SPAWN
Steam sterilisation in spawn production
Mushroom spawn must be axenic. That means that it only contains one species and is not contaminated with any foreign organisms. Any mistake at this point creates disasters further down the line. There is only one proper way to produce axenic spawn: steam sterilisation of the substrate followed by aseptic inoculation.
By Kasper Moreaux, Mycelia S
20°C 1 bar
filter
121°C 2 bar
worst possible place
cold air
worst possible place
Fig 1. Compression of gases in bag during autoclaving.
team sterilisation has overwhelming advantages and is universally used in spawn production. This article briefly explains the production of aseptic spawn recipes, whether in liquid, solid or semi-solid state. Aseptic means that after the treatment, it contains zero foreign organisms. Steam sterilisation of a substrate is a very complicated science and is usually underes- timated. The proper heat treatment of a substrate starts with choosing a production method: bulk, in bags, liquids, which all require different steam sterilisation cycles. But they have a common goal: every particle of the substrate must be sterile. To achieve this,
steam through filter
compressed cold air
Fig 2. Gravity cycle.
each particle must reach a temperature plateau of 121°C or more for a certain time. The amount of time at this plateau depends on the tempera- ture and on the initial spore load of the sub- strate, which is expressed in a so-called “F0-value”. In most spawn substrates, 15 minutes is enough. In bulk and liquids, this is reasonably straightfor- ward, as gases are easily evacuated. In spawn bags, bott- les and other containers however it is much more complicated because gases get
trapped inside the bags (see fig. 1). It is important to keep the sterilisation time at 121°C as short as possible. At these temperatures,
34 MUSHROOM BUSINESS
several chemical reactions take place, such as caramelisation and mayar reactions. These reacti- ons are detrimental for the quality of the spawn: originally available food elements are transfor- med into indigestible complexes and toxins. The spawn producer should apply the shortest possible sterilisation time, while ensuring that all particles are sterilised. The latter can only be determined with the aid of probes. There are three main sterilisation cycles in the production of spawn: the gravity cycle, the liquids cycle and the vacuum cycle/pulsating cycle.
The gravity cycle
The simplest version of a sterilisation cycle is a gravity cycle: the pressure and temperature in the autoclave are increased until they reach the sterilisation plateau at 121-123°C as displayed in fig. 2. This plateau is held stable until all particles are sterile. Then, the temperature and pressure are decreased.
The basic idea behind this cycle is that during the first phase, the air and other gases are replaced by steam by means of gravity and pressure. They must be removed due to the negative effect they have on the sterilisation temperature (see fig. 3) During replacement, the gasses are exhausted through the opened venting valve. This is important, because air and other gases act as insulators, so the temperature of the steam cannot be transferred to the particles. When steam has replaced all gases, the venting valve is closed, allowing the pressure to build up to the plateau. During the plateau, a so-called bleeding valve at the top of the vessel allows for continuous evacuation of gases which
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