There is nearly always a talk at conference urging growers to aim for quality at all costs. However such talks rarely give a clue as to how best to achieve this, probably because quality depends on so many interacting factors.
A definition could be: Quality, is what helps a mushroom sell itself.
An impulse buy
Most mushrooms are bought by women, as an impulse buy. Size matters, so do whiteness, firmness and shelf life. Many consumers think mushrooms are still grown on horse-manure, so any speck of peat, congregation of mites, or a mushroom fly remind them of this. Roundness, whiteness and smoothness add appeal, bruises and discolouration destroy the illusion of perfection.
An average mushroom expands at a rate of at least a 50% increase in diameter every 24 hours. This will make its weight double. Rate of expansion mainly depends on compost and air temperatures. Water availability in the casing and house air humidity also affect expansion rate. Larger mushrooms appear to hold back neighbouring smaller mushrooms. At the height of a flush the bed temperature rises and with it, expansion rate. Getting a longer time to pick is sometimes vital and can be achieved by reducing temperatures and improving picking skills. If some mushrooms are picked very small, at the beginning of the flush, this gives the remainder plenty of space for expansion. Consequently, although the first pick may be uneconomic, it can greatly benefit overall flush weight and quality. If mushrooms are of maximum size for the highest value grade, they will weigh heavy, provided they have the 'correct' moisture content.
Whiteness and firmness
These attributes are linked because lack of firmness leads to bruising and in turn to discolouration.
Firmness is a function of the dry matter (DM) content of the mushroom tissue. It is a term almost interchangeable with quality. It can range from 6% DM to over 12%. At 6% DM a mushroom is slightly pink and easily bruised. At 12% it almost resembles a golf ball and begins to approach it for being hard to chew on! Furthermore, the yield of a 12% DM crop is likely to be only half the weight of a 6% crop. At about 7.5% DM, mushrooms are still 'succulent' and are just about firm enough to avoid bruising, if pickers are careful. Thus 7.5% DM would probably maximise income. To get a better idea of DM, regularly feel the firmness, look for any discolouration, break mushrooms open and nibble them! A low DM mushroom tastes succulent, a high DM one is chewy. Try to achieve a balance.
Mushroom DM depends on a combination of factors. Mushrooms obtain part of their water from the compost, part from the casing. If water is too freely available from either, this will reduce mushroom DM. Compost moisture over 71 % would be considered high. Casing 'wetness' depends on how easily available its water is. If mushrooms are grown slowly, using lower temperatures, this gives them longer to lose moisture, provided there are drying conditions of sufficient air movement and humidity well below 100%.
Cap shape and smoothness
A smooth, uniform, rounded cap is wanted. Any deviation from this is highly undesirable. Smoothness of the cap depends on not exceeding a certain rate of drying. Distortions appear to have a wide variety of causes:- mushrooms too closely packed or damage to pins by a number of physical, or chemical or biological factors.
Discolouration and cap spotting
These flaws are fatal to quality. Breaking deep can lead to discolouration if the casing has some undesirable additive. Usually severe discolouration or sunken spots are due to blotch caused by the mushrooms remaining wet for too long, thus allowing Pseudomonas tolaasi bacteria to multiply on them. However, both distortion and browning can be symptoms of virus x. Furthermore, a variety of fungi, such as several species of Trichoderma, also Verticillium and Dactylium (cobweb) can cause cap spotting. Their spores may be spread by flies, pickers, water splash, or by drips from an infection above.
Casing and mushroom quality
The ease with which casing releases water to the mushroom depends on its original water content, the watering regime, its compaction and in later flushes on its ability to take up water after being partially dried out. Case-run temperatures, casing type and watering treatment will all have an influence on the amount of mushroom mycelium in the casing and this in turn affects casing hardness, water repellence and its ability to take up water for later flushes.
In the recent past, a high proportion of casing was based on brown, springy, moss peat. This released water easily, aiding high yields on relatively poor compost. However, growers now choose black peat because it aids mushroom quality. Black peat may have some brown moss peat mixed in, with the aim of increasing water uptake and shrink resistance.
Black peat that comprises decomposed sphagnum moss holds water well due to its gel like properties. It does not release it too easily and this aids mushroom firmness and quality. Its disadvantage is that it may have such a low dry matter that it shrinks excessively as it gives up water. If it is allowed to get too dry this shrinking can become irreversible, it then does not expand to take up water again. Shrinkage can also make the casing tear away from the compost below.
Fen type black peat is the remains of wet-land plants growing on relatively rich nutrient conditions, possibly reeds, sedges, grasses, irises usually with some tree leaves and twigs. This has a higher DM content, is more friable and less liable to shrink.
If black decomposed sphagnum has dried milled brown peat added, to increase its DM content, this may have the disadvantage of reducing mushroom 'quality ' if the tiny undecomposed leaves of the sphagnum moss stick to mushroom surfaces.
It is important to get to know casing type by ruffling it, squeezing handfuls to 'test' its water content, eventually choosing one that best suits one's own growing method. If casing rewets easily it probably releases water easily and this aids high yields.
If compost is the mushroom's fuel source, then casing is the tap that releases that fuel.
Casing additives that may improve mushroom quality
Certain additives improve mushroom DM and quality. The most popular is sugar-beet lime (SBL). Its particles are so fine that it has clay like properties. Sphagnum moss peat that has become very broken down, over thousands of years, may have similar water retention characteristics. Clay like materials hold water well but do not release it too easily, this aids higher mushroom DM.
SBL is difficult to mix completely evenly into casing. However, provided this unevenness is on a small scale it may help better pin distribution. High SBL content reduces casing shrinkage.
Another additive that is claimed to be beneficial to quality is calcium chloride, added at watering. This salt contains calcium that mushrooms need, but unlike limestone it is in a soluble form. Its presence also raises the osmotic pressure in the casing and this will somewhat hinder water uptake by the mushroom.
However, beware that mushroom DM should not be raised much above 7.5%. Otherwise yields will be correspondingly reduced.
A final complexity is that a balance needs to be struck between nutrient availability from the compost and water availability from the casing. As nutrients start to dwindle in the third flush, mushroom quality will tend to deteriorate. That is, unless the availability of water from the casing is similarly slowed. This usually happens naturally, because as casing hardens, flush by flush its ability to release water diminishes. Unfortunately this process usually goes too far.
>b>Effect of compost quality on mushroom quality
Quality in the third flush in particular, depends on what is left in the compost. The original DM of the compost is one measure of what it once contained. However if some of that DM happened to be sand, for example, this would upset DM as a useful measurement. However, if sand had been part of the compost's DM its presence would have lowered the nitrogen content on a DM basis. This is because there is no nitrogen in sand. Thus a suitable N content on a dry weight basis is another measure of quality. The N value of compost on a wet weight basis combines the above two measures. However, it is still possible to have organic matter containing N in the compost that is not available to the mushroom mycelium. This is where the compost maker's skill is important. The amount of straw used to make a ton of compost and the degree of its breakdown are also important. These are two further simple compost quality measurements.
The balance between compost's DM and moisture content is also vitally important. The mushroom mycelium requires a certain amount of water in the compost to enable it to utilise the DM. Too much DM is just as bad as too little. Each compost maker has ideas as to what is best.
Different growers also get best results from different composts, depending on how well they conserve its water content, how long they spawn run, how long they crop. I like to see bunker compost structure half way between straw and 'powder' with a wet weight N content between 0.65 and 0.80.
A major influence on mushroom quality is freedom from weed or disease moulds in the compost, particularly of Penicillium, as dealt with in last month's article, or of Trichoderma, as dealt with in several previous articles.