InternationalDairyJournal12(2002)255–262 Interpretationoftheirregularlyfluctuatingmicrobialcounts incommercialdairyproducts MichaPeleg* Chenoweth Laboratory, Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA Received13February2001;accepted8June2001 Abstract Microbialrecordsofcommercialrawmilk(totalcounts)andtwoicecreams(standardplatecounts,coliformsandyeasts)werede- rounded.Theirautocorrelationfunctionshowedthatthecountswereindependentandhadnodiscernibletrendorperiodicity.The firsthalfofeachrecordwasusedtoestimatethefrequenciesofhighcountsinthesecondhalf,assumingthatthecountshavea lognormal,log-Laplaceornon-parametricdistribution.Theestimatedfrequencieswereallonthesameorderofmagnitudeofthose actuallyobservedandinsomecasesincloseagreement.Noneofthethreedistributionswasfoundtobeconsistentlysuperiortothe others, most probably because the available records for the analysis were too short. In general, the lognormal and log-Laplace distributions yielded closer estimates than the non-parametric distribution because they allow for high counts at levels not previously recorded. The results strengthen the view that at least, in principle, the irregular fluctuating pattern of the microbial recordsofdairyproductscanbetranslatedintoasetofprobabilitiesoffuturehighcounts,andthatthesecanserveasariskor qualitymeasure. r 2002ElsevierScienceLtd.Allrightsreserved. Keywords: Predictivemicrobiology;Milk;Icecream;Qualitycontrol;Foodsafety 1. Introduction Dairymicrobiologyandindustrialqualitycontrolare both mature disciplines. Thus, there are established methods and procedures to sample and test milk and dairy products in order to assure their quality and microbial safety (e.g., Robinson, 1990; Varnam & Sutherland, 1994; Hubbard, 1996; Erickson & Hung, 1997;Marth&Steele,1998;Walstra,Geurts,Noomen, Jellema,&vanBoekel,1999). Records of routine microbial counts in commercial plantsusuallyshowanirregularfluctuatingpattern(see below).Thecounts,however,usuallyoscillatewithina rangejudgedacceptable,andaslongasexcessivelyhigh countsarenotencountered,thenatureandoriginofthe fluctuations are of little concern. Occasionally, an unexpectedly high count is recorded. In most cases, it can be explained as a result of an accident, like a temporary failure of the refrigeration system, human error,etc.Butinsomecasesthehighcountsappearto happenspontaneouslyandtheycannotbetracedtoany obvious event. Let us examine the cause of such unpredictable ‘‘explosions’’ or ‘‘outbursts’’. Every in- dividual count in the series represents the size of a microbialpopulation,whichisdeterminedbynumerous factors. Some of these are unknown or undocumented and they can vary randomly. The effect of factors like smalltemperaturevariations,non-uniformchlorination, etc.,canpromoteorinhibitgrowth.Usually,theeffect of the factors that tend to inhibit microbial growth balances those of the growth promoting ones but not exactly. Consequently, the counts fluctuate within a relativelynarrowrangemostofthetime.Butthereisa probabilitythatseveralofmanyofthegrowthpromot- ingfactorswillactinunison,inwhichcasetheresultwill beanunusuallyhighcount.(Thereisalsoaprobability that the inhibiting factors will act in unison in which case the result will be unusually low count. But since suchoccurrencesareofnosafetyconcerntheywillnot befurtherdiscussed.)Ithasbeenearliersuggestedthatif this hypothesis is correct then there are circumstances wherethefuture frequency of‘‘outbursts’’exceedingany givenmagnitudecanbeestimatedfromthecounts’past fluctuations pattern (Peleg & Horowitz, 2000). The conditionsare: *Correspondingauthor.Tel.:+413-545-5852;fax:+413-545-1262. E-mail address: micha.peleg@foodsci.umass.edu(M.Peleg). 0958-6946/02/$-seefrontmatter r 2002ElsevierScienceLtd.Allrightsreserved. PII:S0958-6946(01)00103-0