Optimising mushroom quality

Transcription

1 Factsheet 15/04 Mushrooms Horticultural Development Council Bradbourne House East Malling Kent ME19 6DZ T: F: E: Optimising mushroom quality Kerry Burton and Ralph Noble, Warwick-HRI This factsheet collates several HDC funded projects to provide information on cultural and environmental factors that influence mushroom quality. It describes steps which may be taken by growers during growth, or post-harvest, to improve mushroom quality. Background Growing mushrooms of the highest quality usually involves forfeiting maximum yield. In addition, those cultural factors which can influence quality can sometimes have opposing effects. For example, growing first flush mushrooms on wet casing reduces their susceptibility to bruising, but this may affect cleanness adversely. In such instances, the grower must decide which aspects are of particular importance or are causing a problem. Factors other than quality and yield also need to be borne in mind for example, water management of casing materials and ease of handling. This factsheet aims to show growers how to avoid particular quality pitfalls, and how quality can be maintained to the point of sale. Table 1, page 7, summarises the factors influencing mushroom quality, remedial action that grower can take and any cautionary advice to be aware of. 1 Consumers judge mushroom quality on colour, texture, cleanliness, maturity and flavour attributes

2 What is quality? Consumer surveys have concluded that mushroom quality is judged on colour, texture, cleanness, maturity and flavour. Most supermarket rejections for white buttons or cups relate to discolouration or bruising, some for cleanliness and maturity and rarely for texture or flavour. However, the consumers decision to make repeat purchases will depend on all quality factors. The definition of mushroom quality concentrates on the vast bulk of mushrooms sold in the UK which are white buttons or closed cups mushrooms. However, many quality aspects of white buttons or closed cup mushrooms are applicable to flats or browns apart from the obvious ie stage of maturity or cap colour respectively. Mushroom quality attributes are as follows: 1 Colour Mushrooms must be uniformly white, free from any signs of brown discolouration (from bruising, disease), and free from water-soaked areas. 2 Texture Mushrooms must be firm in texture. 3 Clean and damage-free Mushrooms should not be contaminated with casing material, pests or spores and should be free of damage caused by pests or during picking. 4 Maturity Mushrooms caps need to be of regular, approximately circular shape and of uniform size within a batch. 5 Flavour This is the most subjective of all of the quality factors and is part of the reason why different consumer groups prefer buttons, browns or flats. Flavour is considered to increase with maturity. Factors affecting mushroom flavour are not covered in this factsheet. 2 Bruised mushrooms are a major reason for rejection by consumers 3 Mushrooms must be free from discolouration, casing material, pests and diseases 1 Colour When brown discolouration occurs it is usually due to bruising. Postharvest ageing or diseases causing cap spotting may also cause discolouration but to a lesser extent. Cap spotting should be detected during crop growth. It can be caused by various species of Trichoderma, Aphanocladium or Dactylium and locating and controlling the source of spores is the only way to control it. Bruising Mushrooms are fragile items of produce and are highly susceptible 4 Cap spotting causes discoloration and should be detected and acted upon during crop growth

3 to bruising as a result of mechanical damage during picking, packaging and transport. Mushroom bruisability varies from crop to crop and also within a crop and this variation is due to differences in crop agronomy and environment. The factors with the greatest influence relate to water, particularly casing water and humidity. Casing water When first flush mushrooms are grown in wet casing they are more resistant to bruising than mushrooms grown in normally watered or dry casing. Casing water has little effect on the bruisability of second flush mushrooms. However, for the third flush the trend is the reverse of the first flush: mushrooms grown on dry casing are the most resistant to bruising. To minimise the risk of bruising, start the crop by growing in very wet casing (and at high humidity) and then add low volumes of water allowing the casing to dry in time for the third flush. Do not water for at least two days before each flush. Casing materials with a higher water holding capacity can be left unwatered for longer periods before each flush, than casing materials that have lower water holding capacities or a propensity for panning. Humidity Growing in high humidity (92% relative humidity, RH) results in more bruise resistant first and second flush mushrooms than growing in low humidity (85% RH). Compost type Compost type has a small effect on mushroom bruisability. Growing mushrooms on more strawy, less degraded composts results in slightly less bruising than when growing on the darker, more degraded composts. Casing composition Casing composition can have a small effect on mushroom bruisability. In dryish casing, a casing with 30% sugar beet lime results in less bruising than mushrooms grown in 9% sugar beet lime. Mushroom strain Mushroom strains can have an enormous effect on bruisability, but modern hybrid strains have already been selected on the basis of bruise resistance. Picking and handling practices It is necessary to find out when bruising takes place ie in the growing room, packhouse, during transportation from packhouse to retail outlet or in the retail outlet. The degree of bruising should be established ie are all mushrooms bruised or only a few; are there small discrete bruises on mushrooms or is most of the surface of mushrooms discoloured brown? If most of the mushrooms are bruised then it is a sign of too much mechanical damage at or after harvest. If bruising is occasional then it is recommended to check whether this is associated with a particular picker or house. 5 Growers should routinely check casing water potential using a tensiometer or similar equipment as this has the greatest influence on mushroom bruisability 6 If bruising is occurring, identify if it takes placing in the growing room, packhouse or during transportation

4 2 Texture The agronomic and environmental factors which influence firmness (texture) can be different from the factors which influence bruisability. The factors with major influence on texture are casing depth, compost depth and carbon dioxide concentration. Compost and casing depth Mushrooms are firmest when grown on a shallow casing layer or on deep compost. When these factors are combined, the mushrooms produced are very firm. Compost supplements and different casing types have not been shown to affect firmness. Carbon dioxide levels High carbon dioxide levels during growing reduces firmness. The optimum CO 2 level is around 800 ppm. Mushrooms grown above this level eg at 1000 or 1200 ppm CO 2 will be softer. It is likely that this trend continues beyond these values. 3 Cleanness and damage The most important factor affecting cleanness is casing moisture, with a wet casing at picking resulting in dirtier mushrooms than a casing that has dried slightly before picking. Casing type, pinning pests and diseases also influence cleaness and damage. Casing type and pinning Wet-dug brown peat produces the cleanest mushrooms, followed by milled brown or black peats, and then very young blonde peats. Deep pinning is an obvious cause of dirty mushrooms. To avoid deep pinning, avoid low temperatures during case-running (caused by overactive compost), ensure airing does not occur too early and check the calibration of the CO 2 and temperature measuring systems. The rate of application, or type of sugar beet lime or chalk do not affect cleanness. Pests Contamination of mushrooms by pests, even in small numbers, can render a whole consignment unmarketable. Typical symptoms include larval tunnelling in stipes caused by sciarids, and by the presence of white or orange cecid larvae, usually at the junction of the stipe and gills. Adult sciarid or phorid flies are of particular concern in over-wrapped punnets. Contamination of mushrooms with mites, of which red pepper mites are most important, will also render mushrooms unmarketable. Diseases Cap spotting can be caused by a range of fungal and bacterial pathogens. Verticillium and Mycogone also cause distorted mushrooms that are unmarketable. For details of symptoms and identification of mushroom diseases, as well as control measures, see HDC Factsheet 17/00. 7 Deep pinning is an obvious cause of dirty mushrooms 8 Adult sciarid fly larvae create tunnels in stipes leading to consignment rejection

5 4 Maturity and shape Factors influencing maturity and shape include pinning regime, casing type and composition, and compost and casing moisture. Pinning regime Mushroom size is largely determined at pinning. To avoid too many pins and small mushrooms, avoid rapid and severe airing (ie rapid temperature and CO 2 reduction). Casing type and composition The use of milled brown peat casing will increase the number of initials and small button mushrooms. A heavy casing produced from black wet-dug peat and a high rate of sugar beet lime (25%v/v) will reduce the number of initials and produce larger mushrooms. Compost and casing water Premature opening of mushrooms can be caused by insufficient casing moisture or compost nutrition (particularly in the third flush), or by the presence of virus. Check that casing and compost moisture analysis are normal. The exact causes of severe distortions such as split stems, sagging socks and distorted caps are unclear, but they are often most 9 Too many pins can lead to small mushrooms developing so avoid rapid and severe airing prevalent on wet compost and wet casing, particular in inter-flush mushrooms. Leggy mushrooms are a symptom of excess CO 2, and/or dry casing (but can also be caused by virus). Mushrooms with watery blemishes on the caps are caused by a combination of wet casing on dry compost; a high cropping room humidity (95% and above) exacerbates this problem. Air flow humidity, and casing contamination The most common abiotic disorder is probably scaling, an indication of excess air flow and/or low humidity. The causes of hard-gill, where mushrooms have little or no gill tissue, are unknown but some strains are more prone to the disorder than others. Contamination of the casing with hydrocarbons or overdosing with some pesticides can cause rosecomb, where gill tissue develops on the surface of the cap. Vapours from disinfectants and tray dips can cause brown discoloration of mushrooms. 10, 11, 12 Distorted caps, sagging socks and, watery caps and flesh are often due to incorrect water levels Maintaining post-harvest quality Discolouration and texture loss are the two main factors that lead to a reduction in post harvest mushroom quality. Mushroom firmness decreases after harvest, partly as a result of water loss. A number of techniques exist that can minimise reductions in post harvest quality including cooling and packaging. Cooling Cooling is the most effective postharvest technology in conserving mushroom quality after harvest as it stops heat production from the mushrooms and associated water loss (respiration). Heat production continues after harvesting and it is necessary to quickly remove this field heat from mushrooms before storing them at a low temperature. Mushrooms are frost sensitive and so they should never be

6 cooled below 0ºC but they should be cooled as low as possible without running the risk of freezing (ie 2 4ºC). Cooling equipment Equipment to remove field heat includes vacuum coolers and forced ventilation coolers. Vacuum cooling is rapid (25 mins to reduce 5 pallet loads of mushrooms by 10ºC) but incurs significant weight losses (1.8% per 10ºC cooled). Forced ventilation through boxes of mushrooms in conjunction with evaporative coolers can also achieve fast but not quite so rapid cooling as vacuum cooling. Air is normally de-humidified during evaporative cooling which can lead to high weight losses from mushrooms (3% per 10ºC cooled). However, use of wet-air or ice-bank cooling reduces this problem to about 1.3% per 10ºC cooled. Overwrapping Overwrapping films can reduce quality-loss after harvest by modifying the atmosphere in the pack. Growers should routinely check the oxygen concentration in overwrapped punnets to ensure it does not fall below 4% and thus avoid the growth of anaerobic, hazardous bacteria. 13 Forced ventilation through boxes of mushrooms, in conjunction with evaporative coolers, achieves rapid cooling 14 Growers should routinely check the oxygen concentration in overwrapped packs to ensure it does not fall below 4% Quality standards for mushrooms The European Commission introduced marketing standards for fresh cultivated mushrooms on 1 January The provisions concerning quality define minimum requirements, and classification requirements for Extra Class, Class I and Class II. There are provisions concerning sizing and stipe length, and provisions concerning tolerances and presentation (uniformity and packaging). Provisions concerning quality Minimum requirements All mushrooms must be: Intact Sound; produce affected by rotting, severe browning in the stalk or deterioration such as to make it unfit for consumption is excluded Clean, practically free of any visible foreign matter, other than casing material Fresh in appearance, taking account of the typical gill colour of the commercial type Practically free from pests or pest damage Free of abnormal external moisture Free of any foreign smell and/or taste. Classification Mushrooms in the Extra Class must be: Of superior quality in shape, appearance, development, colour and form Free from defects, except very slight superficial defects, providing these do not affect the general appearance or quality of the produce Practically free of casing material. Classes I and II allow slight defects and defects respectively, provided the mushrooms retain their general appearance or essential characteristics as regards quality, keeping quality and presentation.

7 Table 1 Recommendations to optimise mushroom quality during production This table summarises the factors influencing mushroom quality, remedial action that growers can take and any cautionary advice to be aware of when undertaking this remedial action. Quality factor Symptom Influencing factor Recommendations Comments 1 Colour Bruised mushrooms Casing water Start the crop by growing in very wet casing. Then add low volumes of water allowing the casing to dry in time for the third flush. Do not water for at least two days before each flush This is the main factor that influences mushroom bruisability Humidity Grow at high humidity (92% RH) if possible Compost type Grow mushrooms on more strawy, less degraded composts This factor only has a marginal influence on bruising Casing composition If growing dry, use a high percentage sugar beet lime (30%) This factor only has a marginal influence on bruising Picking and handling Establish if bruising takes place pre-or post-harvesting and degree of bruising 2 Texture Mushrooms with poor firmness Compost and casing depth Grow mushrooms on a Be aware that shallow casing shallow casing layer, on deep can sometimes reduce yield compost or a combination of the two CO 2 levels Maintain CO 2 levels during growing around 800ppm Do not allow CO 2 levels to drop below 800ppm as this can result in overpinning. Above 1000ppm CO 2 results in soft mushrooms 3 Cleaness Dirty mushrooms Casing moisture The casing should not be wet and sticky during picking and should not be watered for at least two days before harvest This is the most important factor affecting mushroom cleaness Peat type Wet-dug brown peats produce the cleanest mushrooms Brown peats can lose structure during mechanical handling Air temperature and CO 2 levels (deep pinning) To avoid deep pinning, avoid low air temperatures during case-running caused by over-active compost and ensure airing does not occur too early. Check the temperature and CO 2 measuring systems Delayed airing will cause stroma

8 Table 1 continued Quality factor Symptom Influencing factor Recommendations Comments 4 Maturity and shape Too many initials and small button mushrooms Airing regime, casing type and composition Avoid rapid and severe airing (temperature and CO 2 reduction). Consider using black, wet-dug peat and high sugar beet lime 25% v.v. Premature opening Split stems, sagging socks and distorted caps Casing moisture and compost nutrition Casing and compost moisture Check that casing moisture and compost analysis are normal Check for changes in compost or casing moisture or watering regime, avoid wet casing and wet compost between flushes Leggy CO 2 levels and casing moisture Check calibration of CO 2 measuring system and that casing is not too dry Watery blemishes Casing and compost moisture Avoid wet casing on dry compost and high cropping room humidity (>95%) Scaling Air flow and humidity Reduce air flow and/or increase humidity Low air flows and high humidity can encourage bacterial blotch Rosecomb Casing contamination Avoid contamination of casing with hydrocarbons or overdosing with pesticides Further information This factsheet has been compiled from information in: HDC reports M 8, M 16, M 19, M 19a, M 20, M 20a, M 20b, M 35, M 37, M 40, M 40a, M 100b HDC Factsheets 40/97 Peat for casing (M 20b), 17/00 Mushroom Diseases: Integrated Control. Further information: A full copy of the final reports for all HDC projects listed in this factsheet are available from the HDC office ( ). October 2004 Whilst publications issued under the auspices of the HDC are prepared from the best available information, neither the authors or the HDC can accept any responsibility for inaccuracy or liability for loss, damage or injury from the application of any concept or procedure discussed Horticultural Development Council. No part of this publication may be reproduced in any form or by any means without prior permission of the Horticultural Development Council. Design and production: HDR Visual Communication