Milk Spoilage Research Articles

Detection of antibacterial residues in milk by HPLC-DAD and microbial inhibitor tests

The objective of this study was to assess the detection sensitivity of four commercial microbial inhibition assays (MIAs) in comparison with the results by High Pressure Liquid Chromatography with Diode Array Detection (HPLC-DAD) for residues of three tetracyclines, trimethoprim and sulfadiazine in fortified raw milk samples. MIAs showed fairly good results at detecting sulfadiazine residues, whereas concerning tetracyclines and trimethoprim residues, most of the kits showed insufficient results. HPLC analysis revealed high recoveries of examined compounds with detection limits below their respective maximum residue levels (MRLs). The HPLC method was validated determining linearity, accuracy, precision, detection capability and decision limit. Subsequently, both methods were used to monitor several cow and sheep milk samples for the presence of antibacterial agents. Results obtained showed that MIAs could be successfully used for the analysis of different milk types. However, milk spoilage which was observed in some cases could decrease the sensitivity of the kits, a fact that does not seem to influence the effectiveness of HPLC-DAD method.<br /><br />

Food Sensing: Detection of Bacillus cereus Spores in Dairy Products.

Milk is a source of essential nutrients for infants and adults, and its production has increased worldwide over the past years. Despite developments in the dairy industry, premature spoilage of milk due to the contamination by Bacillus cereus continues to be a problem and causes considerable economic losses. B. cereus is ubiquitously present in nature and can contaminate milk through a variety of means from the farm to the processing plant, during transport or distribution. There is a need to detect and quantify spores directly in food samples, because B. cereus might be present in food only in the sporulated form. Traditional microbiological detection methods used in dairy industries to detect spores show limits of time (they are time consuming), efficiency and sensitivity. The low level of B. cereus spores in milk implies that highly sensitive detection methods should be applied for dairy products screening for spore contamination. This review describes the advantages and disadvantages of classical microbiological methods used to detect B. cereus spores in milk and milk products, related to novel methods based on molecular biology, biosensors and nanotechnology.

Carbon dioxide as a novel indicator for bacterial growth in milk

AbstractHuman milk spoils due to bacterial, yeast, or mold contamination. Current domestic methods of assessing milk spoilage are subjective or rely on time and temperature‐based guidelines. A key unmet food safety need remains the objective assessment of human milk spoilage. Experiments were conducted using a simplified human milk spoilage model based on goat's milk as a human milk surrogate, spiked with a single bacterial strain (Staphylococcus epidermidis), in which pH and carbon dioxide (CO2) concentration were measured along with bacteria count over 160 hr. Bacteria count correlated highly with CO2 but not with pH. A 0.21% CO2 concentration threshold could be defined for milk spoilage (correlating to a bacteria count threshold of 105 CFU/ml), with sensitivity and specificity above 84%. These findings suggest that CO2 measurement is a promising method to detect S. epidermidis growth in milk which merits further investigation for the objective and quantitative assessment of milk spoilage.

Nanosilver in dairy applications – Antimicrobial effects on Streptococcus thermophilus and chemical interactions

Milk spoilage continues to be a major problem. Incorporation of silver into milk packaging might solve this problem. We evaluated the antimicrobial and chemical effects of silver in milk. Antimicrobial experiments were performed by measuring milk acidification by Streptococcus thermophilus at temperatures of 43, 33 and 23 °C and concentrations of silver nanoparticles at 10, 50, 100 and 200 mg/L. Chemical interactions were investigated using potentiometric measurements. Ag+ interacted with milk constituents. Nanosilver was antimicrobially effective at all temperatures and at 100 mg/L. At least 5 mg/L are required by using AgNO3. Practical use of silver for dairy applications is not expected, due to the required high silver concentrations.

Efficacy of Reuterin and Bacteriocins Nisin and Pediocin in the Preservation of Raw Milk from Dairy Farms.

SUMMARYResearch backgroundIn the current scenario of milk production in developing and developed countries, several factors influence the shelf-life of raw milk and add significant numbers of microbial contaminants that drastically lower the initial microbial quality leading to milk spoilage by the time it reaches the processing units.Experimental approachThe present study was undertaken to investigate the biopreservative efficacy of reuterin system along with different combinations of bacteriocins in controlling the initial microflora of raw milk at farm level. Lactobacillus reuteri strain LR47, having effective antimicrobial activity, was shortlisted from our previous study and further characterized for reuterin production and tested in raw milk system.Results and conclusionsPreliminary testing of the cell-free supernatant from L. reuteri LR47 demonstrated significant growth inhibition of the majority of the tested bacterial indicators of milk spoilage. Further genetic analysis of the L. reuteri LR47 revealed the presence of two genes (pduC and dhaB) involved in the utilization of glycerol to produce reuterin via two different pathways. The strain LR47 was also found to possess comparatively higher capacity to convert glycerol into reuterin when checked through colorimetric assay. In the raw milk biopreservation experiment with reuterin alone or in combination with bacteriocins, the highest level of growth suppression in the total bacterial load and coliform counts was observed in the sample that was treated with a combination of reuterin, nisin and pediocin. The treatment combining these three natural biopreservatives at specific concentrations was able to maintain the initial microbial quality and extend the shelf-life of raw milk by 6 h at 37 °C based on the microbial counts and physicochemical properties, viz. pH and titratable acidity. In conclusion, the results confirm that the use of reuterin in combination with bacteriocins is a promising approach for temporary control of the raw milk microflora and extension of its shelf-life until further processing.Novelty and scientific contributionThis study demonstrates for the first time the use of reuterin for the extension of shelf-life of raw milk as an alternative treatment method.

Milk storage system for remote areas using solar thermal energy and adsorption cooling

As there is a time gap between milking and storage, milk spoilage is more in remote areas in India, hence, immediate pasteurization and storing facility is required. For pasteurization heating is compulsory. In India most villages face scarcity of electricity, hence, solar and biomass heat (easily available) is a good option for milk pasteurization. After the pasteurization process for storing (150Ltrs of milk) in low temperature till its distribution, adsorption refrigeration is proposed here as it is run by solar/ waste heat or biogas (easily available from farm waste). During milk heating and storing at low temperature, materials used for containers and piping have chosen stainless steel (SS316), so that materials should not react with milk. For pasteurization of milk, one parabolic collector has been designed and developed. The result shows that at 5LPM flow rate, milk can be easily heated at 75 °C for 30 min. For storing 150ltrs of milk at 15–20 °C for 10–12 hrs, a solar power (flat plate collectors 8 m2) adsorption refrigeration has been designed and discussed. For adsorption refrigeration, selection of adsorption pair is very important. After reviewing different literatures, here activated carbon and methanol have chosen and justified. The experimental result shows that for 400LPH hot water supplied at 90 °C (desorber bed temperature at 80 °C), 35 °C condenser temperature and evaporator temperature 5 °C system’s specific cooling power varies between 5.7 kW/kg to 5.4 kW/kg (variation due to uncertainty analysis) which is sufficient for storing 150ltrs of milk.

Host-parasite translocation: A potential source of zoonoses emergence in Nigeria

Zoonoses are infectious diseases that are spread between animals and people. These diseases are transmitted to humans in many ways, such as direct contacts, indirect contacts, vector-borne, foodborne, and inhalation. Translocation and introduction of animals to new geographic regions correspond to increased human global travel and commerce as underlying factors for infectious disease emergence. In this review, we examined some potential notable driving mechanism of zoonosis in Nigeria. The population explodes, and demand for animal products has resulted in the expansion of animal trade, both local and international, animal and human movements, and intensification of livestock production systems. The above mentioned have an indirect role in zoonotic disease distribution. Animal husbandry, wildlife hunting, and hunting with dogs are potential routes of parasite translocation, most notably when infected animals are killed. Zoonotic diseases cause severe economic loss in the pathogenic spoilage of milk, contaminated animal products, carcass quality, weight loss, infertility, and loss of animal population. The cost of disease control decreases in household income due to a reduction in livestock/product sales. Also, consumption impacts due to reduced food availability, increased household vulnerability where livestock is used as a risk-coping mechanism, and effects on household finance, which influences household savings. Our suggestions for future effective zoonoses control include, an improved surveillance system, well-structured quarantine services, institutionalized one health approach, public enlightenment, interdisciplinary research, and ultimately a strict conservation rules and regulation may be turned into law to avoid transmission of Zoonosis through the consumption of wild animal which is most reservoir of causative pathogen.

An investigation of seasonal variations in the microbiota of milk, feces, bedding, and airborne dust.

ObjectiveThe microbiota of dairy cow milk varies with the season, and this accounts in part for the seasonal variation in mastitis-causing bacteria and milk spoilage. The microbiota of the cowshed may be the most important factor because the teats of a dairy cow contact bedding material when the cow is resting. The objectives of the present study were to determine whether the microbiota of the milk and the cowshed vary between seasons, and to elucidate the relationship between the microbiota.MethodsWe used 16S rRNA gene amplicon sequencing to investigate the microbiota of milk, feces, bedding, and airborne dust collected at a dairy farm during summer and winter.ResultsThe seasonal differences in the milk yield and milk composition were marginal. The fecal microbiota was stable across the two seasons. Many bacterial taxa of the bedding and airborne dust microbiota exhibited distinctive seasonal variation. In the milk microbiota, the abundances of Staphylococcaceae, Bacillaceae, Streptococcaceae, Microbacteriaceae, and Micrococcaceae were affected by the seasons; however, only Micrococcaceae had the same seasonal variation pattern as the bedding and airborne dust microbiota. Nevertheless, canonical analysis of principle coordinates revealed a distinctive group comprising the milk, bedding, and airborne dust microbiota.ConclusionAlthough the milk microbiota is related to the bedding and airborne dust microbiota, the relationship may not account for the seasonal variation in the milk microbiota. Some major bacterial families stably found in the bedding and airborne dust microbiota, e.g., Staphylococcaceae, Moraxellaceae, Ruminococcaceae, and Bacteroidaceae, may have greater influences than those that varied between seasons.

Effect of a pre-milking teat foam and a liner disinfectant on the presence of mesophilic and (proteolytic) psychrotrophic bacteria prior to milking.

Contamination of raw milk by psychrotrophs can lead to the production of heat-resistant proteases and subsequent spoilage of UHT milk. Therefore, this research communication evaluated the effect of a pre-milking teat disinfectant (active components: L-(+)-lactic acid and salicylic acid) and a liner disinfectant (active components: peracetic acid and hydrogen peroxide) on the number of mesophilic and (proteolytic) psychrotrophic bacteria prior to milking. The teat orifices of 10 cows were sampled using a swabbing procedure before and after treatment with a pre-milking teat disinfectant on six subsequent days. On the teat orifices, there was a small but statistically significant decrease in the psychrotrophic bacterial counts between pre and post dipping. No differences were observed for the mesophilic bacterial counts and proteolytic active counts. Liners were also sampled using swabs pre and post disinfection. No statistically significant decrease in the bacterial counts was observed post liner disinfection, although there was a numerical decrease. Sixty-two percent of the proteolytic psychrotrophs were pseudomonads: 16.5% of which were P. fragi, 14.3% P. lundensis, 10.0% P. fluorescens and 2.9% P. putida. Trinitrobenzenesulfonic acid (TNBS) analysis revealed a wide variety in proteolytic activity (from 0 to 55 µmol glycine/ml milk) and the presence of high producers. It can be concluded that there was only a minor effect of teat and liner disinfection on the psychrotrophic bacterial counts indicating that the measures presented did not result in a reduction of the targeted bacteria on teat orifices and liners.

Entrepreneurial Competencies and Performance of Dairy Cooperatives in Machakos County, Kenya

The objective of the study was to establish entrepreneurial competencies and performance of dairy cooperatives in Machakos County, Kenya. Specifically, to establish the extent to which managerial competencies influence performance of dairy cooperatives in Machakos County, Kenya; to determine the extent to which entrepreneurial traits influence performance of dairy cooperatives in Machakos County, Kenya; and to assess the effects of technical skills on performance of dairy cooperatives in Machakos County, Kenya. The study was descriptive. The study was a census of the 15 dairy cooperatives registered under the Lower Eastern Dairy Cooperative Association in Machakos County. The unit of analysis was the cooperatives while the units of observations were the board members of the cooperatives. A standard questionnaire was used to collect primary data from the board members of the cooperatives. Data analysis included frequencies, percentages, and t- tests. The data was analyzed by use of computer statistical packages and data presented in tables and figures. Based on the first study objective, study concludes that there was lack of sufficient evidence to show that managerial competencies have an effect on performance of dairy cooperatives in Machakos County. Based on the second study objective, the study concludes that there is adequate evidence to show that entrepreneurial traits have positive effects on performance of dairy cooperatives in Machakos County. Based on the third study objective, the study concludes that there is adequate evidence to show that technical skills have positive effects on performance of dairy cooperatives in Machakos County. The study recommends development, adoption and full implementation of business and strategic plans for the cooperatives for them to attain sustainable competitiveness and growth. The study also recommends data based product development and diversification. Further training would be critical in building capacity for creativity, risk taking, situational analysis and opportunity identification. Moreover, the study recommends training to enhance employees’ capacity and effectiveness in financial management; procurement; production; and marketing. Particularly, loss of revenue due to spoilage of milk can be controlled by training on best processing and quality control methods while training on marketing, would be critical in enhancing product penetration, reducing customer complaints and stimulating positive growth. Finally, further investigations should be conducted to widening the scope of the study to compare Machakos to other parts of the Country and also to identify reasons for high labor turnover and poor public relations between the cooperatives and communities surrounding them. Keywords: Entrepreneurial competency, Performance DOI : 10.7176/EJBM/11-33-13 Publication date: November 30 th 2019

A rapid and sensitive biosensor for measuring plasmin activity in milk

Plasmin play an important role in milk spoilage and can cause quality issues in ultra-heat treated (UHT) milk and other dairy products. Current methods for measuring plasmin activity in milk have low sensitivity and are too slow to be used in routine testing. We report a bioluminescence resonance energy transfer (BRET)-based biosensor that can measure naturally occurring low levels of plasmin activity in milk within a few minutes. The biosensor incorporates a plasmin-specific peptide target sequence flanked by the resonance energy BRET transduction elements: green fluorescent protein (GFP2) at the N-terminus and a variant Renilla luciferase (RLuc2) at the C-terminus. Complete cleavage of the peptide linker sequence by human plasmin led to an approximately 87% decrease in the starting BRET ratio. Using a 10 min incubation time, the detection limit for human plasmin was 0.25 nM and 0.86 nM for bovine plasmin in 50% (v/v) full fat milk with EC50's of 5.89 ± 0.12 nM and 5.97 ± 0.59 nM, respectively. These detection limits are below the naturally occurring levels of plasmin reported to occur in raw or processed cow’s milk. The plasmin biosensor therefore has the potential to measure naturally occurring plasmin levels directly in raw and UHT-processed milk samples. The protease biosensor described herein is selective and sensitive and retains its function in the complex colloidal matrix of milk. It therefore meets many of the requirements for routine use in production settings. We propose its use for managing plasmin-related quality issues in UHT milk and other dairy products.

Determining the Age of Spoiled Milk from Dried Films Using Attenuated Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy.

Milk spoilage is an inevitable occurrence, which generates waste and can result in food poisoning. When milk spoils, the off-flavor and curdling are due to excessive proliferation of various bacteria which causes pH changes. Time, temperature, environment, and previous handling practice all affect the spoilage rate. There is a need for a fast reliable and accurate method that can identify in situ early spoilage of milk. Here we show the ability of attenuated total reflection Fourier transformed infrared spectroscopy (ATR FT-IR) in conjunction with multivariate data analysis to predict the age of milk. We found that dried films vastly increased the absorbance of important biomolecules within milk such as lipids, proteins, and sugars, compared to an unchanged milk sample. This allowed us to note the minor discrepancies that happened in spoilage. Spoilt milk was characterized by bands associated with increased lipids, proteins, and lactic acid and a decrease in carbohydrates. A semi-quantitative prediction model for milk spoilage at room temperature demonstrated ATR FT-IR spectroscopy can predict milk age with a root mean square error of prediction of approximately 14 h. The model showed poor performance in the first 40 h but the predictions improved significantly after this time. The experimental procedure proposed for detecting biomolecules within milk has the potential to improve common practice. Furthermore, the model would be a starting point for newer and improved methods to predict the spoilage date of milk, with potential commercial uses to reduce food waste and costs to the milk industry.

Application of single and dual fluorophore-based pH sensors for determination of milk quality and shelf life using a fibre optic spectrophotometer

Milk is a high nutrient value food substance consumed by the entire population in several forms. Spoilage and degradation of milk and its products during transportation, processing and packaging are some of the major losses by dairy industries. The change in milk pH with time is an indicating factor of milk degradation. This research reports a rapid, sensitive and facile fluorescence-based technique to study the time-based milk degradation at room temperature. Fluorescein isothiocyanate (FITC)/ FITC-dextran (FITC-dex) and Tris (2, 2′-bipyridyl) dichloro ruthenium (II) hexahydrate (Ru-bpy) were used to develop solution phase and immobilized phase pH sensors. Immobilized phase sensors constituted of alginate microspheres (5 ± 3 μm) produced using an air driven atomizer. The pH sensor works in a linear range of 3–8 pH units with a quick response time of 30 s. The percentage recovery of the system lay in the range of 89.23%–101.28% for detection of milk pH. Accurate detection was possible during entire shelf life of milk (˜24 h) and Inter-sample and inter-day reproducibility was checked over 10 days duration. The results clearly indicate that both single and dual fluorophore-based assay in solution phase and immobilized phases can predict milk pH with good accuracy.

Microbial Growth and Changes in Nutritional Contents of Preserved and Unpreserved Fresh Milk Stored at 4°C

The effects of microbial deterioration and changes in the nutritional contents of preserved and unpreserved fresh cow milk stored at 4°C were investigated. Three different concentrations (0.1%, 0.2% and 0.4%) of preservatives- benzoic acid (BA) and sodium metabisulphite (SM) were evaluated for anti-microbial impact. The bacterial load and nutritional content were assessed and monitored over days for both treated and untreated milk. The results showed that microorganisms would still grow in milk at 4°C with a gradual reduction in nutritional factors. However, upon addition of the selected preservatives, significant reduction in the microbial growth was observed. The efficacy of benzoic acid in reducing bacterial growth was higher across all the levels of treatment especially when 0.4% concentration of both preservatives were applied and compared against untreated milk. The study concludes that the use of chemical preservation can serve the purpose of prolonging the onset of milk spoilage and reduce food safety concerns in dairy production.

A century of gray: A genomic locus found in 2 distinct Pseudomonas spp. is associated with historical and contemporary color defects in dairy products worldwide

Some gram-negative bacteria, including Pseudomonas spp., can grow at refrigeration temperatures and cause flavor, odor, and texture defects in fluid milk. Historical and modern cases exist of gray and blue color defects in fluid milk due to Pseudomonas, and several recent reports have detailed fresh cheese spoilage associated with blue-pigment-forming Pseudomonas. Our goal was to investigate the genomes of pigmented Pseudomonas isolates responsible for historical and modern pigmented spoilage of dairy products in the United States to determine the genetic basis of pigment-forming phenotypes. We performed whole genome sequencing of 9 Pseudomonas isolates: 3 from recent incidents of gray-pigmented fluid milk (Pseudomonas fluorescens group), 1 from blue-pigmented cheese (P. fluorescens group), 2 from a historical blue milk spoilage incident (Pseudomonas putida group), and 3 with no evidence for blue or gray pigment formation (2 from P. fluorescens group and 1 from Pseudomonas chlororaphis group). All 6 isolates collected from products with a gray or blue pigment defect were confirmed to produce pigment using potato dextrose agar or pasteurized milk. A subset of 2 isolates was selected for inoculation into milk and onto the surface of a model cheese for subsequent color measurement. These isolates produced different colors on potato dextrose agar, but produced nearly identical color defects in milk and on model cheese. For the same subset of 2 isolates, the gray color defect in milk was produced only in containers with ample headspace and not in full containers, suggesting that oxygen is vital for pigment formation. This work also demonstrated that a Pseudomonas isolate from cheese can produce a pigment defect in milk, and vice versa. Comparative genomics identified an accessory locus encoding tryptophan biosynthesis genes that was present in all isolates that produced gray or blue pigment under laboratory conditions and was only previously reported in 2 P. fluorescens isolates responsible for blue mozzarella in Italy. Because this locus was found in genetically distant isolates belonging to different Pseudomonas species groups, it may have been acquired via horizontal gene transfer. These data suggest that several past and present gray- or blue-pigmented dairy spoilage events share a common genetic etiology that transcends species-level identification and merits further investigation to determine mechanistic details and modes of prevention.

Modeling American Household Fluid Milk Consumption and their Resulting Greenhouse Gas Emissions

U.S. consumers are the largest contributors to food waste generation (FWG), but few models have explained how households waste food. This study examines how discrete-event simulation (DES) can identify areas for reducing FWG through packaging and consumer milk consumption behavioral changes. Household model parameters included: amount and type of consumption, type and number of containers bought, buying behavior, and shelf life of milk. Simulations comparing the purchase of quart, half gallon, and gallon milk containers were run for 10,000 days to identify which package type reduced waste for 50 1, 2 and 4-person households. Based on consumption averages from the U.S. National Dairy Council, results suggest that if 1 and 4-person households change their purchasing behavior from 1 half-gallon to 1 quart and 2 gallons to 3 half-gallons, they can reduce their greenhouse gas (GHG) emissions from milk consumption by 33% and 12%, respectively, without reducing their total milk consumption. Purchasing enough smaller containers to be equivalent to a larger size decreased spoilage, but not enough to reduce a consumer’s total milk consumption GHG emissions. Results showed that packaging accounts for 5% of the total milk consumption GHG emissions; most of a consumer’s impact comes from milk spoilage and consumption.

Genes Associated With Psychrotolerant Bacillus cereus Group Isolates.

The Bacillus cereus group comprises 18 different species, including human pathogens as well as psychrotolerant strains that are an important cause of fluid milk spoilage. To enhance our understanding of the genetic markers associated with psychrotolerance (defined here as > 1 log10 increase in cfu/mL after 21 days incubation at 6°C) among dairy-associated B. cereus group isolates, we used genetic (whole genome sequencing) and phenotypic methods [growth in Skim Milk Broth (SMB) and Brain Heart Infusion (BHI) broth] to characterize 23 genetically-distinct representative isolates from a collection of 503 dairy-associated isolates. Quality threshold clustering identified three categories of psychrotolerance: (i) 14 isolates that were not psychrotolerant in BHI or SMB, (ii) 6 isolates that were psychrotolerant in BHI but not in SMB, and (iii) 2 isolates that were psychrotolerant in BHI and SMB. One isolate, which was psychrotolerant in BHI broth but was just below the cut-off of >1 log10 cfu/mL increase in SMB was not assigned to a cluster. A maximum likelihood phylogeny constructed with core genome single nucleotide polymorphisms classified all psychrotolerant isolates (i.e., psychrotolerant in BHI) into clade VI (representing B. mycoides/weihenstephanensis). Analysis of correlations between gene ortholog presence or absence patterns and psychrotolerance identified 206 orthologous gene clusters that were significantly overrepresented among psychrotolerant strains, including two clusters of cold shock proteins, which were identified in 8/9 and 7/9 psychrotolerant isolates. Gene ontology analyses revealed 36 gene ontology terms that were overrepresented in psychrotolerant isolates, including putrescine catabolic processes and putrescine transmembrane transporter activity. Lastly, Hidden Markov Model searches identified three protein family motifs, including cold shock domain proteins and fatty acid hydroxylases that were significantly associated with psychrotolerance in BHI broth. Analyses of CspA sequences revealed a positive association between psychrotolerant strains and a previously identified “psychrotolerant” CspA sequence. Overall, our data highlight genetic and phenotypic differences in psychrotolerance among B. cereus group dairy-associated isolates and show that psychrotolerance is dependent on the growth medium. We also identified a number of gene targets that could be used for specific detection or control of psychrotolerant B. cereus group isolates.

Effects of anthocyanin-rich purple and black eggplant extracts on the physical, antioxidant and pH-sensitive properties of chitosan film

Active and intelligent food packaging films were developed by mixing chitosan (CS) with anthocyanin-rich purple eggplant extract (PEE) or black eggplant extract (BEE). Results showed the anthocyanin contents in PEE and BEE were 93.10 and 173.17 mg/g, respectively. Besides, the anthocyanin compositions of PEE and BEE were totally different. PEE and BEE increased the blueness, thickness, UV–vis light barrier and mechanical properties of CS film. Nevertheless, PEE did not change the moisture content of CS film and BEE did not change the water vapor permeability of CS film. Microstructure observation showed low contents (1 and 2 wt%) of PEE and BEE were well distributed in CS matrix. Fourier transform infrared spectroscopy revealed the existence of intermolecular interactions between CS and extracts. X-ray diffraction indicated PEE and BEE somewhat increased the crystallinity of CS film. The antioxidant ability of CS film was remarkably enhanced by PEE and BEE. Moreover, CS-PEE and CS-BEE films were pH-sensitive and showed remarkable color changes in different buffer solutions, which could be used to monitor milk spoilage. Our results suggested CS-PEE and CS-BEE films could be applied as active and intelligent food packaging materials.

Colorimetric detection of volatile organic compounds for shelf-life monitoring of milk

Colorimetric nanosensors for monitoring food quality and shelf life providea an exciting development with obvious economic benefits. In this study, a colorimetric sensor based on silicon dioxide (SiO2) nanoparticles and Schiff's reagent to detect volatile organic compounds (VOCs) generated by the growth of spoilage bacteria in pasteurized whole milk stored at 7, 13, 15, and 19 °C was developed. Volatile organic compounds formed from microbial growth were detected using solid-phase microextraction (SPME) and gas chromatography. Volatile organic compounds levels were correlated with microbial growth (aerobic plate counts) and color change throughout the shelf-life period. Color changes due to VOCs generated by spoilage bacteria were detected at all storage temperatures except 7 °C. Nanosensor response correlated well with microbial growth in milk. Milk spoilage occurred at 32, 60, and 84 h at 19, 15, and 13 °C, respectively and could be detected colorimetrically. Color change, recorded as total color difference (ΔE) for the nanosensor, correlated well with aerobic plate counts (5.0–7.0 log10 CFU/mL) in milk (R2 = 0.81–0.96). This colorimetric sensor could be incorporated into food packaging to predict remaining shelf-life benefitting consumers as well as manufacturers and retailers and reducing food loss.

Insights into Bacterial Milk Spoilage with Particular Emphasis on the Roles of Heat-Stable Enzymes, Biofilms, and Quorum Sensing

Milk spoilage caused by psychrotrophic bacteria and their heat-stable enzymes is a serious challenge for the dairy industry. In many studies, spoilage has been explored based on the simplistic view of undesirable enzymes produced by planktonic cells. Recently, biofilms and quorum sensing (QS) have been suggested as important factors in the deterioration of milk, which opens new avenues for investigation of the processes and challenges. Production and heat stability of enzymes are enhanced in biofilms, mainly because of inherent differences in physiological states and protective shielding by extracellular polymeric substances. QS plays a key role in modulating expression of hydrolytic enzymes and biofilm formation. To date, few studies have been conducted to investigate the complex interplays of enzyme production, biofilm formation, and QS. This review provides novel insights into milk spoilage with particular emphasis on the roles of biofilms and QS and summarizes potential effective strategies for controlling the spoilage of milk.