The production of biogas from anaerobic digestion (AD) of residual agro-food biomasses represents an opportunity for alternative production of energy from renewable sources, according to the European Union legislation on renewable energy. This review provides an overview of the various aspects involved in this process with a focus on the best process conditions to be used for AD-based biogas production from residual agro-food biomasses. After a schematic description of the AD phases, the biogas plants with advanced technologies were described, pointing out the strengths and the weaknesses of the different digester technologies and indicating the main parameters and operating conditions to be monitored. Subsequently, a brief analysis of the factors affecting methane yield from manure AD was conducted and the AD of fruit and vegetables waste was examined. Particular attention was given to studies on co-digestion and pre-treatments as strategies to improve biogas yield. Finally, the selection of specific microorganisms and the genetic manipulation of anaerobic bacteria to speed up the AD process was illustrated. The open challenges concern the achievement of the highest renewable energy yields reusing agro-food waste with the lowest environmental impact and an increment of competitiveness of the agricultural sector in the perspective of a circular economy.

Polyamines (PAs) are ubiquitous substances considered to be bioregulators of numerous cell functions; they take part in cell growth, division, and differentiation. These biogenic amines are also involved in tissue repair and in intracellular signaling; in fact, because of their polycationic character, they interact to a large extent with membrane phospholipids and may play an important role in the regulation of membrane-linked enzymes. The intracellular polyamine content derives from the simultaneous regulation of the synthesis, catabolism, uptake, and elimination of the polyamines; furthermore, PAs are present in all cell types at different concentrations, but the highest levels are found in rapid-turnover tissues. In addition to spermidine, spermine, and putrescine, also agmatine (AGM), deriving from arginine and identified in mammals in the 1990s, is a polyamine and several studies have reported its potentially positive role in the production of secretagogues, and in neuronal, vascular, metabolic, and therapeutic functions. Because of the low arginine decarboxylase (ADC) activity in mammalians, the amounts of AGM found in their tissues can be only minimally ascribed to an endogenous de novo synthesis by ADC, while a substantial quantity of AGM may be of dietary origin. Several food products contain only small amounts of polyamines, while higher concentrations can be found in fermented foods. PAs could also be considered as indicators of freshness in fish and meat products; as these moieties are produced during food storage, it would seem to confirm the main role of microorganisms in their synthesis. In particular, high levels of AGM are present in alcoholic beverages, such as wine, beer, sake, which would seem to confirm the role of yeasts in AGM production. Although many biological functions have been attributed to polyamines, high levels of these compounds in foodstuffs can have toxicological effects; however, no safe level for the intake of polyamines in a diet has yet been established. In this paper the presence of AGM in different foodstuffs is discussed, also taking into account the various factors affecting its presence and concentration.