Ending quality loss

Mushrooms with the same excellent quality from farm to fork should be the aim of the entire chain. But things aren't that simple, as is obvious if you examine the quality of mushrooms on sale in shops or markets. There’s clearly plenty of room for improvement. Where do things go wrong and how can growers make a maximum contribution to premium quality right up to the shop shelves? Jos Hilkens has investigated and inventoried the processes - and what turns out? Here too, the weakest link determines the performance of the whole chain.

Published in Mushroom Business 16, May 2006

By Jos Hilkens, AdVisie 'de champignonteeltadviseurs'
hilkens@mushroomconsulting.nl

When mushrooms are harvested the flow of nutrients ceases and mushrooms cannot become heavier. Normally, quality can only deteriorate post-harvest too .If growers were only able to produce quality I ‘super’ then these mushrooms – under good conditions for the rest of their journey- should also reach consumers as quality I. However, growers also harvest normal quality I mushrooms, that, before they are actually in the fridge waiting to be eaten, have dropped to a quality grade I-2 or II. If growers pick at the lower limit of quality I, then the mushrooms rate a quality III before they are consumed. It's impossible for growers to only supply ‘super’ quality I so addressing the loss of quality in rest of the chain is essential.

What is quality?
If we examine the criteria used by inspectors to grade mushrooms, the main aspects are colour; the development stage (closed, cupped and open); the firmness of the cap and stalk; any blemishes and soiling; patchiness (brown patches, Bacterial blotch etc.); stalk length; size and in particular, size variations within the grade; and finally the general presentation of the mushrooms in the crate or punnet.
Colour is the major criterion when assessing quality and most mushrooms are rejected on this detail, particularly if trade is difficult.
Discoloration is mainly the result of damage to the cell walls caused by infection from moulds and bacteria, mechanical damage, dehydration caused by the climate, over pressure on the cell walls from within and ageing cell tissue. This list demonstrates how much can go wrong, even regarding ‘colour’ alone.

The grower
It’s a noble aim of growers to manage growth to achieve production of 95 % or more quality I mushrooms with perfect keepability properties. Achieving this result is, however, not an easy task. All the individual pieces of the puzzle must fall into place. Especially as there are so many factors on a farm that influence the quality of the final product. Let's take a look at the main aspects. 

Raw materials and filling
Concerning raw materials a good starting point is filling with phase III (incubated) compost. In the Netherlands this has become customary in the past decade, but in the emerging production countries phase II (inoculated compost) is still often used. This gives lower production and usually fewer quality I mushrooms.

With phase III compost, growers aim for an ideal filling weight of 88 to 90 kg/m2 in two flushes and 95 kg/m2 in three flushes. The more compost, the more nutrition, meaning more spontaneous growth and a better chance of good quality. With a 5 kg/m2 higher filling weight it should be possible to harvest 2 % more quality I, in other words 0.8 kg/m2. A higher percentage should normally also be feasible if a higher filling weight is used. A point worth considering. 

The advantages of supplements shouldn't be underestimated either. Supplementing from 14 to 18 kg/ton using products based on soya meal with a raw protein percentage of 42 to 48 % is necessary for well incubated compost. Seen over a year the average production and quality will rise. Good quality compost, a good spread of supplements and controlling the compost temperature are obviously pre-conditions. 
Casing soil remains a neglected area and price is still too often a deciding factor. A missed opportunity, as good quality casing soil can certainly make a big difference. How easily can casing soil absorb water after the first flush, how firm and compact is the casing soil, how well does the upper layer dry out. All key factors to be considered for the good exchange, which is vital for optimal quality.

Filling the beds signals the start of the cultivation cycle and must be done as uniformly as possible. As well as the filling depth, the position of the pressure roller should also be considered. Short structured, wetter compost must obviously be filled more loosely than coarser, drier compost. Growers will also fill compost more compactly  (16-18 cm) if they have more problems controlling the compost temperature. With casing soil how and how many times the soil is handled from the moment it is unloaded from the lorry until it is spread over the actual bed are influential factors. Handling greatly determines the final casing soil structure. The extent of cac-ing also plays a role in the even distribution and amount of mycelium in the casing soil. The aim should be a constant amount of cac-ing.

Cultivation technique
With blow down it’s important to achieve the right number of pieces, all well distributed over the bed. Too many mushrooms on the bed will become rivals. Being packed together results in an accumulation of waste substances and a higher temperature leading to long stalks and causes the mushrooms to start ripening prematurely. Not the desired state of affairs, as the colour and firmness will start to deteriorate from this point. An even spread of fruits is good for the quality, as no more than 5 kilos of mushrooms per square metre need to be picked per day. If 8 to 10 kg/m2 are picked per day, the speed of growth is too high and the compost temperature will sharply rise (more than 2 degrees Celsius per day), with a negative effect on the mushrooms still on the beds.

Watering on the mushrooms is necessary for some growers to maintain quality, but growers that don't spray - or only spray once - harvest on average better quality, with less discoloration and better keepability - the same applies in the second flush. 

Techniques
Growers who improve the climate using cooling and humidity frequently experience more problems relating to mushroom quality. The installation’s reaction speed (for example start dehumidifying at an RH of 1 % too high) is important, and with dehumidifying (a lot of cooling and heating) things often go wrong. Under capacity of the heating units and too low freon/cold water temperatures of 4 to 6 degrees instead of 8 to 10 degrees Celsius and a heating water temperature of 85 degrees instead of 40 to 60 degrees Celsius are also likely to cause problems. The same applies for too narrow outlets.
Temperature fluctuations within the space of an hour can already cause problems too.  A central duct can contribute towards more stable and even control.
Working with plenty of fresh air, water humidification alongside steam humidification and setting a minimum valve position can help solve problems in many cases.
An increasing evaporation during pinheading is very important. If the mushrooms are already discoloured, then it's too late and taking action won’t achieve anything. Measuring more frequently and adjusting the measurement method are necessary to provide continuous insight into evaporation levels in the growing room. Control based on moisture deficit, inlet temperature and RH are all positive, but more can still be done.

Harvesting
The greatest problems with mushroom quality in the first flush occur on the first picking day (grey colour and water blotches) and the last picking day (veils, long stalks and grey discolouration particularly at the edge of the cap). In the second flush most problems occur on the final picking day (ripening) and with the third flush most problems relate to mushroom colour.

The pickers determine the quality and production of mushrooms from the very moment that harvesting starts. The picker decides whether a mushroom has reached the required quality or not and which mushrooms are left on the beds until the following picking cycle or the next day.

It’s vital that supervisors provide clear picking instructions per picking cycle. These instructions should include how to hold and twist the mushrooms from the bed, the correct holding position in the hand, only to pick maximum two to three mushrooms per hand per time, cut off the stalks straight, pay attention to the stalk length, set the picked mushrooms to one side using one hand, face the cap upwards and then not touch the mushrooms anymore. The graze picking system (several times a day).