Bokashi (horticulture)

Inside a recently started bokashi bin. Food scraps are raised on a perforated plate (to drain runoff) and are partly covered by a layer of bran.

The process is typically applied to food waste from households, workplaces and catering establishments, because such waste normally holds a good proportion of carbohydrates. It is applied to other organic waste by supplementing carbohydrates and hence lactic acid production. Recipes for large scale bokashi in horticulture often include rice, and molasses or sugar.[6][7] Any carbohydrate-poor waste stream would benefit from this.

Homolactic fermentation can process significantly more kinds of food waste than home composting. Even items such as cooked leftovers, meat and skin, fat, cheese and citrus waste are, in effect, pre-digested to enable soil life to consume them. Large pieces may take longer to ferment and concave surfaces may trap air, in which cases cutting down is advised in support literature.[8]

Pieces of input are discarded if they are already badly rotten, or show green or black mould. These harbour putrefying organisms which may overwhelm the fermentation.

Homolactic fermentation and similar anaerobic fermentation pathways in general provide a very small amount of energy to the cell compared to the aerobic process. In homolactic fermentation, 2 ATP molecules are made when one glucose molecule (produced by digesting complex carbohydrates) is converted to 2 lactic acid molecules,[9] only 1⁄15 of what aerobic respiration provides.[10] The process will also halt before all available carbohydrates are used, as the acidity ends up inhibiting all bacteria. As a result, a bokashi bucket barely heats up and remains at ambient temperature.[11]

As a waste processing technique, bokashi is notable in that minimal loss of mass in the form of offgassing happens. Compost, which is aerobic, "burns up" much of the carbon into carbon dioxide to sustain the metabolism of microbes as it matures. Biogas production does not burn the carbon, but the bacterial culture is optimized to extract the carbon in the form of methane – a potent greenhouse gas and a useful fuel. In addition, compost can also lose the key plant nutrient nitrogen (in the potent greenhouse gas nitrous oxide and in ammonia), while bokashi almost does not.[11]

When fermentation begins, physical structures start to break down and release some of the input's water content as a liquid runoff. Over time this constitutes more than 10% of the input by weight. The quantity varies with the input: for example cucumber and melon flesh lead to a noticeable increase.

The liquid leaches out a valuable fraction of proteins, nutrients and lactic acid. To recover them, and to avoid drowning the fermentation, runoff is captured from the fermentation vessel, either through a tap, into a base of absorbent material such as biochar or waste cardboard, or into a lower chamber. The runoff is sometimes called "bokashi tea".

The uses of bokashi tea are not the same as those of "compost tea". It is used most effectively when diluted and sprinkled over a targeted area of soil to feed the soil ecosystem. Dilution makes it less acidic and thus less dangerous to plants. Dilution also causes more acid to convert into lactate which is an attractive food for soil microbes. Other uses are either potentially damaging (e.g. feeding plants with acidic water) or wasteful (e.g. cleaning drains with plant nutrients, feeding plants with nutrients in a form they cannot take up).

Household containers ("bokashi bins") typically give a batch size of 5–10 kilograms (11–22 lb). This is accumulated over a few weeks of regular additions. Each regular addition is best accumulated in a caddy, because it is recommended that one opens the bokashi bin no more frequently than once per day to let anaerobic conditions predominate.

In horticultural settings batches can be orders of magnitude greater.[6][11] Silage technology may be usable if it is adapted to capture runoff. An industrial-scale technique mimics the windrows of large-scale composting, except that bokashi windrows are compacted, covered tightly and left undisturbed, all to promote anaerobic conditions. One study suggests that such windrows lose only minor amounts of carbon, energy and nitrogen.[11]

Bokashi is inherently hygienic in the following senses:

• Lactic acid is a strong natural bactericide, with well-known antimicrobial properties.[12] It is an active ingredient of some toilet cleaners.[13] As more is produced, it eventually suppresses even its own makers, the acid-resistant lactobacilli, such that bokashi fermentation eventually slows and stops itself. There is also evidence that mesophilic (ambient temperature) fermentation kills eggs of the Ascaris worm – a parasite of humans – in 14 days.[14]
• The fermentation bin does not release smells when it is closed. A household bin is only opened for a minute or so to add and inoculate input via the lid or to drain runoff via the tap. At these times the user encounters the sour odour of lacto-fermentation (often described as a "pickle") which is much less offensive than the odour of decomposition.[15]
• When closed, an airtight fermentation bin cannot attract insects.
• Bokashi literature claims that scavengers dislike the fermented matter and avoid it in gardens.[16]

Fermented bokashi is added to a suitable area of soil. The approach usually recommended by suppliers of household bokashi is along the lines of "dig a trench in the soil in your garden, add the waste and cover over."[17]

In practice, regularly finding suitable sites for trenches that will later underlie plants is difficult in an established plot. To address this, an alternative is a 'soil factory'.[18] This is a bounded area of soil into which several loads of bokashi preserve are mixed over time. Amended soil can be taken from it for use elsewhere. It may be of any size. It may be permanently sited or in rotation. It may be enclosed, wire-netted or covered to keep out surface animals. Spent soil or compost, and organic amendments such as biochar may be added, as may non-fermented material, in which case the boundary between bokashi and composting becomes blurred.

A proposed alternative[19] is to homogenise (and potentially dilute) the preserve into a slurry, which is spread on the soil surface. This approach requires energy for homogenisation but, logically from the characteristics set out above, should confer several advantages: thoroughly oxidising the preserve; disturbing no deeper layers, except by increased worm action; being of little use to scavenging animals; applicable to large areas; and, if done repeatedly, able to sustain a more extensive soil ecosystem.