A method of positioning and sealing a bag in a vacuum chamber, the method comprising: loading the bag by placing a product in the bag to produce a loaded bag, the bag including an upstream end including a bag mouth, the bag further including a patch adhered thereto, the patch including a fluorescent material, the bag and the patch both being transparent to infrared radiation; placing the loaded bag on an infeed conveyor; advancing the loaded bag, on the infeed conveyor, to an infrared sensing apparatus including an infrared detector and a first radiation source, the infrared detector being disposed on an opposite side of the infeed conveyor from the first radiation source; detecting a trailing edge of the product inside the loaded bag by interrogating, through the loaded bag, infrared radiation emitted from the first radiation source, using the infrared sensing apparatus; advancing the loaded bag to a fluorescence sensing apparatus including a fluorescence detector and a second radiation source; detecting a trailing edge of the patch by interrogating fluorescence emitted by the patch using the fluorescence sensing apparatus, wherein radiation emitted by the second radiation source excites the fluorescent material; acquiring information from detecting the trailing edge of the product and detecting the trailing edge of the patch, and transmitting the information to a controller; controlling a distance of advancement of the loaded bag to a sealing position in the vacuum chamber including a heat seal assembly (364, 366), using the controller, based on the information acquired from detecting the trailing edge of the product and detecting the trailing edge of the patch; and closing the loaded bag by heat sealing the loaded bag, using the heat seal assembly (364, 366), so that a heat seal is applied between the trailing edge of the product and the bag mouth and between the trailing edge of the patch and the bag mouth. loading the bag by placing a product in the bag to produce a loaded bag, the bag including an upstream end including a bag mouth, the bag further including a patch adhered thereto, the patch including a fluorescent material, the bag and the patch both being transparent to infrared radiation; placing the loaded bag on an infeed conveyor; advancing the loaded bag, on the infeed conveyor, to an infrared sensing apparatus including an infrared detector and a first radiation source, the infrared detector being disposed on an opposite side of the infeed conveyor from the first radiation source; detecting a trailing edge of the product inside the loaded bag by interrogating, through the loaded bag, infrared radiation emitted from the first radiation source, using the infrared sensing apparatus; advancing the loaded bag to a fluorescence sensing apparatus including a fluorescence detector and a second radiation source; detecting a trailing edge of the patch by interrogating fluorescence emitted by the patch using the fluorescence sensing apparatus, wherein radiation emitted by the second radiation source excites the fluorescent material; acquiring information from detecting the trailing edge of the product and detecting the trailing edge of the patch, and transmitting the information to a controller; controlling a distance of advancement of the loaded bag to a sealing position in the vacuum chamber including a heat seal assembly (364, 366), using the controller, based on the information acquired from detecting the trailing edge of the product and detecting the trailing edge of the patch; and closing the loaded bag by heat sealing the loaded bag, using the heat seal assembly (364, 366), so that a heat seal is applied between the trailing edge of the product and the bag mouth and between the trailing edge of the patch and the bag mouth.