Factors influencing the accuracy of the plating method used to enumerate low numbers of viable micro-organisms in food

This study aims to assess several factors that influence the accuracy of the plate count technique to estimate low numbers of micro-organisms in liquid and solid food. Concentrations around 10 CFU/mL or 100 CFU/g in the original sample, which can still be enumerated with the plate count technique, are considered as low numbers. The impact of low plate counts, technical errors, heterogeneity of contamination and singular versus duplicate plating were studied. Batches of liquid and powdered milk were artificially contaminated with various amounts of Cronobacter sakazakii strain ATCC 29544 to create batches with accurately known levels of contamination. After thoroughly mixing, these batches were extensively sampled and plated in duplicate. The coefficient of variation (CV) was calculated for samples from both batches of liquid and powdered product as a measure of the dispersion within the samples. The impact of technical errors and low plate counts were determined theoretically, experimentally, as well as with Monte Carlo simulations. CV-values for samples of liquid milk batches were found to be similar to their theoretical CV-values established by assuming Poisson distribution of the plate counts. However, CV-values of samples of powdered milk batches were approximately five times higher than their theoretical CV-values. In particular, powdered milk samples with low numbers of Cronobacter spp. showed much more dispersion than expected which was likely due to heterogeneity. The impact of technical errors was found to be less prominent than that of low plate counts or of heterogeneity. Considering the impact of low plate counts on accuracy, it would be advisable to keep to a lower limit for plate counts of 25 colonies/plate rather than to the currently advocated 10 colonies/plate. For a powdered product with a heterogeneous contamination, it is more accurate to use 10 plates for 10 individual samples than to use the same 10 plates for 5 samples plated in duplicat

Interpolation and optimal monitoring in space and time

This thesis shows how statistics can be used for both analysing data and for determining the (optimal) design for collecting data in environmental research. An important question is often where to place monitoring stations to meet the objective of measuring as good as possible. In this thesis it is shown how existing monitoring networks can be adjusted on the basis of quantitative criteria. These criteria are based on aspects of spatial(-temporal) interpolation.A case study of climate variables in Jalisco State of Mexico is used to investigate the use of interpolation techniques. The climate variables monthly maximum temperature and monthly mean precipitation are predicted on a regular grid of points on the basis of measurements at climate stations. Four forms of kriging and three forms of thin plate splines are discussed. From these techniques, trivariate regression-kriging and trivariate thin plate splines performed bestThe optimal adjustment of existing monitoring networks is investigated for three case studies with different criteria. In the first place, a monitoring network adjustment is investigated for estimation of the semivariance function, whereby the criterium is based on the theory of optimal design of experiments. Secondly, we develop and apply a methodology to reduce an existing monitoring network to find an optimal configuration of a smaller network. In this case a criterion based on locally weighted regression with two different weight functions is used. The methodology is applied to the Dutch national SO 2 network and offers the possibility to include different politically relevant options in the model by weight criteria. As a third case study, a monitoring network for groundwater level is considered. It focusses on a possible reduction of the number of measurementsat this monitoring network without losing much information about the groundwater level at the different piezometers. Investigation of a reduction of the number of measurements is based on a geostatistical spatial-temporal model. The results show that the monitoring effort of the network can be reduced.Finding optimal designs involves several optimization problems. In this thesis several methods are developed and applied to solve these problems. For small problems full enumeration of all possible configurations is possible with a branch-and-bound algorithm. In this way, it is ensured that the global optimum is found. If full enumeration of all possible monitoring networks is impossible, a search algorithm is applied to find a (sub)-optimal solution

Stakeholderanalyse "Oogsten in Ketens"

Uitgegaan wordt voor "Oogsten in Ketens" dat er in co-innovatieprojecten van AKK kennis is ontwikkeld en ervaring is opgedaan. Doel is dan ook deze kennis en ervaring te verzamelen, deze expliciet en algemeen toepasbaar te maken, waardoor deze overdraagbaar wordt naar een bepaalde doelgroep. AFSG heeft een methode voor kenniscirculatie ontwikkeld, genaamd "Wageningen Experience Box" (WEB)

Rapport : bemonsteren en analyseren van dierlijke mest op een vrachtauto

Dierlijke mest is uitermate lastig materiaal om te analyseren met NabijInfraRood Spectroscopie (NIRS), want het is donker materiaal, hoofdzakelijk bestaand uit vocht en het is niet homogeen qua samenstelling. Toch wordt in literatuur beschreven, en ook in eerder onderzoek onder gecontroleerde omstandigheden aangetoond, dat stikstof- en fosfaatgehaltes in dierlijke mest kan worden gemeten met behulp van NIR spectroscopie met acceptabele niveaus qua betrouwbaarheid. De doelstellingen van het in dit rapport beschreven project waren: - Kan met Nabij Infrarood Spectroscopie bij belading en tijdens het lossen van mesttransport vrachtwagens een analyse van stikstof en fosfaat worden gerealiseerd die zich qua betrouwbaarheid ten minste kan meten met de huidige laboratorium-analyses? - Is de Nabij Infrarood Spectroscopie geschikt om toe te passen op voertuigniveau waarbij behoud van de betrouwbaarheid van analysen een voorwaarde is? - Is de Nabij Infrarood Spectroscopie een techniek die qua kosten en handhaving op grotere schaal in de praktijk toegepast kan worden

Distribution of Cronobacter spp. in industrial batches of powdered infant formula and the impact of sampling approaches

Objectives - The three objectives of this study are: first, to investigate how Cronobacter spp. are distributed throughout a recalled and a normal batch of powdered infant formula (PIF); second, to investigate the occurrence of conglomerations of Cronobacter spp. cells, since conglomerations of cells with high concentrations may impact on risk assessment and public health; and third, to investigate the performance of typical sampling plans. Results - In the recalled batch the concentrations Cronobacter spp. versus the filling time was assessed by 415 samples of 333 g using the MPN technique. In 58 % of the samples, concentrations were below the detection limit of -2.52 log CFU/g. However, specifically within three time intervals MPN concentrations were estimated varying between -2.52 and 0.66 log CFU/g. In addition 2290 samples of 1 g were investigated by plating, and 8 were found to contain Cronobacter spp. above the detection limit of 0.52 log CFU/g. These samples were originating from packages filled at the time interval with also the highest MPN concentrations. The two largest conglomerations were 123 and 560 cells in 1 g of PIF. In 99% of the samples from the normal batch, concentrations were below the detection limit. Various sampling plans were evaluated for the contamination data from the recalled batch. Keeping the total sample weight constant at 300 g and increasing the number of random samples from 1 to 300, increased the probability of detection from 0.38 till 1.00. Conclusions - Cronobacter spp. was distributed heterogeneously within a recalled batch of PIF with parts of the batch with no detectable contamination, and parts of the batch with concentrations up to 2.75 log CFU/g. The presence of conglomerations of Cronobacter spp. cells occurred with a low frequency. Taking more and smaller samples, keeping the total sampling weight constant, considerably improved the performance of the sampling plans to detect such a type of contaminated batch