RESUMEN
Ultrasound, alone or in combination with natural antimicrobials, is a novel food processing technology of interest to replace traditional food decontamination methods, as it is milder than classical sterilisation (heat treatment) and maintains desirable sensory characteristics. However, ultrasound efficacy can be affected by food structure/composition, as well as the order in which combined treatments are applied. More specifically, treatments which target different cell components could result in enhanced inactivation if applied in the appropriate order. The microbial properties i.e. Gram positive/Gram negative can also impact the treatment efficacy. This work presents a systematic study of the combined effect of ultrasound and nisin on the inactivation of the bacteria Listeria innocua (Gram positive) and Escherichia coli (Gram negative), at a range of cavitation conditions (44, 500, 1000â¯kHz). The order of treatment application was varied, and the impact of system structure was also investigated by varying the concentration of Xanthan gum used to create the food model systems (0 - 0.5% w/v). Microbial inactivation kinetics were monitored, and advanced microscopy and flow cytometry techniques were utilised to quantify the impact of treatment on a cellular level. Ultrasound was shown to be effective against E. coli at 500â¯kHz only, with L. innocua demonstrating resistance to all frequencies studied. Enhanced inactivation of E. coli was observed for the combination of nisin and ultrasound at 500â¯kHz, but only when nisin was applied before ultrasound treatment. The system structure negatively impacted the inactivation efficacy. The combined effect of ultrasound and nisin on E. coli was attributed to short-lived destabilisation of the outer membrane as a result of sonication, allowing nisin to penetrate the cytoplasmic membrane and facilitate cell inactivation.
Asunto(s)
Escherichia coli , Listeria , Antibacterianos/farmacología , Recuento de Colonia Microbiana , Nisina/farmacologíaRESUMEN
The effect of W1/O/W2 emulsion incorporation in set-type yogurt on the acidification process, physicochemical properties, bacterial growth kinetics and structural characteristics was investigated. The W1/O/W2 emulsion was formed by using a two-step homogenisation process and milk as the W1 and W2 phases, and stability was monitored with optical microscopy and cryo-SEM. Adding the W1/O/W2 emulsions reduced the acidification rate, viscosity and water retention capacity. Texture (adhesiveness, cohesiveness, hardness, and gumminess) differed in yogurts containing W1/O/W2 emulsion compared to controls during the acidification process, however, trends became stable during storage. The growth of S. thermophilus during the acidification process of yogurt was reduced in the presence of W1/O/W2 emulsion while L. bulgaricus trended higher during storage. This study shows that yogurts containing W1/O/W2 emulsion are feasible subject to processing modification.
Asunto(s)
Emulsiones/química , Manipulación de Alimentos/métodos , Aceites/química , Agua/química , Yogur , Adhesividad , Animales , Fermentación , Dureza , Concentración de Iones de Hidrógeno , Cinética , Lactobacillus delbrueckii/crecimiento & desarrollo , Lactobacillus delbrueckii/metabolismo , Viabilidad Microbiana , Leche/química , Streptococcus thermophilus/crecimiento & desarrollo , Streptococcus thermophilus/metabolismo , Viscosidad , Yogur/microbiologíaRESUMEN
Nano-emulsions (typically droplet diameter<1µm) are common in foods, and have been extensively reported to present antimicrobial activity, however, the mechanism is not well defined, and some studies reported no effect. A review of the literature was conducted and revealed strongly contradictory reports regarding the antimicrobial effect of nano-emulsions even in reference to similar microbial species and formulations. Following up, this study aimed to investigate the effect of nano-emulsions on four bacterial species (Staphylococcus epidermidis, Bacillus cereus, Lactobacillus acidophilus and five Escherichia coli strains) possessing different surface charge and hydrophobicity. Model oil-in-water (O/W) emulsions with different size of oil droplets were prepared with sunflower oil stabilised by polysorbate 80 (Tween80) emulsifier (hydrophilic), using high shear mixing followed by ultrasonication. The viability of bacteria was monitored by culture, membrane integrity was assessed with flow cytometric analysis with propidium iodide (PI) staining and fluorescence microscopy monitored the spatial distribution of cells within the O/W emulsions. The stability of the nano-O/W emulsions in the presence of bacteria was assessed by monitoring the droplet size [D (4, 3)] and creaming height. In contrast to other reports the survival and growth of bacteria was not affected by the size of the oil droplets, no damage to the bacterial membrane was evident with flow cytometry and emulsion stability was not affected by the presence of bacteria during 7days of storage. Furthermore, the antimicrobial activity of caprylic acid (CA) was compared between O/W coarse and nano-emulsions while varying the concentration of the hydrophilic surfactant Tween80. The activity of CA was similar in nano-emulsion and coarse emulsion; however, it was higher than in bulk oil and was reduced with increasing Tween80 concentration, suggesting that its efficacy is dictated by formulation rather than oil droplet size. The results demonstrated no enhanced antimicrobial activity due to nano-sized oil droplets and that conclusions on nano-emulsions should be taken with caution.