Preliminary Investigation Of The Use Of Raman Spectroscopy To Predict Beef Microbial Counts

Due to its essential nutrients and high-protein meat is an important food,but also can easily cause microbial spoilage.The traditional methods for microbial analysis mainly rely on laboratory-based investigations with a complex and time-consuming sample analysis.In order to improve the efficiency of detecting and ensure food safety,the meat industry needs rapid and non-destructive analytical procedures to detect microbial spoilage indicators and predict the shelf life of meat.Raman spectroscopy is a vibrational spectroscopy method based on Raman scattering,which has clear potential for non-destructive and rapid determination.This technology provides the possibility of online analysis in meat quality and become a promising technology in meat industry.Consequently,the potential of Raman spectroscopy to detect beef spoilage was examined in a preliminary investigation.In this paper twenty Chinese crossbred yellow cattle?Luxi�Simmental?were slaughtered in a commercial abattoir at 24 months of age.The M.longissimus lumborum?LL?was removed from the left side of the carcasses and cut into 3cm steak,packaged randomly and individually in vacuum packaging?VP?or modified atmosphere packaging?MAP,50%O₂/30%CO₂/20%N₂?.Steaks from each LL were randomly assigned to the combinations of two packaging methods and different storage times?day 0,3,7,14 and 21?in 0-4?storage.The Raman spectra,pH values,meat color,microbial counts and microbial diversity analysis of the cold-stored steak were measured at different storage time.All the values were regressed against Raman spectra using partial least squares regression?PLSR?to build predictive model.Models to predict traits were fitted PLSR analysis with leave one out cross validation?LOO-CV?with the optimal PLSR model selected based on the minimum cross validated root mean squared error of prediction?RMSEP?and the lowest number of latent variables?LVs?.The Raman shift range was restricted to a range of 500-1800cm^-1.The main results were as follows:?1?The packaging method had no significant effect on the pH value of the beef steaks?P>0.05?,but it had significant effect on a*and b*in day7 and day14?P<0.05?.Compared with VP group,MAP group showed the better color protection effect in day7 and day14.The MAP group showed the better microbial inhibitory effect for beef steaks than VP group.MAP group had lower total viable counts,Pseudomonas spp.and lactic acid bacteria compared with VP group?P<0.05?.Among these two packaging methods,microbial counts did not reach the recommended legal spoilage threshold during the 21-day storage period.The microbial diversity of beef steaks in both two packaging methods decreased with storage time.MAP group beef steaks had a more complex bacterial diversity than VP group.The initial microbial community was mainly dominated of Acinetobacter,Ochrobactrum and Thermus,but from day14,the relative content of Brochothrix in the MAP group increased rapidly and become the dominated bacteria.Acinetobacter,Lactobacillus and Carnobacterium dominated alternately in the VP group over time,among which Carnobacterium dominated after day14 of storage time.?2?Raman spectroscopy has a good ability to predict L*for day0 steak(R²_cv=0.78,RMSEP=2.61),pH value for first 7 days storage time(day0:R²_cv=0.99,RMSEP=0.071;VP in day3:R²_cv=0.35,RMSEP=0.065;MAP in day3:R²_cv=0.48,RMSEP=0.066;VP in day7:R²_cv=0.56,RMSEP=0.067;MAP in day7:R²_cv=0.56,RMSEP=0.061;),total viable counts and lactic acid bacteria counts up to 21d post-mortem(TVC in VP:R²_cv=0.99,RMSEP=0.61;TVC in MAP:R²_cv=0.90,RMSEP=0.38;LAB in VP:R²_cv=0.99,RMSEP=0.54;LAB in MAP:R²_cv=0.75,RMSEP=0.60).The predictive ability of pH value and L*of long-term storage need to be explored deeply,in the same storage time,the predictive ability of Raman spectroscopy on the L*value for MAP group is higher than VP group.The characteristic peak of Raman bands used to explain the variation trend of various indexes of steak during storage were:tyrosine doublet(Raman shift 827 and853 cm^-1);PO₃^2-stretching vibration bands of lactic acid and phosphate at 974 cm^-1and 1045 cm^-1;Raman bands for amide?at 1653 cm^-1,amide?at 1319 and 1271cm^-1;1450cm^-1?scissoring mode of methylene CH₂?and 1653cm^-1?double bond stretching C=C?are related to lipid oxidation;1080 and 1126 cm^-11 belong to C-N stretch;937cm^-11 which was matched with C-CN structure for Lysine?Lys?.Comparison of the spectra reveals that amide?at 1653 cm^-11 and amide?(1319 and1271 cm^-1)was decreased and intensity in doublet(Raman shift 827 and 853 cm^-1)increased as the storage time increased.By comparing the average spectra of different storage times and combining the intensity differences of the above Raman bands,the spoilage of meat at different storage times can be explained to some extent.