Cucumbers

Whereas cucumbers (Cucumis sativus) retailed for their direct use are customarily bred to have tough skins, those targeted for pickling need to have a thin and relatively tender coating. They are harvested at a relatively immature stage, before the seeds have matured and before the area around the seeds has gone soft and starts to liquefy through the action of polygalacturonases on cell-wall hemicelluloses. The most valuable cucumbers are also the smaller ones. The cucumbers are sorted according to their diameter, and those that are too long are cut to a length that will readily fit into jars.

Other breeding criteria include disease resistance, yield, the growth locale and a relatively small seed area. Cucumbers should be straight and uniform with a length to diameter ratio of 3 : 1. They should be firm, green and free from internal defects. Chemical parameters include the level of cucurbitacins, which afford bitterness, sugars (which are the substrates for the fermentation), malic acid (relevant to the extent to which 'bloaters' are produced during fermentation) and the level of polygalacturonase. Opportunities for molecular biology in the optimisation of these parameters are being explored.

Cucumbers that are grown locally are processed within 1 day, whereas those grown further afield are refrigerated on shipping. If brined, they can be transported internationally.

Pickling cucumbers are preserved by one of three methods. Some two-fifths are preserved by fermentation, possibly accompanied by pasteurisation. Pasteurisation alone (reaching an internal temperature of 74°C for 15 min) is applied to another 40% of the total, while the remainder rely solely on refrigeration. For pasteurised and refrigerated processing, acid (produced separately, i.e. not through in situ fermentation) is usually added, perhaps accompanied by sodium benzoate.

Table 15.1 Stages of microbial involvement in vegetable fermentation.

Stage

Microbial events

Start A range of Gram-positive and Gram-negative bacteria present

Primary Most bacteria inhibited in the acid conditions created by the lactic acid fermentation bacteria. Lactic acid bacteria and yeast are able to thrive

Secondary Lower pH now inhibiting lactic acid bacteria, but not yeasts growing fermentation fermentatively

Post-fermentation Surface growth of oxidative bacteria, moulds and yeasts in open tanks.

However, if in sealed anaerobic tanks, no growth if pH is low enough and salt concentration high enough

Based on Fleming (1982).

Most commercial cucumber fermentations rely on a natural microflora. Sometimes, however, the natural microflora is heavily depleted by hot water blanching (66-80°C for 5 min), in which case there may be seeding with Lactobacillus plantarum. The various stages of microbial growth are indicated in Table 15.1. When the flower has withered, it tends to have increased levels of micro-organisms and, furthermore, the flowers also contain polygalactur-onase that plays a significant role in softening cucumbers by hydrolysing the polysaccharide matrix. The major fermentation sugars are glucose and fructose and these are metabolised to lactic acid, acetic acid, ethanol, mannitol and carbon dioxide. Lb. plantarum is normally the predominant organism in the natural microflora, mostly producing lactic acid. A malolactic fermentation is important in converting malate in the cucumbers to lactic acid.

The fresh cucumbers are immersed in brine in bulk tanks. The control parameters are pH, temperature and the level of salt. The brine is typically lowered to a pH of around 4.5 with either vinegar or lactic acid. This facilitates the loss of carbon dioxide (by shifting the equilibrium from bicarbonate towards carbonic acid). Furthermore, it has a major impact on which organisms grow, for instance, the growth of Enterobacteriaceae is suppressed at the lower pH whereas lactic acid bacteria are able to thrive in the absence of competition from organisms not able to tolerate these acidic conditions. The optimum salt level is 5-8% sodium chloride with the temperature in the range 15-32° C. The species involved are listed in Table 15.2.

During fermentation, the brine is purged with either nitrogen or air to prevent bloater formation, and the cucumbers are maintained submerged. Whereas air is the cheaper option, nitrogen is preferable as there is then less yeast and fungal growth, fewer off flavours and less colour development. Potassium sorbate (0.035%) is typically added to inhibit the growth of fungi. It is critical that the end product should possess a firm, crisp texture. Furthermore, as lactic acid is deemed too tart for products such as hamburger dill, a draining stage is employed with replacement of the brine by vinegar.

Pasteurised products typically contain 0.5-0.6% acetic at a pH of 3.7. The relative content of acid and sugar is adjusted depending on the desired sourness/sweetness balance.

Table 15.2 Lactic acid bacteria involved in fermentation of vegetables.

Homofermentative

Enterococcus faecalis Lactobacillus bavaricus Lactococcus lactis Pediococcus pentosaceus Heterofermentative Lactobacillus brevis Leuconostoc mesenteroides Mix

Lactobacillus plantaruma aThis organism uses hexoses homofermentatively but pentoses heterofermentatively.

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