Georgofili World

Newsletter of the Georgofili Academy

What is enzymatic browning?

Appearance is significantly impacted by colour and is one of the first attributes used by consumers in evaluating food quality. 
Polyphenoloxidase (PPO) - also referred to as phenoloxidase, phenolase, monophenol oxidase, diphenol oxidase and tyrosinase - is a copper-containing enzyme which catalyzes the conversion of phenolic substrates into brown-colored polymers and so plays an important role in the undesired browning of damaged tissues of fruits, vegetables and seafood. Moreover, enzymatic browning leads to nutritional losses due to the degradation of phenolic substrates and the decrease of their antioxidant capacity. In this regard, enzymatic browning is the main physiological disorder that impairs the sensory properties of minimally processed fruits and vegetables such as fresh-cut products which are characterized by an important and growing market. In brief, economic and quality losses for agricultural and food industry suggest that the control of PPO from harvest to consumer has always been a challenge. 
Numerous studies have focused on preventing PPO activity using several physical and chemical methods that aim to eliminate one of the essential reaction components such as oxygen, enzyme, copper ion or substrate. 
The main physical techniques include heating, high pressure, combined high pressure-thermal treatments, ultrasound associated with other methods, UV-C light treatment and cold plasma. Non-thermal technologies are an alternative to thermal ones which significantly affect the organoleptic and nutritional quality of heat sensitive products. 
Among the chemical agents, various forms of sulfite-containing compounds, ascorbic and citric acids are traditionally used. However, the synthetic additives may have adverse side effects including off-flavours, potential health hazards or low stability when exposed to oxygen and water. 
Extensive research has been focused on browning control with food additives obtained from natural sources. The inhibitory effect of dog rose, pomegranate and onion extracts, anthocyanin and green tea on  PPO activity has been reported. There is also a growing interest in the exploitation of the residues generated by the food industry that may be useful for the control of enzymatic browning throughout a given product/service life cycle. The effectiveness of agro-food industry by-products is mainly attributed to their high content of bioactive components belonging to organic acids, glucosinolates and polyphenols. In particular, these latter molecules represent a diverse group of compounds containing multiple phenolic functionalities and identified as specific natural PPO inhibitors. Products enriched with these bioactive compounds may be an effective tool not only  for improving food quality and safety by inhibiting PPO and protecting phenolic compounds but also  for developing functional foods. 

Anna Lante, Università di Padova (