Fermentation

Some blending of juices may occur prior to fermentation and additions made. In particular, there may be a blending with sugars or AJC, to arrive at a specific gravity of 1.08-1.1. The FAN level may be raised to 100 mgL-1 by the addition of ammonium sulphate or ammonium phosphate. Thiamine may be added, perhaps at 0.2 ppm, but this must be separate from the addition of sulphite as the latter will destroy it. Other B vitamins that are required are pantothenate (2.5 ppm), pyridoxine (1 ppm) and biotin (7.5 ppb). Such materials are especially likely to be limiting if the cidermaker is using significant quantities of AJC or sugars.

Another potential problem with AJC is the generation of O- and N-containing heterocyclics within it (by Maillard reactions - see Chapter 1), which are inhibitors of yeast. They can be removed by the treatment of AJC with activated charcoal. If the apple juice and its additions are too 'bright', then it will be necessary to add some solids (e.g. bentonite) to act as nucle-ation sites, the escaping CO2 relieving inhibition of the yeast and also serving to maintain agitation in the fermenter. We have already encountered this for the fermentations of beer and wine.

Pectolytic enzymes are sometimes added to initial fruit pulp or to the juice immediately prior to fermentation.

SO2 is traditionally added to prevent the growth of contaminating microorganisms (Table 5.4). It is less critical from that aspect with the advent of dried wine yeast, but it is still important from a flavour perspective and is not without significance for antimicrobial protection. The effectiveness of SO2 increases as the pH decreases because it is the undissociated form of bisulphite which has the antimicrobial properties. The pH is lowered to less than 3.8 by the addition of malic acid prior to the addition of sulphite.

Healthy fruit generally will only contain low levels of sulphite-binding agents and should have sufficient SO2 to offer effective resistance to spoilage before addition of yeast. If, however, the fruit is in less good condition, then it

Table 5.4 The quantity of sulphur dioxide that should be added to cider apple juice.

pH

SO2 to be added (mg L 1)

3.0-

3.3

75

3.3-

-3.5

100

3.5-

3.8

150

>3.

8

150 (after blending or acid addition to

achieve a pH < 3.8)

Based on Lea & Drilleu (2003).

Based on Lea & Drilleu (2003).

may contain materials such as 5-ketofructose or diketogluconic acid as a result of bacterial activity. This type of substance binds SO2 and therefore reduces the endogenous protectant level. Furthermore, if ascorbic acid is oxidised to 1-xylosone, this also binds SO2. Finally, if AJC is depectinised, this will yield galacturonic acid that will also diminish SO2.

In traditional cider making, the yeast was delivered adventitiously with the fruit or the equipment (Saccharomyces does not naturally inhabit cider apples - but it is to be found on presses). SO2 suppresses the growth of most microbes other than Saccharomyces. Traditionally a succession of microflora in juice that had not been sulphited was involved in metabolising apple juice to cider. Saccharomyces was significant relatively late in the process. The introduction of SO2, however, rendered Saccharomyces as being vastly more important in the process. Since the 1960s, though, the vast majority of cider fermentations have been seeded. Juice should be held at <10°C prior to the addition of that yeast in order to prevent native flora from kicking off fermentation. Many of the cultures now added were originally isolated from the cider factories themselves, but some cidermakers use wine yeasts with well-defined characteristics, including the spectrum of flavour compounds that they produce and their flocculation behaviour. Since the 1980s, there has been widespread use of active dried wine yeast, which simply needs mixing with warm water, freeing the cidermaker from the need for in-house propagation. Some will employ an aerobic yeast incubation period so as to ensure that the yeast membranes are in good condition in order that the yeast will be capable of effecting very high levels of alcohol production.

Frequently the inoculum is a mixture of Saccharomyces pastorianus and Saccharomyces bayanus. The former is felt to give a lively start to the fermentation, whereas the latter performs better later in the process, and ferments to dryness.

Where temperature control is effected (this is not universal), this is likely to be within the range 15-25°C. If the fermentation displays sluggishness, then a portion of the goods may be warmed to 25°C by pumping through a heat exchanger. Most fermentations will be complete in 2 weeks.

Ciders are subjected to a malolactic fermentation as in the case of some wines (see Chapter 3). This is effected by heterofermentative Leuconostoc oenos, together with other lactobacilli. This is favoured if there is no sulphiting in fermentation and storage and also by autolysis of yeast when the cider is allowed to stand unracked on its lees. As sulphiting is so widespread these days, the malolactic fermentation is probably less significant than it once was. Furthermore, there is a lessening tendency to leave cider on the lees. In the malolactic fermentation, there is a conversion of malic to lactic acid and the release of carbon dioxide. The resultant cider will tend to have a more rounded, complex flavour that is less acidic. The process is inhibited if the pH is too low.

A range of sulphite-binding compounds are produced during fermentation, but the most potent binder of SO2 is acetaldehyde (Fig. 5.3). Essentially,

Undissociated Form S02
Adduct

Fig. 5.3 Adduct formation.

until all of this is bound to SO2, no free SO2 can remain to bind other components. Indeed, SO2 bound to carbonyls such as acetaldehyde has little antimicrobial action, which is why cidermakers try to minimise the level of carbonyls. The addition of thiamine reduces the production of pyruvate and of a-ketobutyrate. Pantothenate can reduce the production of acetaldehyde.

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