R-N=C=S Isothiocyanates

R-C=N Nitriles + sulfur well as a crop rotation with cereals; wheat or barley following canola is the preferred sequence. Most fields are not cropped with canola more than once every four years. This practice permits a break in disease and insect pest cycles, while preserving the soil structure. Canola is susceptible to several fungal diseases such as blackleg (Leptosphaeria maculans), stem rot (Sclerotinia sclerotiorum), damping-off (Fusarium spp.), root rot (Rhizoctonia spp.), and black spot or leaf spot CAlternaria spp.); disease severity varies with year and location (19). A number of different insects can also cause serious damage to canola (eg, flea beetle) (20). Crop rotation is effective in controlling fungi and insects. Several herbicides are also available for a variety of weed species (12,21).

Harvest and Storage Practices

The date and method of canola harvesting are important determinants of yield and seed quality (12,18,22). Immature seeds have higher chlorophyll and free fatty acids contents. On the other hand, if the harvest is delayed, canola is susceptible to shattering of the pods, particularly in windy areas. To avoid this, swathing (leaving cut plants to dry in the field) is applied when the majority of the seeds are at a firm doughlike stage (moisture content 35-40%). Swathing facilitates uniform maturation and yields seeds of good color and high in oil and protein. Premature swathing (seed moisture >45%) results in reduced yields, whereas late swathing (moisture <20%) increases shattering losses and the possibility of frost damage. Canola is ready to combine when the moisture content is below 15%. Freshly harvested seeds normally require up to six weeks to become dormant; storage in dry and cool environments minimizes respiration losses. Canola is usually transported after harvest to dryer-stocker companies and therefrom to oil-processing plants. For storage, silos of various sizes are used, while on the farms the seed is stored in bins. Immature and field-germinated seeds should not be stored for long periods because they deteriorate rapidly. A high percentage of damaged or fragmented seeds also gives rise to a rapid increase in free fatty acids (22,23).

The moisture content of the seed, because of its importance to enzyme activity and the growth of microflora, is the main criterion for safe storage. In the case of oilseeds, including canola, moisture is held by the nonfatty components, thus bringing the critical level of overall seed moisture content much below that required for safe storage of cereal grains (15%). The maximum moisture content for storage of clean canola seeds in a cool environment (< 15°C) is 10% (24); others sources (22,25) even suggest lower optimum levels of moisture (8-9%) for storage over periods of several months. Above this level, the seeds must be air dried; air temperatures must be kept below 38°C to prevent heat damage. Aeration during storage, although at much lower airflow rates than drying, is also required to prevent moisture gradients and to cool the stored grain. Figure 2 shows how storage is affected by both temperature and moisture content. Canola is particularly prone to microbial spoilage, with severe losses in seed and oil quality. The rate of spoilage rapidly increases if nonuniform distributions of moisture and temperature exist in the stored grain. An important consideration regarding moisture content of canola is that before solvent extraction of the oil, the seed must be cooked rapidly at temperatures between 80 and 100°C and at water contents between 6 and 10% to inactivate myrosinase; above 10% moisture, hydrolysis of glucosinolates would occur; below 6%, heat inactivation of the enzyme is difficult.

Chlorophyll is another important quality parameter of canola. Besides imparting an undesirable color to the oil, chlorophyll promotes oxidative rancidity in salad oils. Several environmental factors and agronomic practices, including length of growing season, temperature during seed

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