Effect of ultrasound pretreatment combined with different drying methods on the quality and bioactive compounds of broccoli stems

The effects of ultrasound (US) pretreatment combined with drying process on storage quality maintenance and bioactive compound accumulation of broccoli stems were investigated. Results indicated that US pretreatment (λ = 8.5 mm) delayed the decline of L* value and ascorbic acid content but inhibited the increase of a* and b* values, browning index, and microbial counts of fresh-cut broccoli stems. Furthermore, the US application enhanced total phenolics content by activating phenylalanine ammonia-lyase, cinnamate-4-hydroxylase, and 4-coumarate-CoA ligase in phenylpropane metabolism. Meanwhile, it also promoted γ-aminobutyric acid (GABA) accumulation by upregulating activities of diamine oxidase, polyamine oxidase, and aminoaldehyde dehydrogenase in GABA shunt and polyamine degradation pathway, respectively, which in turn, accelerated the biosynthesis of GABA from polyamines and glutamate. In addition, compared with other three drying methods, the combined vacuum freeze drying (VFD) could further retain the sensory attributes and bioactive compounds to a maximum extent in dehydrated broccoli stems. Hence, current findings illustrated that US pretreatment combined with VFD process is beneficial for commercial quality maintenance and nutritional value enhancement, thus providing new perspectives on the comprehensive utilisation of broccoli stems.