Investigations of Physical-Chemical and Microbiological Deterioration of Chocolate Pralines: Development of Predictive Spoilage Models

Research output: Book/ReportPh.D. thesisResearch

  • Cecilie Lykke Marvig Nielsen
Chocolate pralines, defined as soft fruit, sugar or fat based fillings covered with a chocolate shell, are multidomain, complex food products. The chocolate shell consists of a continuous fat phase in which sugar and cocoa particles are dispersed. The fillings can vary in composition and texture and can consist of one or moreb types of fillings in the same praline. The chocolate of a freshly produced chocolate praline is expected to have a high gloss, a perfect snap and melt directly when placed in the mouth, which are all properties given by the fat crystals. However, also the particle size of the sugar and cocoa particles plays a role in the sensorial assessment of the chocolate. Fillings for chocolate pralines varies a lot and no common quality parameters can be set for these, but needs to be defined for the individual filling. The shelf life of chocolate pralines is determined by numerous and complex interactions between intrinsic and external parameters, and spoilage can be of either microbial or physical-chemical character or a combination of the two. Chocolate belong to the group of low moisture food products and is only susceptible to physical-chemical spoilage, while the fillings has intermediate moisture content and are spoiled both by growth of microorganisms and physical-chemical changes. Microbial spoilage is experienced as production of off-flavours, formation of slime, gas production leading to cracking of the chocolate shell, or visible growth on the surface of the filling. Physical-chemical spoilage can be seen as fat bloom, sugar bloom, cracking or softening of the chocolates. The objective of this study was to obtain a deeper knowledge and understanding of both physical-chemical and microbial spoilage mechanisms of chocolate pralines. The main focus of the PhD thesis was on microbial spoilage with the aims of (i) identifying potential spoilage organisms of chocolate pralines, (ii) screen for their tolerance towards the individual stress factors present in chocolate praline fillings, such aw, pH, ethanol and presence of sorbic acid and acetic acid, and finally (iii) develop predictive growth/no growth models to be used in product development in the confectionery industry. In regard to the physical-chemical spoilage, the focus was on moisture migration in chocolate pralines containing liquid fillings with and without alcohol. Physical-chemical spoilage can be caused by fat migration, fat bloom, sugar bloom or moisture migration. Moisture migration is caused by water activity gradients within the product and can be described by principles of diffusion. The moisture migration in chocolate pralines containing liquid sugar fillings with and without alcohol was studied by following the migration in chocolate praline models containing different combinations of the components of chocolate for 86 weeks. The migration of sucrose, ethanol and water was followed by HPLC analysis and analysis of water content. In addition, the fat crystals were determined during the process by differential scanning calorimetry (DSC) and small angle X-ray scattering (saxs). The structural changes were tested by confocal laser scanning microscopy (CLMS) and determination of particle sizes. Three different storage temperatures were tested (15°C, 22°C and 25°C). The cocoa particles in the chocolate were found to be the driving force of the moisture migration. Presence of sucrose also gave rise to minor moisture migration. It was hypothesised that the water adsorb to the sugar and cocoa particles followed by diffusion into the porous cocoa particles, which make them swell. This was confirmed by determination of particle measurements before and after moisture migration and confocal laser scanning microscopy (CLSM) pictures. In samples with chocolate models consisting of only cocoa butter, no migration was observed. The amount of liquid fat in the fat phase was important for the moisture to come in contact with the hydrophilic particles and migration occurred much faster at 25°C where the content of liquid fat was high compared to 15°C where much less liquid fat is present in the chocolate. No correlation could be found between the polymorphic transition of the cocoa butter crystals and the moisture migration, which might be due to lack of sufficient samplings. The microorganisms responsible for microbial spoilage of chocolate pralines have previously been only sparsely investigated. Increased knowledge on which spoilage organisms are present in the chocolate praline fillings and their tolerance to the stress factors present in the fillings is important for the confectionery industry when developing new products. 677 micro-organisms, belonging to bacteria, yeast, and moulds and capable of growing at sugar rich media, were isolated from 40 chocolate pralines and 25 sugar syrups used for chocolate pralines production. The isolates were identified by conventional microbial analysis and by sequencing of relevant genes. Fifteen isolates were screened for their ability to grow in presence of low aw (0.60-0.90), low pH (2.0-7.0), ethanol (0-15% (w/w), and sorbic acid (0-1500 ppm) relevant for chocolate pralines. The yeast Zygosaccharomyces rouxii was found to be the most important organism associated with spoilage, since it was most frequently isolated, isolated in high numbers (up to 107 CFU/g), and most tolerant to the stress factors tested. For physiological characterisation of Z. rouxii and as a tool to be used for product development by the confectionery industry, predictive growth/no growth models for Z. rouxii in conditions imitating chocolate praline fillings were developed. Since fillings for chocolate pralines cover a broad range of products, two sets of models were developed; (i) one for aw 0.76-0.88, pH 5.0-6.2, ethanol 0-15%, sorbic acid ±1500 ppm, acetic acid ±1%, and temperature 8-25°C and one for (ii) aw 0.65-0.80, pH 2.5-4.0, ethanol 0-15%, sorbic acid ±250 ppm, and time 0-90 days. The second set of models included two different strains of Z. rouxii, while only one of the strains were included in the first set of models. The models predicted that for aw >0.66, addition of other preservation methods was necessary to ensure microbial stability of chocolate pralines. The effect of pH was negligible (pH 3.0-6.2) unless organic acids were present. Also, the effect of temperature was only significant in the lower temperature range (8-15°C) for the least stringent conditions. However, for lower aw (<0.80) or higher ethanol concentrations (>9%) a major shift in the growth/no growth boundary was seen already between 15 and 22°C. The major effect of the environmental stress conditions could be observed within 60 days of incubation for the most stringent conditions and earlier for less stringent conditions. From screenings performed on individual stress factors it can be difficult to conclude which strain is most tolerant to the combined effect of the same stress factors. Therefore, several strains need to be included in predictive models. A validation of the developed models still remains to be performed in chocolate praline fillings to confirm the accuracy of the predictions in a real food product.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Number of pages157
Publication statusPublished - 2014

ID: 125690139