When twist-off jars are closed manually with screw caps (cam screws) it may occur that the jars aren't tight even before, during or after the sterilisation process. You should check the closing safety for screw caps to exclude technical causes and faulty products. For this purpose the manual process and the expected tightness should be analysed.
The compound in screw cams that are suitable for sterilisation (maximum temperature up to 121°C) is harder than the compound in screw caps that are suitable for pasteurisation (up to 97°C). Sterile compounds consist of non-foamed materials to satisfy the migration regulations for the use of fatty filling goods. These caps are frequently made for (partially) automatic or rather industrial filling. The tolerance range for the optimal closing safety for screw caps (equivalent to tightness) has decreased in the last years. Manual treatment is more difficult and requires a higher physical effort!
Fig. 1 First search the seam at the glass mouth to check the closing safety for screw caps.
Fig. 2 Mark the cam at the glass mouth.
Fig. 3 Mark the beginning of the cam at the corner of the lid and close the jar.
Fig. 4 Measure the distance between both marks. Like this you identify the distance between seam and cam after the closing process.
Recommendation for the distance between seam and cam:
Lid TO 53-70 = 2-5 mm
Lid TO 82-110 = 2-7 mm
With some practice you can check the distance between seam and cam on the closed jar taking a subsequent glance under the corner of the screw cap.
Checking the closing safety for screw caps place a vertical marking on lid and jar when the jar is closed (fig. 6). Open the jar and screw it until it's finger-tight (i.e. when you perceive the first resistance). The distance between the markings on the jar and lid gives the so-called closing safety (fig. 7). The closing safety for screw caps should be 4 to 7 mm and 1 to 5 mm after cooling down (fig. 8).
Before use heat the lids in a warm water bath at a temperature of about 65°C for at least 15 minutes to assure the closing safety for screw caps. Please note that the refilling the water bath with "cold" screw caps reduces the temperature significantly. So please pay attention that the temperature is kept constantly on a level of 65°C.
Always keep glass mouth and screw cap (compound, cams) tidy.
Constant filling temperature
Ensure a constant temperature of the filling good.
No air pockets
Avoid air pockets in the filling good.
Check if there is enough head space (air volume) between the filling good and the screw cap because otherwise no vacuum can be created.
Internal and counter pressure
Using the autoclave the difference between internal and counter pressure must be less than 0,7 bar.
Reason for leakage => insufficient safety level
Vacuum is missing or too small
Overtightened screw cap