Filtering Maple Sap
Table of Contents
To produce high quality maple syrup every effort must be made to maintain high quality sap that is relatively free of microorganisms from the taphole to the evaporator.
Various species of bacteria, yeast, and mould may be found in maple sap. Sap is an ideal growth medium for microorganisms because it contains sugars (largely sucrose), minerals, and amino acids suitable for microbial growth and reproduction.
Growing microbial populations have two effects on sap. Firstly, enzymes secreted by microorganisms break down sucrose into glucose and fructose, which causes a darkening in syrup colour and a caramel taste; and secondly, microorganisms can cause off-flavour and increase maple syrup viscosity. These effects are intensified as the temperature warms and microbial growth increases significantly.
Research suggests that early sap flow reduction and stoppage in tapholes is also related to excessive growth of microorganisms. Therefore limiting, controlling and reducing microbial activity can have a two-fold benefit for maple producers: more sap and better quality sap and syrup.
There are a variety of methods to control or reduce microbial activity in maple sap. They include sanitary tapping, keeping sap cool in the sugar bush and storage tanks, boiling sap soon after it runs, keeping buckets, gathering tanks and storage tanks properly covered to keep out debris, use of germicidal ultraviolet irradiation, cleaning and sanitizing equipment, and filtration of sap by various means.
Ensuring that the sap collection system is properly installed and maintained will also help control microbial activity.
Figure 1. Bucket covers help keep out debris that can introduce microorganisms into the sap.
Filtration involves pouring or pumping sap through a filter or series of filters to remove suspended materials, including some of the microorganisms. Suspended material in sap may include small bits of bark and wood, dust or dirt, insects and any other debris that might fall into sap buckets or open storage tanks. It is important to remove this material from the sap by filtering as soon as possible since debris in sap can be a source of microbial contamination. Filtration of sap is accomplished by using gravity and/or pressure type filters at appropriate locations in the sap transfer system.
The major objective of filtering maple sap is to maintain and/or improve the quality of the sap and the maple products made from it. To date no scientific studies have been conducted to evaluate the extent of the improvement. However, maple producers have reported that filtering sap increases the quality of their maple syrup by as much as one full grade by improving the colour class.
The activity of microorganisms influences the length of time sap can be stored. To increase the safe storage period for sap requires either complete sterilization of the sap (ultraviolet irradiation) or control of the microbial population by keeping it at a low level so that any biochemical changes due to microorganisms in the sap before processing are minimal. Maple sap filtration will not overcome spoilage caused by microbial activity occurring in sap collection system. However, if filtration is carried out properly and storage conditions are unfavourable for microbial growth, it will maintain the quality of sap during storage for a longer period of time.
Microorganisms in sap range in size from hundreds of microns to less than one micron. Organisms less than 40 microns cannot usually be seen without the aid of a microscope, while organisms smaller than 1 micron cannot usually be seen without the use of an electron microscope. A micron is a metric unit of measurement, which is one millionth of a metre.
Microorganisms can grow rapidly when conditions are favourable and some species will even grow below freezing. Growth of microorganisms normally refers to the growth of populations of cells, which is the increase in the number of cells not the growth of individual cells. Limiting and/or reducing the number of microorganisms in sap improves the quality of it and the syrup produced from it will be lighter in colour.
The filtration of sap to remove microorganisms has been improved by developments in filtration and purification of water. Both small pore water filters and diatomaceous earth (D.E.) filters are being used to improve sap quality before boiling since they are effective in removing some of the microorganisms.
The presence of microorganisms in sap does not constitute a hazard since they are destroyed during the final boiling process. However, filtration, along with other methods of controlling, limiting and reducing microbial growth, are essential for the production of high quality maple syrup and should be considered as an important goal by all maple producers.
A 1999 Ontario survey indicated that 71% of maple producers filter their sap, while a 2003 survey of nearly 500 producers indicated 81% filter sap. The most common filtering method was using a coarse screen, following by pop-on filter bags. These surveys indicated that there is still considerable room for expansion and improvement of sap filtering in the industry since every producer could benefit from it. The coarse filtering methods most commonly used could be improved by adding sap-filtering equipment that removes some of the microorganisms.
This Factsheet provides information to assist maple producers initiate or upgrade sap-filtering methods.
Placing covers on buckets to keep out rain and debris and using steel strainers in gathering tanks to remove twigs, buds, insects etc. has been practiced by maple producers for well over a century. To remove finer materials some producers placed closely woven burlap, flannel or fine screens over the metal strainers. Removing this material from the sap is important since these materials are sources of microorganisms that grow and reproduce in the sap, reducing its quality.
The practice of filtering sap as it was being dumped into storage tanks became popular 40-50 years ago. Producers made their own filter frames and used a press cloth prefilter or several layers of muslin over a hardware cloth screen.
Synthetic flat filters (Orlon« felt) were first used for filtering syrup in 1959. A non-woven rayon material called miracle cloth or maple prefilter paper was introduced at the same time. Synthetic prefilters and filters became popular with producers and gradually replaced most other materials.
Figure 2. Proper installation and maintenance of tubing systems along with thorough cleaning and sanitation procedures are necessary for controlling microbial growth in them.
As tubing systems replaced buckets there was less chance of coarse foreign materials getting into the sap. However microorganisms developing at the taphole and in the tubing were a problem. Thorough cleaning and sanitation procedures are important in controlling microorganism growth and development in the tubing systems.
The ability to control and destroy microorganisms in sap improved around 1960 when ultraviolet irradiation to sterilize sap was tested. A commercial inline ultraviolet unit was successfully tested on sap in 1963. However, this equipment was considered too expensive and did not become popular until the early 1980s.
In-line ultraviolet sterilizing units are effective in killing bacteria
and other microorganisms in sap. Research has found them more effective
in destroying bacteria than yeasts.
It was found that sap contaminated with microorganisms could be stored, after irradiation, for several days at temperatures up to 4°C (39║F) without any further deterioration due to microbial fermentation. The use of inline ultraviolet lights expanded in the 1980s along with the increase in the use of Reverse Osmosis machines, which required high quality clear sap to operate efficiently.
Reverse Osmosis was first tested for the concentration of sap at the USDA Maple Research Laboratory in 1966. Results from tests conducted in 1966 and 1967 indicated that Reverse Osmosis could be used for partial concentration of maple sap. This research also recognized that microorganisms could cause damage to the modified cellulose acetate membranes of the Reverse Osmosis modules or form slime deposits on the membranes, which were capable of causing a marked decrease in the flux or flow rate of water through the membranes. It was therefore necessary to devise means of reducing the microbial population in the sap to the lowest possible level before it was supplied to the Reverse Osmosis unit. Furthermore, methods had to be developed for the control of microbial growth in the Reverse Osmosis machine during periods of idleness.
Reverse Osmosis machines are equipped with one or more in-line sap filters
to remove contaminants prior to concentrating sap.
The first commercial Reverse Osmosis system for concentrating maple sap was installed in 1971. However, adaptation of Reverse Osmosis in the industry was slow with only six machines installed in the 1970s. This changed quickly in the early 1980s and by 1983 there were as many as 100 machines operating in maple operations in the United States and Canada. Reverse Osmosis machines are equipped with a small pore prefilter system capable of removing microorganisms down to 5 microns. Some producers use inline ultraviolet treatment of the sap before and after concentrating the sap. Others have installed small pore filters in the system before the sap reaches the Reverse Osmosis machine. Regardless of the methods used to reduce or destroy microorganisms, the efficiency and operation of the Reverse Osmosis unit will be enhanced and the resulting grade of maple syrup produced will usually be improved.
The introduction and use of small pore filters in the early 1980s was a major advancement in sap filtering. In addition to being used as a prefilter in Reverse Osmosis machines, small pore filters are used in various locations in sap delivery systems before reaching the evaporator. Inexpensive inline water filter housings made of reinforced polypropylene were found to be good for sap filtration and with disposable filter cartridges down to 5 microns, they greatly improved sap filtering by removing some of the microorganisms.
About 1988 an inexpensive sap prefilter known as a pop-on or pop filter was introduced. It is a bag type filter with a plastic adapter head that attaches to a sap line. It can be installed on the end of a mainline, on the vacuum extractor discharge or a pump station pipe. The bag filters used are in 50-100 micron range and will remove debris from sap prior to storage tanks.
Around 1990 diatomaceous earth (D.E.) sap filters were offered for sap filtering. This equipment was developed to filter water for swimming pools. When using it as a sap filter it is necessary to use food grade diatomaceous earth and to ensure that the filter is made of food grade approved material. The D.E. coating on the filter elements is called filter cake. It will remove micro-organisms down to the 1-3 micron range depending on the grade of D.E. used. The filter will clear up milky or cloudy sap, but several passes through the filter are necessary to do this. Maple producers using D.E. sap filters have found them to be effective in improving the clarity of maple sap and the quality of maple syrup. To date no specific research projects have been conducted on their use in the maple industry.
Microbiology is the science that deals with the study of microscopic forms of life generally referred to as microorganisms. In sap, several species of bacteria, yeast, and mould may be found anywhere from the taphole to the evaporator.
Research confirms that sap is sterile (free of microorganisms) while flowing in maple wood xylem vessels. Following tapping microorganisms are found in the sap coming from the tapholes early in the season due to contamination from tree bark. Microbes are introduced into the sap by unsanitary tapping procedures and unsanitary collection and storage practices induce the growth of microorganisms in sap. Further contamination of sap occurs when buckets, gathering tanks, tubing and storage tanks are not completely emptied, cleaned and sanitized between sap runs, especially during the warm periods that commonly occur during the latter part of the sap season.
Microbial growth and development starts out slowly in the early part of the season when temperatures are cool and increases considerably during warm periods, especially when the temperature of the sap rises above the 7-10°C (45-50°F) range. Bacteria account for most of the contamination. The growth of microorganisms is generally affected by three factors: temperature, time and nutrient availability. Increased temperatures favour rapid division of microorganisms, thereby increasing their population over time. As they divide and increase in number, they use sugars in the sap as an energy source.
It is important to realize that although microbial contamination can sometimes largely be responsible for the production of lower quality syrup, other factors relating to sap chemistry and processing are often the cause.
The appearance of sap provides a clue to its condition. Clear, colourless sap with few microorganisms will produce lighter grades of maple syrup. Turbid, milky sap is fermenting due to the presence of microorganisms and will produce darker-coloured maple syrup. Some bacteria and yeast species produce pigments discharged into the sap, causing it to become green or reddish in colour, depending on the microorganisms involved.
Bacteria are microscopic single cell organisms that reproduce by division into two identical cells. Under suitable conditions they multiply rapidly by each cell dividing into two daughter cells every 20-30 min., resulting in thousands of bacteria in a few hours. Some species of bacteria are capable of movement in sap while others can develop spores (e.g. Bacillus). Other species are able to form gummy capsules around the cell, which allows them to cling together in slimy stringy strands.
Genera of bacteria found in maple sap in early research projects included Pseudomonas, Achromobacter, Flavobacterium and Bacillus.
Low temperatures (cool sap) will reduce their growth and keep down the number of bacteria. However, low temperatures will not entirely prevent the growth of these organisms since some species can continue to grow slowly at temperatures below freezing since the sugar in the sap prevents it from freezing when water freezes.
Early research on microorganisms in sap found that some bacteria affect syrup flavour; others syrup colour and still others do not affect either. (More research on the presence of bacteria and their effect on maple sap and syrup quality is needed.)
They are microscopic single cell organisms. They reproduce asexually by budding or sexually by the pairing of two mother cells that can lead to spore formation. Yeasts are chief agents of fermentation. Most yeasts do not develop the high population levels in sap that occurs with bacteria, but because yeast cells are larger than bacteria, fewer are required to produce turbidity in the sap. Yeasts can grow with or without oxygen.
The two most numerous genera of yeasts identified in sap are Rhodotorula and Trichosporon. Other yeasts found as contaminants of tapholes include species of Candida, Torulopsis and Cryptococcus.
The presence of yeast in sap is more directly related to syrup colour than the bacteria populations. Syrup colour darkens or lightens as yeast numbers increase or decrease.
They are microscopic organisms that can be one or many cells. They are complex microorganisms that grow to the extent of being visible to the naked eye. Moulds produce thread-like filaments called hyphae, which branch like the roots of a plant. A collection of hyphae as seen with the naked eye is called mycelium.
They reproduce mainly by forming spores that can be transported by air currents, water or sap. Moulds usually grow on the surface of the sap in colonies forming a mat that is readily visible. They produce acids from the sugar in the sap and impart a characteristic moldy odour.
Several genera of moulds have been identified in sap, the main ones being Aspergillus and Penicillium. Low mould counts in maple tapholes have been found early in the sap season when the count of all microorganisms is quite low. They did not increase to high numbers later in the season like bacteria and some yeasts.
The two basic categories of sap filters used by maple producers are gravity flow and pressure. Gravity filters are commonly used in bucket operations where the sap is poured into gathering or storage tanks. Pressure filters are used in tubing and vacuum operations and in other situations where sap is pumped into storage tanks or between tanks. They are also used as prefilters in Reverse Osmosis machines.
The number of sap filters and types used in a maple operation vary depending on producer preference and the way in which sap is moved from the tree to the evaporator. To be most effective, sap should usually be filtered several times. In some situations producers pump sap through the same filter more than once in an effort to reduce or control microorganisms. Gravity filters remove mainly coarse materials from the sap and in doing so reduce the sources of contamination. In addition to coarse materials, pressure filters can also remove some of the microorganisms making them more effective in improving sap quality. In general, the finer the filter, the more effective the filtering process. When very fine filtration is desired, a series of filters down to as low as 0.5 microns are used to perform the filtering process in steps.
Regardless of the type of filter used, regular maintenance using proper procedures is necessary for them to be most effective in limiting and/or reducing contamination by microorganisms. Proper washing and drying of filters to avoid the risk of cross-contamination or damage to the filter and replacement of worn filters are important aspects of maintenance.
Gravity Sap Filters
Gathering Tank Strainers
In a bucket operation the first opportunity to filter the sap is when the sap is dumped into the gathering tank. All commercial metal gathering tanks made for sap collection have a metal strainer for removal of ice, bark, leaves, wood chips, and other coarse debris. To remove finer debris some producers use a fine nylon screen, which they place in a filter frame over the strainer in the gathering tank. Sap strainers in gathering tanks are important because the removal of this debris reduces the sources of contamination early in the process of moving the sap from the tree to evaporator.
Figure 5. Commercial gathering tanks are equipped with a metal strainer for removing ice, bark, leaves, wood chips and other coarse particles.
Sap should not be left in the bottom of the gathering tank between runs as this sap can contaminate fresh sap dumped in the tank on the next run. It is important to wash out the gathering tank and strainer at the end of each run with clean potable water.
Flat Sap Filters
Commercially produced sap filter frames have been available for many years. They are used to filter sap into storage tanks. Stainless or nylon screens are used in the filter frames to support a synthetic flat filter that is usually polypropylene. A polyester flat prefilter can also be used in flat sap filter frames. Wash any synthetic filter materials often in hot water to remove contaminants. Do not wring out the filter as this can damage it and reduce its effectiveness in filtering. Air-dry the filters in the sun when possible.
Recently many producers have replaced their flat filters with pop filters or bag filters introduced in the late 1980s.
Pop Sap Filters
Pop sap filters are effective inexpensive sap filters known as a pop-on or pop filter. It is a polyester or polypropylene bag type filter with a plastic adapter head that allows attaching sap lines of several diameters to it by using the correct reducer bushings. The bag is equipped with a stainless steel ring that literally pops over the plastic head to hold it on. Pop sap filters, sometimes called bag filters, are used to remove coarse debris from the sap prior to storage.
Figure 6. Pop sap filters are used to remove coarse particles from the sap prior to storage.
They are supplied in 50 and 100-micron ratings and they do not filter out microorganisms suspended in the sap. Pop filters can be installed at the end of a mainline or the vacuum extractor discharge or on a pump station pipe. They are washable and can be used several times. They should be washed every day after the sap run or more often if they become plugged. Washing them after the sap run each day removes the debris and microorganisms that may contaminate the next run of sap.
When pop sap filters are used on the end of pressure lines, make sure the filter bag does not come off as it becomes clogged.
Pressure Sap Filters
Economy In-line Sap Filters
Economy in-line sap filters are available from maple equipment suppliers. They are inexpensive, easy to install and maintain. Most are water filters that work well for filtering sap.
Figure 7. In-line sap filters are commonly called small pore filters. They are effective in removing contaminants including some microorganisms.
There are many manufacturers of the filter housings and several types of filters. The most common cartridge filter is polypropylene. The economy in-line type sap filter housings or casings are often made of reinforced polypropylene as well. They are available with a pressure gauge and relief valve installed. Stainless steel filter housings are available but they are expensive.
In-line sap filters can be installed in the sap transfer system where the sap is under pressure. They are also used in Reverse Osmosis machines to prefilter the sap before going through the Reverse Osmosis membranes. These filters are commonly referred to as small pore filters.
Disposable filter cartridges are available that filter down to 0.5 microns. Re-usable filter bags are also available but producers do not use them as often. Small pore filters are rated according to the pore size of the material in the filters. The pore size is measured in microns; the lower the number, the smaller the pore size and the more effective the filter is in removing contaminants including some microorganisms. Some producers use multiple in-line sap filters in series starting with a larger pore size and gradually reducing it to one micron in the last filter.
Protect in-line sap filter housings against freezing. Do not leave sap in sap filters between runs as it may con-taminate fresh sap. Flush the filter daily with cold potable water to remove sap. The level of maintenance depends on the rate of flow of the sap, the nature and quantity of the contaminants and the type and pore size of the filter. Always follow manufacturer's instructions regarding their use and maintenance.
Since cartridge filters are disposable replace clogged filters with new ones.
Diatomaceous Earth (D.E.) Sap Filters
Diatomaceous earth (D.E.) sap filters are the most recent type of filter used by maple producers. Developed to filter water from swimming pools, they are effective filters that trap tiny particles and microorganisms.
Figure 8. Diatomaceous Earth (D.E.) sap filters are the most recent filter type used for sap filtering. They can trap tiny particles as well as some microorganisms.
Use a food-grade diatomaceous earth powder, available from maple equipment dealers. D.E. powder is microscopic silica skeletons of Diatoms that appear like tiny sponges when viewed under a microscope. When added to the filter the powder coats the filter elements or septums and forms a layer called filter cake. When the sap passes through the microscopic openings in the diatomaceous earth, particles including microorganisms 1-3 microns or larger are trapped.
The type of D.E. filters used by maple producers are classed as regenerative, i.e. the flow of sap through the clogged filter can be restored to a near original flow without taking the filter apart and washing it. The clogged filter cake is removed from the filter elements, then mixed internally and reapplied to them. The method varies depending on the manufacturer. After regenerating the flow several times all makes of D.E. filters must be taken apart and washed, new diatomaceous earth put in and reassembled for operation.
Many producers prefer to buy larger size filters. A larger unit increases the time between cleanings, which require shutting off the pump and taking the filter apart. Protect all D.E. filters against freezing. If the D.E. filter clogs up quickly it usually means the filter is doing a good job in removing particles and contaminants. In all cases follow the manufacturer's instructions regarding set up, operation and maintenance.
Some maple producers have found D.E. sap filters clean cloudy or milky sap better than any other filter. However, it usually requires more than one pass through the filter to do this. Sap at the end of the season should be put through the filter several times. No matter how cloudy the sap is, you can usually make it clear with enough passes through the filter.
Sap filtering technology in the maple industry has greatly improved since the development of synthetic filters, followed by the use of pressure filters.
Many producers found that filtering sap is an effective method of increasing the quality of maple syrup by up to one full grade by improving the colour class. The effect of filtering on flavour is less apparent. A research project to evaluate the effect of filtering sap on the flavour of maple syrup is needed to determine if there is any significant improvement.
All maple producers should carefully plan the sap filtering requirements of their operation. The number and types of filters used depend on producer preference and the manner in which the sap is from the tree to the evaporator.
Filtering sap does not purify it completely. It does not remove all microorganisms but it is effective in controlling microbial development by removing debris from sap. Some filter types can remove or trap some of the microorganisms. Microorganisms remaining in the sap after filtering are not harmful since they are destroyed when the sap is boiled.
Combine sap filtering with other preventative measures to control or reduce microbial activity in maple sap. These include sanitary tapping, proper installation and maintenance of tubing, keeping sap cool in the sugar bush and storage tanks, boiling the sap soon after it runs, keeping buckets, gathering tanks and storage tanks properly covered, using germicidal ultraviolet lights, and proper cleaning and sanitizing of all equipment used in the sap transfer system.
The production of better quality maple products results in increased financial returns and improved customer satisfaction - two important goals of every maple producer.
The author wishes to acknowledge the assistance of Clarence Coons, Agroforestry consultant with the preparation of the Factsheet. Input provided by maple producers and others who reviewed the draft manuscript is appreciated. Assistance provided by the North American Maple Council and the Eastern Ontario Model Forest is also gratefully acknowledged.
For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047