A spectroscopic method, preferably Raman scattering, is used to rapidly determine the general health or stress status of living plants and plant products, including agricultural crops, forests, and harvested fruits and vegetables. In the preferred embodiments, carotenoid levels are used to provide an indication of oxidative deterioration. Based upon the results of the analysis, further action may or may not be taken, for example, in terms of choosing, picking, harvesting or sorting the agricultural product in accordance with the carotenoid level. Concentration levels of an analyte substance can be determined relative to an external standard, to each other, or relative to another substance in the item being analyzed. Carotenoids such as lycopene, beta-carotene, lutein, violaxanthin, neoxanthin, antheraxanthin, and zeaxanthin are determined according to the invention, through other plant components may be analyzed such as other terpenes, polyenes, chlorophyll, proteins, starches, sugars, overall nitrogen levels, flavonoids, and vitamins. The hardware associated with the invention may be field portable or mounted on a piece of equipment such as a harvester or sorter.