6.1 Fresh Mushrooms 135

6.1.1 Introduction 135

6.1.2 Commercial Growing Practices 136

6.1.3 General Composition 137

6.2 Microbiology of Fresh Mushrooms 138

6.3 Spoilage of Fresh Mushrooms 139

6.3.1 Sources of Microorganisms Causing Spoilage 142

6.3.2 Cultural (Growing) Practices Favoring Spoilage 142

6.3.3 Cultural Practices to Suppress Spoilage of

Fresh Mushrooms 142

6.3.4 Postharvest Conditions Favoring Spoilage of

Fresh Mushrooms 146

6.3.5 Postharvest Practices to Suppress Spoilage of

Fresh Mushrooms 147 Packaging 148 Washing Treatments 149 Irradiation 150 Pulsed Ultraviolet Light Treatment 151

6.4 Conclusions 152

References 153

6.1 FRESH MUSHROOMS 6.1.1 Introduction

Based on phylum classification, fungi are classified as Ascomycota, Basidio-mycota, Chytridiomycota, Deuteromycota, and Zygomycota [1]. While edible fungi such as truffles and morels belong to the phylum Ascomycota, most commercially cultivated edible fungal genera including agaricus, lentinula, and pleurotus belong to the phylum Basidiomycota.

Mushrooms, the common name for a large group of edible fungi, are a common and popular food product. The reproductive portion or the fruiting body of the mushroom usually lies above the growing substrate. It is the portion that is commonly used for consumption. Because of their unique earthy aroma and taste, many wild mushroom species have been traditionally consumed. However only a few mushroom species have been extensively cultivated on a commercial basis [2].

Agaricus bisporus (J. Lge) Imbach (button mushroom) is the most widely cultivated species of edible mushroom, representing approximately 32% of world production in 1997 [3]. China, the U.S., and the Netherlands are the top three producers of A. bisporus in the world [4]. Lentinula edodes (Berk.) (shiitake) and Pleurotus ostreatus (Jacq.:Fr) Kumm. (oyster mushroom), the second and third most cultivated edible mushrooms, account for approximately 25 and 14% of world production, respectively [3,4]. Commercial mushroom production makes a significant contribution to the total agricultural output of the U.S. In 2002-2003 the U.S. mushroom crop totaled 844 million pounds, valued at $889 million. White and off-white A. bisporus mushrooms still have by far the largest market share, in particular in the western hemisphere, accounting for about 80% [5].

Since agaricus is the major genera of cultivated mushrooms around the world, this chapter mainly describes the microbiology and microbial spoilage of the cultivated button mushroom, A. bisporus.

6.1.2 Commercial Growing Practices

The agaricus mushroom growing process is unique in that it requires decomposed organic matter as both a substrate for growing and as a source of essential nutrients. A typical growth substrate contains straw-bedded horse or chicken manure, hay, corn cob, brewer's grain, cotton seed, cocoa seed hull, and water. The substrate mixture is aerobically fermented under semicontrolled conditions [6,7], a process known as Phase I composting. Ingredients are mixed and placed in aerated bunkers or formed into long rows that are periodically turned, watered, and reformed. Rapid microbial growth over a 15- to 25-day period causes the substrate (compost) temperatures to reach as high as 175°F (80°C). During the Phase I process substrate nutrients are converted into forms efficiently assimilated by the mushrooms.

Phase II composting begins when the finished substrate is transferred in bulk into controlled atmosphere tunnels, or in trays into controlled atmosphere rooms where further microbial activity and nutrient conversion occur. Phase II includes a controlled pasteurization step designed to eliminate mushroom and human pathogens [8], weeds, and insect pests. A successful crop requires that the compost temperature reach 130 to 140°F (60°C) for at least 2 hours [9].

Agaricus mycelial starter cultures grown on cereal grains, commonly known as mushroom spawn, are then mixed into the substrate and allowed to

Agaricus fruiting body Casing layer

■#> Myceiia colonizing compost > Compost

FIGURE 6.1 Schematic diagram of the mushroom substrate (compost), the peat-based casing layer, and fruiting bodies.

grow throughout the compost for 14 days. Following complete colonization of the substrate by A. bisporus mycelia, a two-inch casing layer (consisting of peat soil amended with calcium carbonate and water) is applied on top of the colonized substrate bed (Figure 6.1). The casing soil enhances retention of irrigation water on the growing beds, and promotes mushroom fruit body formation.

Mycelial growth occurs throughout the substrate and into the casing layer. After 14 to 21 days, mushroom primordia are formed at the fruiting stage known as pinning. The primordia develop into mature fruiting bodies over a one-week period. During the development process, the growing beds are irrigated to maintain substrate moisture, prevent disease, and maintain postharvest mushroom quality [2,10,11]. At maturity, the mushrooms are harvested, stipe-trimmed, packaged, and moved into cold storage.

6.1.3 General Composition

Edible mushrooms, especially A. bisporus (button), tend to be high in moisture. Mattila et al. [12] reported that the dry matter (percent solids) content of A. bisporus grown in Finland was 7.7%. These values for A. bisporus mushrooms are similar to those normally experienced in North America, but moisture can be as high as 95% when mushrooms are excessively irrigated [13].

Mushrooms contain large amounts of carbohydrates including polysac-charides (such as glucans and glycogen), monosaccharides, and disaccha-rides (such as trehalose), sugar alcohols (such as mannitol), and chitin. Mattila et al. [12] reported that A. bisporus contained 4.5% (fresh weight) total carbohydrates. Most of the polysaccharides are structural components of the cell walls — chitin and glucans — and are indigestible by humans and can be considered as dietary fiber. The A. bisporus mushroom species is also known to contain significant amounts (20 to 30%, dry weight) of the sugar alcohol mannitol, and 1 to 3% of the disaccharide trehalose [14].

While mushrooms contain only low levels of crude fat (0.31 to 0.35%, fresh weight) [15,16], they contain a significant amount of protein, vitamins, and minerals. Mattila et al. [12] found that A. bisporus mushrooms contained about 2.0% net protein (fresh weight). These mushrooms are also known to be

FIGURE 6.1 Schematic diagram of the mushroom substrate (compost), the peat-based casing layer, and fruiting bodies.

high in the B-complex vitamins: niacin, folate, pantothenic acid, and riboflavin [17]. It was found that A. bisporus mushrooms contained almost 0.4% riboflavin (fresh weight) [12]. With respect to minerals and trace elements, A. bisporus mushrooms contain relatively high concentrations of potassium (0.36% fresh weight) [12], copper (0.22% fresh weight) [18], and selenium (3.2 and 1.4mg/kg, dry weight for brown and white A. bisporus strains, respectively) [12].

From the standpoint of nutrients, fresh mushrooms are capable of supporting growth of microorganisms. Agaricus mushrooms have a neutral pH value, and fall in the category of foods with a water activity of 0.98 or higher. These factors favor the growth of microorganisms, leading to the microbial-induced quality degradation and spoilage of fresh mushrooms.

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