from Antarctic soils, for exam-ple, having a maximum growth temperatureof 17°C, whilst some from tropical environ-ments will grow at greater than 40°C.Some yeast species are strict aerobes

whilst others also have a fermentativemetabolism. In the case of a contaminatedfruit juice for example, fermentative yeastswill cause alcoholic fermentation in the bulkof the product, whilst aerobic yeasts willproduce a film or pellicle on the surface ofthe liquid. In most food spoilage it is theanaerobic fermentative yeasts that cause themajor issue. In sealed food containers, anyoxygen will be rapidly consumed creating ananaerobic environment, and it is here thatthe spoilage organisms will grow, the mostcharacteristic spoilage event being gas pro-duction. Flexible containers will become dis-tended, whilst rigid containers appearunaffected until opened when rapid pres-sure release can result in a forcible ejectionof the contents.The osmophilic or xerotolerant yeasts

specialise in growing in environments of highosmotic pressure, due to the presence ofsalt or sugar. Many of the yeasts isolatedfrom high salt environments will grow read-ily under salt-free conditions, but this is notthe case for those organisms isolated fromhigh sugar environments, which grow poorlyor not at all on standard growth agars.Syrups, jams, conserves, fondants are all sus-ceptible to spoilage by xerotolerant yeasts.

Moulds

Like the yeasts, moulds can also produceboth positive and negative effects in foods.Their negative effects are well known –mould contamination of products containinghigh sugar or of low pH is obvious with theorganisms tending to grow as colonies onthe surface of such products. Their positive

effects are also well known. Moulds will beseen on the surface of mould ripenedcheeses such as brie and camembert, andwithin blue veined cheeses like Stilton andDanish Blue. Some species are a key part of some fer-

mented food products from Japan, whilstthe meat replacement known as Quorn, isproduced from a Fusarium mould that is cul-tured and treated to form a ‘meat-like’ tex-ture. On the negative side, mould like yeastscan affect low pH and low water activityfoods. Indeed this group can grow at verylow water activities (Aw) causing spoilageproblems in products with Aw values below0.75.Moulds are slow growing organisms and

will be rapidly out competed by bacteria andeven yeasts in normal conditions. Theycome into their own when the pH and/orAw is reduced and other groups are unableto grow. Then the moulds can take over,forming hyphal mats or colonies on the sur-face of food products. It is important torealise that moulds are aerobic organisms,they need oxygen to grow, so they will usu-ally be confined to the surfaces of foods,forming easily visible, often colouredcolonies.

Airborne yeasts

Yeasts which are present in the air must becapable of surviving very harsh conditions.Airborne yeasts can be present in the formof ascospores and although these are moreresistant than vegetative yeast cells, themajority of yeast particles present in the airwill not be viable. Soil, dust, drains, equip-ment surfaces, raw materials and ventilationducts can all release yeasts into the air.Viable counts from settle plates reflect a

dynamic situation of yeast particles becom-ing airborne and then settling again. The sizeof the yeast particle determines how far itwill travel before settling, with most air-borne particles being between 2-20µm insize. As a source of product contamination,airborne yeasts are of most concern forsuch operations as aseptic filling plants and,in these cases, control of the air quality bymeans of air filtration will be necessary.

Processing equipment

A lack of attention to the hygienic design offactory plant and cleaning and sanitationprocedures can lead to yeast and mouldcontamination of products. This is particu-larly problematic in plants producing highsugar/low water activity/low pH products.Factories producing fruit products, bakedgoods, confectionary, fermented dairy prod-ucts etc can be at real risk from yeast andmould contamination.In such plants, care must be taken to

ensure that equipment such as proportion-ing pumps, hose connections and valves,storage chillers, and other product contact

surfaces are adequately sanitised. Factoriesshould be designed in such a way as toreduce the risk of cross contamination ofthe finished product by the raw product. Inmany cases it is assumed that low pH prod-ucts are quite stable due to an inability ofbacteria and many yeasts to grow in suchconditions. However it only takes very low levels of

specialist spoilage yeasts and moulds to cre-ate major spoilage problems in these prod-ucts. Such organisms are ideally suited tothe environmental niches in these produc-tion environments, hence the need for goodhygiene in such areas.

Packaging materials

It is of considerable importance to considerthe hygienic status not only of the productand production equipment, but also of anycontainers and packing materials, as thesetoo can be a source of spoilage yeast andmoulds. Any purchased containers, for example

jars and bottles and container closures(lids/caps), have to be assessed for theirlevel of contamination and any need fordecontamination before use. The storage ofsuch containers/closures before use is alsoimportant. On a number of occasions thestorage of opened pallets of containers inopen/semi-open environments drasticallyincreases the potential for contamination.Cardboard may be highly contaminated

with environmental yeast and mould sporesand even foils and plastics may show lowlevels of contamination. Yeast numbers mayincrease substantially during storage,unrolling and moulding due to static electric-ity attracting dust from the environment.

Resistance and illness

Moulds and some forms of yeast are able toproduce spores, and these can be heatresistant. This makes the determination ofany heat process used on food productsvery important. It is not unknown for heat-processed, low pH/low Aw products to bespoiled by moulds whose spores have sur-vived heat processes able to destroy vegeta-tive bacteria. Generally, yeast and mouldspores are not as resistant as bacterialspores, but are much more resistant thanvegetative bacterial or yeast cells.

Companies wishing to establish heatprocesses able to destroy such organismsare advised to contact specialist advisorswho can help to establish suitable heatingprocesses.Whilst yeasts are not usually acknowl-

edged to cause foodborne illnesses, somemoulds can produce a range of mycotoxinsthat can have chronic effects. Unacceptablelevels of many mycotoxins are detailed inlegislation, and this should be consulted ifmoulds are considered a potential problemin particular foods types.Yeasts and moulds can be major problems

within certain sectors of the food industry.Any food or ingredient that has a reducedpH, a low water activity, or has been chemi-cally preserved may be open to attack byyeasts or moulds. They are able to grow in pH and Aw con-

ditions that will prevent all bacterial growth,they can be resistant to many chemicalpreservatives, and some fungal spores canexhibit heat resistance. Some yeasts haveeven been seen growing as small visiblecolonies on the surface of chilled proteina-ceous products such as chilled sausages orchicken portions giving rise to a ‘white spot’spoilage.

Testing methods

It is of great importance to ensure rawmaterial, the production environment andthe final product are subject to testing forpotential spoilage yeasts and moulds. This,however, is not as easy as it sounds. Whilst ‘normal’ yeasts and moulds may be

easily grown on media such as Rose BengalChloramphenicol Agar (RBCA) or MaltExtract Agar (MEA), specialist spoilageorganisms present in low water activity orlow pH environments may not. It is usual forlow water activity foods to be tested usingmedia such as DG18, and even the initialdilution of the foods may need to be doneusing a specialist diluent, containing levels ofsugar. This will prevent the yeasts and moulds

being injured by osmotic stress. Injured cellsmay not grow, will not be detected and thusa possible problem will remain hidden, untilproduct begins to spoil! It is recommendedthat when setting up testing regimes forsuch organisms, expert help is consulted onmethods required.Identification of yeasts and moulds is a

specialist area, requiring specialist knowl-edge. The development of molecular meth-ods has been moving the basis foridentification away from fermentation basedand morphological assessment, towards theanalysis of the DNA within the organism.Again if identification is required, specialistlaboratory help should be requested. nb food.testing.admin@thermofisher.com

References are available from the author on request

International Food Hygiene — Volume 24 Number 4 11