Head cabbage

Dense-headed white or Dutch autumn-maturing cabbage (B. oleracea var. capitata) crops are stored for up to 10 months and ultimately sold all year-

round on to the fresh market or for processing into coleslaw or as part of prepared salads. Such crops are grown extensively in north-west Europe (Germany, the UK and The Netherlands) and North America. Extended storage of cabbage is detrimental where it is going to be processed further, as in the production of coleslaw.

Increasingly, cabbage is used as a part of mixtures of vegetable products in what has become known in marketing jargon as the 'added-value chain' of 'minimally processed products'. Traditionally, crops such as cabbage have been held in low esteem because they demand considerable preparation for use in the home. This perception is changed by processing outside the home to a limited extent before sale to the retail customer. Naturally such processing increases the price charged to the consumer and is termed 'value-added'. The amount of processing is normally small ('minimal'), usually consisting of cutting into portions. Raw cabbage can be shredded and mixed with other vegetables such as diced carrot to provide a dry mixture that is stored in MAP for 10-14 days. Adding mayonnaise to the shredded mixture forms coleslaw. There are significant differences in the suitability of cultivars for this trade (Cliffe-Byrnes and Beirne, 2005).

Late-maturing, dense-headed, red leaf types (B. oleracea var. capitata f. rubra) may be similarly stored for subsequent processing, especially in The Netherlands. The stage when cabbages are suitable for cutting and storage is difficult to determine solely by visual estimation. Characteristics such as head firmness, density, size, days of growth from transplanting, estimates of heat units and summation of solar radiation from planting have all been tested as means of determining readiness for storage. The estimation of sucrose content has also been offered as a means of identifying maturity and has considerable attractions since this character is easily tested in the field with the type of portable, pocket refractometers used to test sugar content of maturing beet roots.

Maturation during the harvesting period is characterized by dry matter and sugar accumulation and increasing firmness, with the highest quality in white cabbage developing towards the end of the autumn harvesting period (Suojala, 2003) (see Fig. 8.1).

Quality during storage declines gradually as a result of disease incidence, dehydration and metabolic changes. Firmness diminishes during storage as a result of dehydration and dry matter losses caused by continued transpiration and respiration. As a result, juiciness and crispness are lost. Other sensory indicators of quality, however, such as chewiness, sweetness, lack of bitterness, intensity of flavour and lack of off-flavours seem to be retained. During storage, there appears to be a shift of energy reserves from the head leaves to the core. Sugars in particular are concentrated in the core towards the end of storage. This may be a physiological preparation for regrowth, demonstrating that the harvested cabbage head retains a seasonal rhythm despite being severed from the roots and placed in cool temperatures. These

processes will be under genetic control, and unravelling them (after Suojala, 2003) enables plant breeders to improve quality and longevity.

Storage technology has developed from traditional clamp methods as used for root vegetables. Use is made of barns that are equipped with forced night air ventilation for cooling and dispersal of field heat. This permits preservation for 3-4 months until late February or early March. More sophisticated methods are required for longer term storage involving purpose-built refrigerated stores in which cabbages are packed into palletized containers.

White cabbage has been stored for extended periods under refrigeration in nitrogen atmospheres modified with 2-6% carbon dioxide and 1-5% oxygen. These result in less storage and trimming losses, longer retention of fresh colour, flavour and texture, and lower pathogen-induced spoilage. There may be indirect benefits from modified atmosphere storage since reduced time is required in preparing the product for processing and marketing, and the storage period can be extended to 10 months.

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