EVALUATION OF MICROBIAL, ACIDIFICATION, PROTEOMICS AND AROMATICS PROPERTIES OF CULTURED DHANAAN: CAMEL MILK FERMENTED USING COMMERCIAL STARTER CULTURES

Abstract

The study was conducted to evaluate and characterize cultured Dhanaan (camel milk fermented using commercial starter cultures) with the following objectives; evaluation of the microbial safety of traditionally fermented Dhanaan, evaluation of the acidification activities of commercial starter cultures in camel milk, proteomic/peptidomics characterization of cultured Dhanaan and aromatic characterization of cultured Dhanaan. Shotgun metagenomic approach of 16S rRNA gene amplicon was targeted and ion torrent semiconductor sequencing technology was used to evaluate the microbial safety of the traditional Dhanaan samples. The acidification properties of the commercial starter cultures in camel milk were characterized by acidification rate. The physicochemical properties (acidification properties, textural properties) and microbiological qualities (coliform count, yeast/mould count) of the cultured Dhanaan were determined. Further chemical properties of the cultured Dhanaan have been characterized by peptidomics/proteomics and aromatics technologies. Dynamic head space GC/MS (gas chromatography- mass spectrometry) method was used to characterize the volatile compound profile of the cultured Dhanaan samples. (LC-MS/MS) Liquid chromatography tandem mass spectrometry set up to Q Exactive Orbitrap mass analyzer, was used to analyze the peptidomics characterization of the products. The mass spectra of the peptide data were searched using the sequest searching algorithm against the Camelus dromedarius uniprot database. Metagenomic profiling showed the presence of classical lactic acid bacteria (LAB) (streptococcus, lactococcus, weissella, pediococcus, lactobacillus and enterococcus), non classical LAB (Streptococcus lutetiensis) and pathogenic microorganisms (klebsiella, enterobacter, acinetobacter and clostridium) in the traditional Dhanaan samples. The presence of significant amount of pathogenic microorganisms in all the Dhanaan samples indicates the need for the transformation of the local practices of Dhanaan manufacturing to an improved and safe production system. Streptococcus lutetiensis might be technologically important lactic acid bacteria for the fermentation of camel dairy products. However, further safety evaluation and characterization need to be conducted on this species as the QPS (qualified presumption of safety) status of the species is not determined yet. All the evaluated cultures were able to acidify camel milk and reached a final pH at a level similar to what was achieved in bovine milk, xv but the speed of acidification was generally lower in camel milk indicating fermentation difficulties in camel milk. This could be due to inhibitory substances in camel milk or due to reduced availability of nutrients. Experiments using mixtures of camel and bovine milk or supplementation with casein hydrolysates were used to distinguish between these possibilities. High acidification rates were obtained in camel milk mixed with bovine milk or supplemented with casein hydrolysates. This demonstrates that the cultures are not inhibited by camel milk and the growth rates of these cultures in pure camel milk are limited by the rate of proteolysis. Neither heat treatment nor application of commercial starter cultures could improve the weak texture properties of fermented camel milk. The Cultured Dhanaan showed microbiological stability during the storage time. Peptidomics/proteomics characterization of cultured Dhanaan indicated that lower amount of peptides were observed from the high heat treated (90°C, 30 min) samples than the low heat treated (65°C, 30 min) samples. However, higher acidification rate was observed from the H-samples than the L-samples. Cultured Dhanaan is found to be dominated by peptides derived from beta caseins and 77% of the total peptides were found to be short peptides of a size <3 kDa fraction. This indicates the potential of the functional properties of camel milk peptides. Aromatic characterization of the cultured Dhanaan indicates that a total of 105 volatile compounds have been identified in the fermented camel milk products. These compounds were; alcohols (28), aldehydes (17), ketones (12), terpenes (12), alkanes (8), carboxylic acids (5), esters (4) and other compounds (19). The analysis showed that fermented camel milk products are rich in volatile flavor compounds. The most abundant characteristic compounds found in the products were aldehydes (30.73%) and ketones (29.68%) followed by alcohols (23.00%). Effect of heat treatment of high temperature pasteurization (90°C, 30 min) was observed to generate more volatile compounds than the low pasteurization temperature (65 °C /30 min).