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article 03, issue 01

Human Powered Juice Making

Theo Schmidt
August 21, 2004

Abstract

A traditional hand-powered juicing method is described.  An innovative human-powered preservation method is presented.
 

Introduction

Juice making provides an important way of preserving large quantities of fruit and vegetables for human consumption.  This can be done with electrically powered machines, but many situations exist when human-powered juicing represents the most practical option.  In my case this begins with an apple tree which during several weeks in August delivers 5 -10 kg of fruit per day. The apples are a curious variety, which must be processed within a day or two to avoid spoiling.  This amount is ideal for traditional hand-powered equipment.  A local juicing service, usually not available until later in the harvest season, requires a minimum quantity about ten times larger .  A hand-operated pulper and press allow me to utilize more of the tree's apples and saves making trips to the commercial juicer. Much of the information here also applies to grapes and other fruit.

 

[Figure 1] The apples are washed in water.

[Figure 2] Pulper.

[Figure 3] Press and bowl.

The juicing process

The juicing process involves gathering, sorting, washing [fig. 1], pulping [fig. 2], pressing [fig. 3], and preserving for storage unless quickly consumed.  Pulping represents the key step, accomplished with a hopper-fed rotary rasp. The pulper's wooden cylinder is covered with over 50 sharp stainless steel studs (visible dimensions about 4 x 4 x 4 mm) [fig. 4]. The interior hopper dimensions are about 10 x 10 x 10 cm, perfect for small apples.  The interior cylinder diameter is 21 cm, the height 24 cm. The resulting pulp is put into a fabric bag placed inside the press, where about 40% of the weight is extracted as juice  [figs. 3 and 5]. 

 

[Figure 4] Detail of the pulper's cylindrical  rasp.

[Figure 5] Overview of press components. Note the cloth bag.

 

Human powered juice preservation

Fermentation and traditional preservation

Traditionally apple juice may be consumed fresh or preserved by pasteurization in various stages of fermentation (conversion of sugars to alcohol) ranging from sweet through hard cider to apple wine and ultimately apple vinegar. The dominating yeast cultures influence the taste. Sometimes other microbial action takes over to render the juice unpalatable.  Hence, modern standards of taste conformity and hygiene insist on tighter control than was the custom in the past. Today, heating to about 80 C pasteurizes fresh-pressed sweet apple juice. Afterward the juice is cooled allowing excessive solids to settle forming a sediment.  For harder cider, pasteurized juice ferments under controlled conditions utilizing specific yeast cultures. Further pasteurization halts the fermentation at a chosen point.  All this heating is of course energy consuming.  There is another less energy intensive way.

 


[Figure 6] Quarter-liter plastic and glass bottles. Note bicycle valve.

Preservation by pressure

Fermentation slows or stops at above 5 - 9 bar, depending on storage temperature and the composition of gases present.  Because a living yeast culture outgases carbon dioxide, a tightly closed container will self-pressurize. This increased pressure will temporarily slow the fermentation rate. This is not true "pressure pasteurization", which requires about 5000 bar.

 I simply fill the raw juice into bottles made of glass or plastic (PET).  After a few days, the juice within a closed container begins to build up considerable pressure.  Ordinary bottles aren't suited for this and could be dangerous.  Out of a approximately one hundred, I have had one glass and one plastic bottle of the 1-liter size explode.  Thankfully I wasn't around. Thus, I  recommend using bottles less than 1-liter, unless the juice is to be consumed within a short while after bottling.  Smaller, thus stronger, bottles have never ruptured.  The greatest danger is to the eyes in the event of a glass container breaking without warning. Wearing heavy-duty safety goggles could prevent serious injury when handling or working around sealed bottles.  The pressure preserved juice tastes sweet,  with a slight alcohol content and a carbonated tang.  I love it!  Fermentation can be halted even more quickly and thus with a lower alcohol content by increasing the initial pressure from an external source and using cooler storage conditions.  I do this by mounting bicycle tire valves in bottle caps and pressurizing with a tire pump [fig 6].  Opening the bottles later can be messy and exciting. A significant volume of carbon dioxide may be released causing the fluid to foam considerably.  In these cases the reward of juice making is a glass of apple-champagne!  Other times faulty bottle tops leak at just the right rate for the juice to preserve and then lose pressure without the formation of hard cider or vinegar.  These fortunate "accidents" have not been consistently reproducible.

[Figure 7] A 30 liter stainless steel container. Working pressure 9 bar.


[Figure 8] Approximately 9 small apples are needed to make a glass of juice.

More efficient and far safer than bottles are special stainless steel kegs.  Mine has 30 liters volume.  Ideally this container would be filled completely then sealed to help reduce the amount of oxygen present, which contributes to the formation of vinegar. In order to preserve quantities of juice less than 30 liters, I pressurize the less-than-full steel container using an air pump. However pumping in carbon dioxide rather than the ambient air may be more suitable.  Future plans include injecting the juice day-by-day under pressure by using a feed pump.  All of these notions include human-power of course!
 

Data

A typical run consists of processing 100 small apples (6 kg)  into 2 - 2.5 liters of juice.

The pulper operates most comfortably  at about 100 rpm.  The corresponding force on the crank is 40 - 50 N,  giving a torque of 7.6 - 9.5 Nm with the 0.19 m crank.  Thus, the power delivered by the operator equals 80 -100 W.  One filling of the hopper requires about 9 turns, i.e. about 5 seconds.  The force on the push lever is also 40-50 N, but nearly static.  So not much power is used.

The power required to operate the press is also quite small.  Most effort is spent preparing and handling the tools and materials and general cleaning.


Resources

A good site for amateur cider making is:

http://ourworld.compuserve.com/homepages/andrew_lea/content2.htm

 

 

 

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