Reference: López et al. (1977), cited by Pimienta (1990).
iological disturbance is typical of some tropical and subtropical fruits and vegetables (Saltveit and Morris 1990; Wang 1990) as well as nopalitos (Ramayo-Ramírez et al. 1978a; Cantwell 1991) and leads to surface discoloration and softening, which, in turn, usually promotes microbial infections. The susceptibility to chilling injury and its consequences vary with species and stem age, the harvest method, and the method of packing as well as with the atmosphere, temperature, relative humidity, and duration of cold storage. Nopalitos of Opuntia spp. packed in vented polyethylene bags (leading to a modified atmosphere inside the bag) may show signs of chilling injury at 21 days of storage at 5°C, whereas cactus stems packed in wooden crates and also stored at 5°C show chilling injury at 15 days (Rodríguez-Félix and Villegas-Ochoa 1998). Stems of Nopalea cochenil-lifera are more susceptible to chilling injury during storage at 4°C than are those of Opuntia spp. Outside of bags, they develop symptoms of chilling injury at 7 days, but if stored in plastic bags, an additional 4 days of storage is gained before symptoms appear (Nerd et al. 1997).
In general, "minimally processed" horticultural products are prepared and handled to maintain their freshness while providing convenience to the consumer. Producing minimally processed products involves cleaning, washing, trimming, coring, slicing, and shredding (Brecht 1995; Schlimme 1995). Other terms used to refer to minimally processed products are "lightly processed," "partially processed," "fresh-processed," and "preprepared" (Cantwell 1992). According to Avena (1996), minimum processing includes the operations generally used for canned, frozen, or dehydrated food products, but without scalding for inac-tivation of enzymes. Most of these products are sold as ready-to-eat foods, which is a major advantage. "Pre-cut," "minimally processed," or "fresh-cut" can describe a special modality for postharvest handling of fresh nopalitos.
circular form, another nopalito layer is placed upon the first one, and so forth (Corrales-García 1997). Pacas have proved practical, especially when periods of commercialization are short (1-3 days). If the period is longer, considerable heat is generated in the center of the packing units by respiration of the nopalitos (Cantwell 1991), reducing the quality of product.
Refrigeration atmospheres with reduced O2 and/or elevated CO2 concentrations extend the storage life of many fruits and vegetables by reducing respiration rates. However, postharvest deterioration can result from many factors besides high respiration rates, including the biochemical changes associated with respiratory metabolism, ethylene production and action, compositional changes, physiological disorders, and pathological breakdown (Kader 1986). Furthermore, under certain conditions, atmospheric changes shift cladodes from aerobic to anaerobic respiration, leading to fermentation and the accumulation of ethanol and acetaldehyde (Chang et al. 1982) and causing unpleasant flavors and odors.
Storing at low temperatures extends the shelf life of nopalitos and maintains their vitamin content. This is especially true under modified atmospheres, which also implies low O2 availability for oxidation and browning, low degradation of vitamins, and, in general, low enzymatic activity. Other factors, such as harvest technique, storage duration and relative humidity, and packing technique also affect the shelf life of nopalitos (Cantwell 1995). Ramayo et al. (1978b) found that 21% of pads packed in wooden crates and stored at io°C (at 80-85% relative humidity) showed decay at the cut surface at 10 days; however, if carefully harvested, the shelf life can be extended to 2i days without decay development under the same storage conditions.
Nopalitos are susceptible to chilling injury when exposed to nonfreezing temperatures below i0°C. This phys
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