DEVELOPING A SENSOR TO DETECT VOLATILE COMPOUNDS IN MILK TO MONITOR SHELF LIFE

There is great interest from food industries, and food safety controlling bodies to develop accurate, cost effective, rapid, and reliable methods of evaluating the shelf life and quality of food. This dissertation is focused on developing a Silicon dioxide (SiO2) based sensor capable of detecting VOCs to monitor shelf life. In this study, pasteurized milk (3.9% fat) was stored at 5,7,10,13,13,15 and 19 (±1°C) up to the end of shelf life. Microbial counts including aerobic plate, psychrotrophic bacteria, and Bacillus spp. were done throughout the storage life. The pH, titratable acidity, lipase activity, and protease activity were also measured. The shelf life of pasteurized milk, as determined by aerobic plate count (APC) and acidity indicators, was 24, 36, and 72 hours at 19, 15, and 13°C, respectively. Milk stored at 5, 7, and 10 °C had shelf lives of 30, 24, and 12 days, respectively.The relationship between bacterial counts and VOC formation in whole (3.9% fat) milk stored under various storage conditions was investigated. In addition, VOC formation was measured using solid-phase microextraction (SPME) with gas chromatography. Microbiological data well correlated with both off-flavors and gas chromatography–mass spectrometry (GC-MS) data. After inception, VOCs increased, with bacterial counts of 5.0 – 7.0 CFU/ mL. Development of a colorimetric VOC detection system for stored milk was the overriding goal of this dissertation. Spoilage bacteria were detected by the nanosensor colorimeter at all storage temperatures. The nanosensor response correlated well with microbial growth counts in the milk samples. Milk spoilage was determined with the sensor to have occurred at 32, 60, and 84 hours at 19 , 15, and 13°C, respectively. Color change, recorded as total color difference (ΔE), correlated well with ABC levels (6.0 -7.0 log10 CFU /mL) in milk (with the exception of the 7°C samples), demonstrating that chemical interactions occur between the nanosensor and the VOCs. These results suggest that the nanosensor has good potential as a real-time VOC sensor for monitoring microbial spoilage in milk during storage and transport. Further development of rapid detection methods for spoilage microorganisms can be based upon the present sensor

Isolation and Characterization of Lactic Acid Bacteria from Libyan Traditional Fermented Milk "Laban"

Fermented milk product "Laban" in Libya is one of the most a traditional fermented milk product consumed a refreshing drink, particularly in the warm season The average values of the physicochemical including titratable acidity, pH, total solids, and fat were 0.73%, 4.16, 8.12%, and 1.54% respectively. Coliform, yeast and mold counts were 21&amp;times;10⁴, 39&amp;times;10⁴, and 41 &amp;times;10&amp;sup3; cfu/ ml., respectively. Most strains of coliform bacteria were Serratia odorifera, Escherichia coli 1, E. coli 2. and Klebsiella oxytoca. The average Lactococcus, Streptococcus, Mesophilic Lactobacillus / Leuconostoc and Thermophilic Lactobacillus counts were 99 &amp;times;10⁷, 96 &amp;times;10⁷, 93 &amp;times;10⁷ and 15 &amp;times;10⁷ cfu / ml. respectively. A total of 142 lactic acid bacteria (LAB) isolates were identified to the genus level as Lactobacillus (48.59%), Lactococcus (43.66%), Streptococcus (4.93%) and Leuconostoc (2.82%). Sugar fermentation tests revealed the most frequent Lactobacillus species found to be Lactobacillus delbrueckii ssp. lactis (62.32%), followed by Lactobacillus plantarum (31.88%). Furthermore, other selected LAB isolates were identified by API 50 CH test as Lactococcus lactis ssp. lactics, Lactobacillus pentosus, Lactobacillus brevis, and Leuconostoc mesenteroides ssp. cremoris. Thus, our research documented the lactic acid bacteria strains and will provides fundamental basic and useful information for further studies of strain selection starter culture, with regard to the industrial production of fermented dairy milk products.</jats:p