#315 STS Lab Note: Hot vs. Cold Pasteurization – The Straw Guide

Cold pasteurization using hydrated lime achieves the necessary pH shift to inhibit competitors without the high energy costs
or equipment required for traditional heat-based methods. Compare high-heat and chemical-based pasteurization techniques for straw cultivation. This guide offers low-tech, cost-effective solutions for achieving professional sterilization results without the need for expensive boilers or pressure vessels.

Summary

Introduction

For the first time grower it is advised to grow White Oyster mushrooms, Grey Oyster or Brown Oyster on pasteurized straw. The straw can be hot or cold pasteurized and will also mention a third way which combines both and currently my preferred prep method – Bundu Tek. Among the many benefits of pasteurized straw cultivation are:

1. Straw is cheap and readily available, can get your local co-op.
2. No need to pre-treat substrate, such as compost for button mushrooms can take up to a month to ‘ferment’ correctly.
3. No sterilization required as supplemented sawdust or spawn, drastically reducing overheads and time/effort.
4. With pasteurization minimal products (overheads) required. Very cost-effective using buckets and Bundu Tek which eliminates waste as buckets can be cleaned and reused.

Hot Pasteurization

The partial sterilization of a product, such as milk or wine, to make it safe for consumption and improve its keeping quality. “Pasteurization reduces the risk of food poisoning.”

Oxford Languages

In the summer of 1864 while on holiday in Arbois, France, Louis Pasteur decided to remedy the acidity of the local wines… he found out experimentally that it is sufficient to heat a young wine to only about 57 °C (135 °F) for a short time, and that the wine could subsequently be aged without sacrificing the final quality. Thus started “Pasteurization” and was modified to many other foodstuffs including milk. Milk is pasteurized by heating it to about 145°F (63 °C) for 30 min and as not boiling it does not affect the taste.

During his 1864 summer vacation in Arbois, France, Louis Pasteur resolved to address the acidity issue in local wines. His experimental findings showed that heating young wine to about 57 °C (135 °F) for a brief period allowed it to age without compromising the end quality. This marked the beginning of “Pasteurization,” which was later adapted for various other foods, including milk. Milk pasteurization involves heating it to approximately 145°F (63 °C) for half an hour, a process which does not boil the milk, thus preserving its taste.

For mushrooms this leaves a more ‘natural’ substrate as mycelium grows in symbiosis -often called a ‘selective’ substrate. In practice you will need a large steel drum as your ‘pot’ -with Gas burner beneath. Put straw in bag that can take boiling temperatures or steel chicken wire basket, and weigh down with brick/cement block, and fill drum till water over straw container. Start gas burner and monitor temperature, when temperature reaches 60°C start your timer. Leave burner on and periodical monitor temperature, when the temperature raises over 70 °C, close burner. If temperature drops below 65°C turn burner on again until the time is up.

To put it simply, the harmful bacteria are eliminated, while the beneficial bacteria, known as thermophilic bacteria, are left untouched. These bacteria create a biological shield that allows mycelium to grow on the substrate. I believe the thermophilic bacteria that forms endospores, known as Bacillus Subtilis or the ‘Hay Bacillus’, plays a pivotal role. However, it’s important to note that the notorious ‘Wet Spot’ or ‘Sour Rot’ is also a type of Bacillus, yet it still functions in the microbial process.

Cold Pasteurization of straw.

There is another way and this is the so-called ‘Cold Pasteurization’ of straw, which uses Calcium Hydroxide [Calcitic hydrated lime] to rapidly raise the pH of the substrate and this kills the bacteria. This method was initiated by Dr Stoller, later this was bought over by Aloha Medicinals. Take note that must use Calcium hydrated lime – not builders lime, or lime high in Magnesium. This method is often also referred to as “Low Tek“

Herewith a simple protocol for cold pasteurization:

1. Cut straw into lengths of 1-5cm. This can be achieved by placing the straw in a rubbish drum and using a bush-cutter to chop it. This phase of the task is the messiest. The straw can be utilized as it is, but the yield will be reduced and bagging will be more challenging. Cutting the straw into smaller pieces enhances the surface area and physically disrupts the cell structure, making it easier for the mycelium to process nutrients. Additionally, it improves the straw’s water-holding capacity, which is crucial since mushrooms are 90% water. Other options to consider include gaining access to a hammer mill or acquiring straw pellets.
2. Soak straw in lime water overnight [12-24 hours], in old pillowcase, add 1 tablespoon lime per 10L water used [Or .
3. Drain pasteurized straw – can be done on large sieve, such as shade-net – or in old pillowcase, until no more excess water draining out.
4. Combine the mushroom grain spawn with substrate, in a 2-3% spawn ratio for substrate dry mass. As a guide, a 20 Litre bucket can accommodate 1 kg of dry chopped straw. Although the straw can absorb water and increase to 5 kg, it only requires 20-30g of spawn per bucket [equivalent to 2-3 Tablespoons].
   We also spawn at a higher level of 3% based on the wet weight for other non-oyster genus mushrooms (150g).
5. Stuff pasteurized straw / spawn mixture into an appropriate fruiting container. Quite often poly bags used [from lay-flat to black rubbish bags], but can be done with Buckets. Note that holes must be made about every 5cm in staggered layout, also make sure some holes at ‘bottom’ to allow excess water to drain.
6. Let colonize at room temperature in dark place, e.g. cupboard. This should take 7-14 days and will be seen with “mycelium running” through inoculated pasteurized straw substrate until a white web envelopes all. Oyster mushrooms will ‘pin’ naturally near holes which shows time to expose to fruiting conditions.
7. Move to a fruiting chamber or some other appropriate environment to initiate mushroom fruitbody growth. This can be trickiest part of the operation as have to have 80-90 % Relative Humidity and high fresh air exchange [FAE of 4-8 room volume per hour]. Though mushrooms must have light, try to keep in shade – cut slits in bags for mushrooms to grow out of.
8. If one of the pigmented Oyster grown, such as Pink Oyster, then have to expose to light to bring out color. Also been found that the pigmented varieties develop a richer color if grown on hard wood.
9. Harvest and enjoy!

Both methods work, but for the ‘cold pasteurization’ must use straw not hay [hay = straw + seeds]. This is not a deal-breaker – the image on top was using hay and will get some seeds sprouting but this will not affect mushrooms. These methods can also be used for other ‘artificial’ substrates such as Coir. Traditionally these methods are used with fast colonizers like oyster mushroom. But we have grown Shiitake, in filter bags.

This is the traditional methods of pasteurizing substrates, both are used in Bundu Tek. Please read Blog on Bundu Tek.

Pitfalls of Pasteurization

The main issue with both pasteurization methods is the moisture level, particularly when dealing with processed materials like feed pellets or protein supplements. It’s critical to ensure that the pasteurized substrate is thoroughly drained. This process might take a few hours if supplemented, as it’s been observed that the supplement – ranging from high-protein pellets to bran – can turn into a soggy mess and not drain properly. It may require manually squeezing out the excess water, a task that is not only inconvenient but also raises the risk of contamination.

If you’ve completed pasteurization, inoculated with spawn, and bagged the substrate, it’s crucial that there’s absolutely no water pooling at the bottom of the bag. This can cause anaerobic conditions and result in bacterial contamination. A simple solution is to include up to 10% dry vermiculite by volume which can absorb the extra water and restore an aerobic environment for the mycelium runs.

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Socratic Questions:

1. What is the difference between hot and cold pasteurization for mushrooms?
2. How to perform cold lime pasteurization on straw?
3. Best low-cost pasteurization methods for small-scale farmers?
4. How to prevent contamination on pasteurized straw?
5. Does hot pasteurization produce higher yields than cold?